US20070125653A1 - Multilayer electroform, methods of making multilayer electroforms, and products made therefrom - Google Patents

Multilayer electroform, methods of making multilayer electroforms, and products made therefrom Download PDF

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US20070125653A1
US20070125653A1 US11292475 US29247505A US2007125653A1 US 20070125653 A1 US20070125653 A1 US 20070125653A1 US 11292475 US11292475 US 11292475 US 29247505 A US29247505 A US 29247505A US 2007125653 A1 US2007125653 A1 US 2007125653A1
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master
sub
electroform
μm
layer
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US11292475
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Dennis Coyle
Kenneth Zarnoch
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SABIC Innovative Plastics IP BV
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General Electric Co
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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D1/00Electroforming
    • C25D1/0033D structures, e.g. superposed patterned layers
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D1/00Electroforming
    • C25D1/006Nanostructures, e.g. using aluminium anodic oxidation templates [AAO]
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D1/00Electroforming
    • C25D1/10Moulds; Masks; Masterforms, e.g. mandrels, stampers

Abstract

In one embodiment, the method for making an electroform comprises: passivating a sub-master to form a passivation layer, disposing the passivated sub-master into an initial electroplating solution, applying a first current, depositing a first metal onto the passivated sub-master to form an initial coating on the sub-master, moving the initially coated sub-master from the initial electroplating solution to a subsequent plating solution, applying a second current, depositing a subsequent layer of a second metal onto the initial coating to form a multilayer, and removing the multilayer to form the electroform.

Description

    BACKGROUND
  • This disclosure generally relates to electroforming and methods for forming an electroform.
  • Electroforming involves an electrochemical process that uses an anode (which may supply metal for deposition), an electrolyte, and a substrate (which acts as a cathode). An electrical current to the anode and cathode is controlled to manage the deposition of the metal onto the substrate to create a metal replica of various shapes and textures. In another example, electroforms can be made from a complex micromachined master. The replicas (or micromachined master) can be used to mass-produce plastic articles with precise microstructure using processes such as printing, embossing, and casting. For example, these replicas can be employed in the production of data storage media such as CDs, DVDs, and the like.
  • In backlight computer displays or other display systems, optical films are often used to direct light. For example, in backlight displays, light management films use prismatic structures (often referred to as microstructure) to direct light along a viewing axis (i.e., an axis substantially normal to the display). Directing the light enhances the brightness of the display viewed by a user and allows the system to consume less power in creating a desired level of on-axis illumination. Films for turning or directing light can also be used in a wide range of other optical designs, such as for projection displays, traffic signals, and illuminated signs. The prismatic structures are generally formed in a display film by replicating a metal tool, mold, or electroform having prismatic structures disposed thereon, via processes such as stamping, molding, embossing, or UV-curing. It is generally desirable for the display film and the mold to be free from defects so as to facilitate a uniform luminance of light. Since such structures serve to strongly enhance the brightness of a display, any defects, even if they are small (on the order of 10 microns), can result in either a very bright or very dark spot on the display, which is undesirable. The mold and the display films are therefore inspected to eliminate defects.
  • Molds such as, for example, electroforms are generally used for manufacturing light management films such as prism sheets for use in liquid crystalline displays. In general, such light management films have at least one microstructured surface that refracts light in a specific way to enhance the light output of the display. Since these films serve an optical function, the surface features must be of high quality with no roughness or other defects. This microstructure is first generated on a master, (e.g., a silicon wafer, glass plate, metal drum, or the such) and is created by one of a variety of processes such as photolithography, etching, ruling, diamond turning, or others. Since this master tends to be expensive to produce and fragile in nature, tooling or molds are typically reproduced off of this master, which in turn serve as the molds from which plastic microstructured films are mass-produced. These tools can be metal copies grown via electroforming processes, or plastic copies formed via molding-type processes. Tools copied directly from the master are called 1st-generation (sub-master), copies of these tools are called 2nd-generation (sub-master), etc. In general, multiple copies can be made of every tool made at any generation, leading to a geometric growth in number of tools with each generation—i.e. a “tooling tree” is produced. Each generation is an inverted image of the previous generation. If the desired final product is a “positive” geometry, then any generation of tooling that is a negative can be used as a mass-production replication tool. If the master is manufactured as a negative, then any even-generation mold can be used for mass-production.
  • One difficulty always present when a manufacturing process, such as the optical display film manufacturing process, uses a component or subprocess in a subsequent step of the process is the systemic defect. If a major component, such as a shim tool or a master tool, is defective, then every subsequent mold and film replicated from those components will be defective. In prior attempts to alleviate this problem, the optical display film manufacturing process has been separated into three semi-independent manufacturing processes, the master tool, the shim tool and the display film manufacturing processes. Each primary manufacturing process has had an independent inspection and defect correction process that identifies a defective component or product at that particular step in the process and then removes it from the process chain. These processes are intended to prevent a defective master tool from being made into a defective shim tool, a defective shim tool from being made into defective film samples, and defective film samples from being sold.
  • Depending upon the size of the replica, the size, geometry, and amount of features to be replicated, and the materials of the master and sub-master, the degree of successful replication can vary greatly. There is a constant need to make the electroplating process more efficient (e.g., reduce the plating time), and more effective (e.g., improve the accuracy of the replication, enhance the separation of sub-master from master, and reduce the amount of yield loss). These needs are especially difficult when the articles made from the electroform serve an optical function, making tolerances critical and very small defects unacceptable.
  • BRIEF SUMMARY
  • Disclosed herein are methods for making electroforms, electroforms formed therefrom, and products made from the electroforms.
  • In one embodiment, the method for making an electroform comprises: passivating a sub-master to form a passivation layer, disposing the passivated sub-master into an initial electroplating solution, applying a first current, depositing a first metal onto the passivated sub-master to form an initial coating on the sub-master, moving the initially coated sub-master from the initial electroplating solution to a subsequent plating solution, applying a second current, depositing a subsequent layer of a second metal onto the initial coating to form a multilayer, and removing the multilayer to form the electroform.
  • In one embodiment, the electroform comprises: a surface layer comprising a surface layer composition, and a backing layer comprising a different composition than the surface layer composition.
  • In one embodiment, a method for making a film comprises: applying a coating to a surface of an electroform, curing the coating, and removing the coating from the electroform to form the film. The electroform comprises multiple layers, and has microfeatures having nanoscale resolution disposed in the surface of the electroform.
  • The above described and other features are exemplified by the following figures and detailed description.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • Refer now to the figure, which are exemplary, not limiting, and wherein like numbers are numbered alike.
  • FIG. 1 is one embodiment of an electroforming process map.
  • FIG. 2 is a cross sectional view of a backlight display device.
  • FIG. 3 is a perspective view of an optical substrate comprising a surface characterized by a cross section of a prism having a curved sidewall or facet.
  • FIG. 4 is a first cross sectional view of an optical substrate comprising a surface characterized by a cross section of a prism having a curved sidewall or facet.
  • FIG. 5 is a second cross sectional view of an optical substrate comprising a surface characterized by a cross section of a prism having a curved sidewall or facet.
  • FIG. 6 is a cross sectional view of a compound angle prism and of the geometric parameters of the curved sidewall or facet of FIGS. 4 and 5 as described by a segment of a polynomial function.
  • FIGS. 7 and 8 are scanning electron micrographs of an electroform with a multilayer portion and a single layer portion.
  • FIG. 9 is a diagram illustrating the areas of the electroform that were imaged in FIGS. 7 and 8, namely area 2 and 5, respectively.
  • DETAILED DESCRIPTION
  • Ranges disclosed herein are inclusive and combinable (e.g., ranges of “up to about 25 wt %, or, more specifically, about 5 wt % to about 20 wt %”, is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt % to about 25 wt %,” etc). Furthermore, the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context, (e.g., includes the degree of error associated with measurement of the particular quantity). The suffix “(s)” as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., the colorant(s) includes one or more colorants).
  • FIG. 1 is a process map of one embodiment of an electroforming process for making a sub-master and a 2nd generation sub-master. The process comprises forming a master drum having a pattern disposed in an external surface thereof. The pattern can be produced in various fashions, e.g., photolithography, machining, etching, cutting, milling, scribing, among other techniques. The master can be cleaned, e.g., washed with organic solvents, water, acid, and/or base. To enable the removal of the electrodeposited layer from the master, a release point can be formed and the drum can be passivated. The release point can be an area of the master that is masked, such as with tape, to prevent metal deposition on that area. The area is chosen to be outside of the pattern, and has a length to facilitate removal of the layer from the drum. For example, the tape can be disposed longitudinally across the master such that, once the layer has been disposed on the master, the tape can be removed, exposing an edge of the sub-master. The sub-master can then be removed by peeling it from around the drum. It is desirable to uniformly remove the sub-master from the drum without twisting or torquing the sub-master. Twisting, torquing, and other non-uniform removal can damage the pattern on the surface of the sub-master and potentially even damage the surface of the master.
  • To further facilitate the separation, the surface of the master can be passivated which helps to prevent the replica (i.e., sub-master) from adhering to the surface of the master. Possible passivation techniques include the formation of a separation layer (such as an oxide and/or hydroxide layer) over the master surface, electrostatic cleaning, and/or by chemical passivation techniques. Formation of a separation layer can comprise an electrolytic oxidation process wherein the electrolytic current and voltage are applied to form a controlled thickness separation layer. Chemical passivation can comprise immersing the master surface in a solution for a controlled period of time. The particular solution is dependent upon the master composition. Some possible solutions include alkali metal hydroxide solutions, chromate (such as potassium dichromate), among others.
  • For example, the surface of the master can optionally be rinsed with Simple Green solution (commercially available from Sunshine Makers, Inc., located in Huntington Beach, Calif.) and then sprayed with a saponin solution to promote wetting of the surface. A potassium dichromate solution (e.g., about 5 grams per liter (g/l)) can be applied to the surface of the master (e.g., poured over the surface). The potassium dichromate is then rinsed from the master surface to form a passivated master. Optionally, the saponin and potassium dichromate applications can be repeated as desired.
  • The passivated master can then be plated in various processes, including an electroforming process. The electroforming process can be performed in an electroforming tank where the outer surface of the master functions as the cathode through electrical contacts. The anode can be constructed from various metals, including the metal to be deposited during metallization. For example, a nickel anode or nickel alloy can be used if nickel is the desired metal in the metallization process. For example, the passivated master can be placed into an electroforming solution and optionally rotated (e.g., up to about 10 revolutions per minute (rpm) or so) to more uniformly deposit the metal. A rectifier in electrical communication with the anode and cathode can be maintained constant during this process or it can be adjusted. The electroforming can be accomplished in up to about 24 hours.
  • The solution in the electroforming tank can be an aqueous solution comprising a surfactant agent, a pH of less than or equal to about 6, and optionally a hardening agent. The solution will further comprise the metal(s) to be deposited. One embodiment of a solution can comprise about 60 grams per liter (g/l) to about 100 g/l of metal sulfamate (e.g., the metal to be deposited), sufficient acid to attain a pH of less than or equal to about 6, a sufficient amount of surfactant agent to affect wetting of the metallic surface to be coated, and optionally a hardening agent, e.g., to control stress in the deposit. For example, the solution can comprise about 70 g/l to about 90 g/l nickel sulfamate, about 25 g/l to about 35 g/l boric acid, and sufficient sulfamic acid to attain a pH of about 2 to about 5.0.
  • When a current is applied to the system, the anodic metal oxidizes to form metal ions which then flow to the cathode (the outer surface of the passivated master) and deposit thereon. The cathode then reduces the metal ion into elemental metal. The following shows the reactions at the anode and cathode for nickel:
    anode: Ni0−2e→Ni2+
    cathode: Ni2++2e−→Ni
    Electroforming of other metals also go through similar reactions at the anode and cathode. Some of the possible metals for the electroforming process include, but are not limited to, nickel (Ni), cobalt (Co), copper (Cu), silver (Ag), iron (Fe), aluminum (Al), titanium (Ti), iridium (Ir), gold (Au), chromium (Cr), beryllium (Be), tungsten (W), tantalum (Ta), molybdenum (Mo), platinum (Pt), palladium (Pd), gold (Au), among others, as well as alloys comprising at least one of the foregoing metals, and mixtures comprising at least one of the foregoing metals. Some possible alloys include a nickel-phosphorus (NiP) alloy, a palladium-phosphorus (PdP) alloy, a cobalt-phosphorus (CoP) alloy, a nickel-cobalt (NiCo) alloy, a gold-cobalt alloy (AuCo), and a cobalt-tungsten-phosphorus (CoWP) alloy.
  • Electroforming process parameters include solution temperature, composition, and rectifier voltage. Regarding the temperature, the solution in the electroforming tank can optionally be heated to about 30° C. to about 80° C., or, more specifically, about 35° C. to about 60° C., or, even more specifically, about 40° C. to about 50° C. The rectifier can be used to apply a sufficient voltage to the electrodes to induce an electric current to cause anodic oxidation of the metal to be deposited, and to reduce the metal ions at the cathode. For the formation of a Ni or Ni alloy layer, for example, the current density can be about 2 amperes per square foot (ASF) to about 100 ASF or so, or, more specifically, about 5 ASF to about 60 ASF or, even more specifically, about 10 ASF to about 30 ASF.
  • The exposure time in the electroforming tank while the current is applied can be determined based upon the particular metal layer to be formed and the desired thickness of that layer. The layer thickness can be based upon a desired structural integrity to enable the layer to be removed from the master as well as to be used to produce next generation sub-masters, and based upon the size of the features formed in the surface of the layer. Thicknesses can be up to and exceeding about 500 micrometers (μm) or so, or, more specifically, about 50 μm to about 400 μm, or, even more specifically, about 100 μm to about 300 μm, and, yet more specifically, about 150 μm to about 250 μm.
  • By controlling the processing parameters of the electroplating, the thickness of the deposited metal layer can be adjusted. The thickness of this metal layer can be calculated from the equation: T = ( M · I · t Z F ρ A )
  • where: T=thickness of the electroformed layer;
      • M=the molar mass of the metal;
      • I=the current;
      • t=the time of electroformation;
      • |Z|=the absolute value of the valence of the metal;
      • F=Faraday constant;
      • ρ=the density of the metal; and
      • A=the surface area to be covered by the metal.
        This equation gives a theoretical maximum thickness assuming 100% efficiency of the cathode. However, because electrodes are not always 100% efficient, the actual thickness is usually less than that calculated by the equation. Generally, the efficiency of an electrode is about 95% to about 99% depending on the material used as well as other factors.
  • Once the desired thickness is achieved, rectifier is switched off and the cylinder is removed from the electroforming tank. Optionally, the coated master is rinsed, e.g., with water (such as deionized water (i.e., water that has been treated with an ion exchange resin to remove ions therefrom)), and retained in an inert environment (e.g., an environment that does not chemically interact with the sub-master surface to change the surface chemistry under the environmental conditions). Some possible inert environments include nitrogen, argon, helium, vacuum, and others, depending upon the environment.
  • The sub-master, comprising a negative of the structures on the master, can be separated from the master. For example, if a separation tape (also know as plater's tape) has been disposed on the master, the tape can be removed from the master, exposing the master as well as an edge of the sub-master. The sub-master can then be peeled from the master. The master can again be used to produce additional generations of sub-masters by repeating the masking, passivation, plating, and separation.
  • The sub-master thus produced, can then be used to make next generation sub-masters (e.g., shims). Prior to employing the sub-master in the plating process, the sub-master can optionally be annealed. The annealing can be employed to flatten the sub-master into a sheet-like form, for example, for ease in production of a subsequent generation of sub-masters. The sub-master, once removed from the master, has a rounded shape, e.g., a partial cylinder. Therefore, the sub-master can be heated to a sufficient temperature to change the shape of the sub-master from a cylinder-like to a sheet or plate-like sub-master. The particular annealing temperature is based upon the sub-master composition as well as the annealing time. The particular temperature employed is sufficiently high to soften the sub-master such that it forms a sheet in a chosen time, while sufficiently low to avoid undesirable surface reactions as well as adversely affecting the sub-master's surface features. The upper temperature limit is based upon the melting temperature of the sub-master material, as well as the possible reactions and adverse effects that the heat may have on the surface features, while the lower temperature limit is based upon a practical amount of time to convert the overall shape of the sub-master to a sheet-like form. At the higher temperatures, an inert environment can be employed.
  • With Ni and Ni alloy sub-masters, for example, annealing can be accomplished at a temperature of about 200° C. to about 400° C., or, more specifically, about 200° C. to about 300° C., or, even more specifically, about 225° C. to about 275° C. These temperatures can be employed for periods of time of about 3 hours to about 10 hours, or, more specifically, about 4 hours to about 7 hours.
  • Optionally, the annealed sub-master can be mounted to a stiffener plate to enhance the structural integrity of the sub-master. The stiffener plate can comprise any material that will provide the desired structural integrity to the sub-master, will not react with the plating solution and/or the sub-master under the plating conditions. Possible stiffener plates include materials such as aluminum, ferrous materials (e.g., stainless steel), and so forth, as well as combinations comprising at least one of the foregoing materials.
  • The sub-master can be mounted to the stiffener plate with various adhesives, such as a non-conductive vinyl adhesive, double-faced tape (e.g., pressure sensitive, double-faced tape). The tape adhesive can comprise rubber, acrylic, silicone, and combinations comprising at least one of the foregoing adhesives. The adhesives can be applied to the plate and contacted with a rubber roll to attain good adhesion. Optionally, the plate can then be placed in a vacuum bag where a vacuum can be applied to evacuate air from the space between the sub-master and the plate, eliminating air voids. In other words, the pressure in the bag can be reduced to below the pressure of the atmosphere surrounding the bag.
  • In another embodiment, the sub-master can be attached to the stiffener plate using magnetic force. For example, the plate could have permanent and/or electromagnets imbedded therein, and/or magnetic sheeting (e.g., vinyl magnetic sheeting) can be adhered to the surface of the stiffener plate.
  • The mounted sub-master can then be masked to prevent the subsequent deposition of the metal layer onto the stiffener plate, as well as onto undesirable areas of the sub-master. Various masking materials compatible with the plating environment as well as the stiffener plate and sub-master, can be employed. Some exemplary materials include vinyl tape, polyimide tape, and polyester tape, among others. The polyester tape has been found to be chemically resistant and consequently reusable for several plating cycles. The adhesive on the tape can comprise various adhesives compatible with the plating environment, such as silicone. The mask can be applied to the back of the plate and the perimeter of the front of the plate to define the plating area for the electroforming step.
  • Connection areas can then be cut through the tape, e.g., at the short edges of the sub-master, to expose the mounting plate. Conductive material can then be employed to connect the stiffener plate and the sub-master. The conductive material can be a metal such as nickel, aluminum, stainless steel, copper, and combinations comprising at least one of the foregoing metals. For example, metal foil tape can be applied to the connection area and conductive sealer (e.g., conductive paint) can be disposed around the copper tape to ensure conductivity between the sub-master and the stiffener plate (wherein the foil tape has a conductive adhesive).
  • In order to form sub-masters with a taping area, e.g., to inhibit surface area yield loss in the subsequent generations of sub-masters, the sub-master can optionally be mounted on an oversized stiffener plate. This stiffener plate can have a larger surface area than the sub-master such that, on the side of the stiffener plate where the sub-master is mounted with adhesive (e.g., double-faced tape), there is an area of stiffener plate surface that extends beyond the perimeter of the sub-master; e.g., that forms a boarder around the sub-master. The size of the desired border is application dependent. A border can be formed having a width of up to about 5 inches (12.7 centimeters (cm) or so, or, more specifically, about 0.25 inches (0.6 cm) to about 4 inches (10.2 cm), or, even more specifically, about 0.5 inches (1.3 cm) to about 3 inches (7.6 cm), and, yet more specifically, about 1 inch (2.5 cm) to about 2 inches (5.1 cm).
  • A conductive rim can then be disposed on the exposed surface of the stiffener plate, overlaying the edges of the sub-master. The conductive rim can be formed with any conductive material that is compatible with the plating environment and materials, and that can be adhered to the stiffener plate. Possible conductive materials include stainless steel, copper, nickel, and silver, among other conductive materials. Possible forms for the conductive material include sheets, tapes (such as pressure sensitive tape), paintable liquids and/or pastes containing metals, among others.
  • If the adhesive on the tape used to form the conductive rim is not electrically conductive, a conductive material such as a conductive paint can be applied to the seam between the sub-master and the conductive rim. The conductive material can be dried (actively or passively), sanded smooth to remove over-painted regions, thereby forming an extended sub-master. The mounted sub-master can then be masked as described above, leaving at least a portion of the conductive rim exposed. Here, due to the presence of the conductive rim, when the electroform is deposited (e.g., via chemical vapor deposition, plasma spraying, in an electroforming bath, or otherwise), the next generation sub-master produced will have a non-surface feature area that forms a periphery or frame around the surface feature area. This periphery can then be used for the masking and adhesion in the production of subsequent generations of sub-masters whereby the loss of surface area in the surface feature region is minimized because the non-surface feature frame can be sacrificed if trimming of the periphery is needed.
  • The mounted sub-master can be connected to electrode(s) through the back of the stiffener plate. For example, copper electrode(s) can be secured to the back of the stiffener plate with copper screws.
  • The masked sub-master can then optionally be disposed in a box (e.g., a frame such that the mounted sub-master forms the back of the box), where the edges of the frame are sealed to the plate with sealant. For example, the mounted sub-master is placed in a box (such as a pre-machined box formed from an electrically non-conductive material such as glass, plastic (e.g., polyvinyl fluoride), and/or the like) and sealed to the masked sub-master with sealant such as a silicone sealant. The frame may help facilitate the even distribution of the metal layer on the sub-master, inhibiting buildup at the edges thereof.
  • The sub-master can be used in the plating process to create additional electroforms so long as the surface to be plated is properly passivated. As noted above, passivation helps to prevent the next generation sub-master from adhering to the surface of the prior generation sub-master once formed. Controlling various parameters of the passivation layer affect the life of the sub-master (e.g., the number of copies that can be made from the sub-master while retaining macroscale, microscale, and nanoscale resolution). Macroscale refers to the reproduction in the next generation sub-master of the overall geometry of the replica, such as flatness and visual uniformity. This macrostructure has a size of approximately 1 millimeter (mm) to about 1 meter (m) or the entire size of the part being formed; i.e. of a size scale easily discerned by the human eye. Microscale refers to the reproduction in the next generation sub-master of microstructures on the surface, such as hemispheres, corner-cubes, prisms, or the like. These microstructures have a size of less than or equal to about 1 mm, or, more specifically, greater than 100 nanometers (nm) to about 1 mm. Nanoscale resolution refers to the reproduction in the next generation sub-master of nanostructures forming part of a surface feature, such as at or near corners or a peak or valley of a surface feature, or the optical smoothness of a facet of a feature. These nanostructures have a size of less than or equal to about 500 nm, or, more specifically, less than or equal to about 100 nm, or, even more specifically, less than or equal to about 20 nm, and yet more specifically, about 0.5 nm to 10 nm.
  • The parameters that can be controlled that can affect the life of the sub-master include the chemical composition, the thickness, density, and distribution of the passivation layer. By controlling the passivation layer, greater than or equal to about 50, or, more specifically, greater than or equal to about 75, or, even more specifically, greater than or equal to about 100, and even expect hundreds of sub-masters from nickel alloy electroform replicas having the nanoscale resolution can be produced.
  • The particular passivation employed for the sub-master is dependent upon the sub-master material. Passivation can be accomplished, for example, by contacting the sub-master with a solution (e.g., an aqueous solution) and anodically charging the sub-master. The aqueous solution can comprise a surfactant and be alkaline (e.g., have an alkalinity of greater than or equal to pH 8, or, more specifically, greater than or equal to about 10, or, even more specifically, a pH of about 12 to about 14).
  • The surfactant can be any material that reduces the surface tension of water and aids the wetting of the metal surface. Possible surfactants include cationic, non-ionic, anionic, as well as combinations comprising at least one of the foregoing surfactants. Anionic surfactants include, for example, carboxylates, sulfonates, sulfates, and phosphate esters. Cationic surfactants include, for example, amines and quaternary salts. Non-ionic surfactants include, for example, polyoxyethylene derivatives of fatty alcohols, carboxylic esters, and carboxylic amides.
  • The alkalinity can be attained with a material capable of promoting or causing the formation of metal oxides and/or metal hydroxides on the surface of the sub-master, e.g., an oxidation species. Some possible oxidation species that can be employed include alkali metal hydroxides (such as sodium hydroxide, potassium hydroxide, and the like), as well as combinations comprising at least one of the foregoing.
  • Once the sub-master is disposed in the aqueous solution, it is anodically charged to convert metallic components on the surface thereof to metal oxides and/or metal hydroxides, thereby forming a passivation layer. The sub-master can be charged until the passivation layer has a thickness of about 10 Angstroms (Å) to about 500 Å, or, more specifically, about 15 Å to about 60 Å, or, even more specifically, about 20 Å to about 40 Å. The amount of current applied to the sub-master can be about 1 ASF to about 40 ASF, or, more specifically, about 5 ASF to about 25 ASF, or, even more specifically, about 5 ASF to about 10 ASF. This current can be applied for a period of about 1 minute to about 5 minutes, or, more specifically, about 1 minute to about 3 minutes.
  • This passivation technique has been found particularly useful with Ni containing sub-masters (e.g., comprising Ni and/or a Ni alloy (e.g., NiCo, NiCr, among others)). For the Ni containing sub-masters, a greater number of successful replication of the nanostructures was achieved when employing a solution comprising an alkali metal hydroxide instead of a chromate. Possible alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and the like, as well as combinations comprising at least one of the foregoing.
  • Once passivated, the sub-master can optionally be dried (passively and/or actively), and then plated. Plating can be in various fashions that are capable of applying a layer of metal onto the surface of the electroform and thereby replicating the surface features thereof. For example, the passivated electroform can be placed in a solution in the electroforming tank. Electrical connection is made to the surface of the sub-master comprising the surface features so that it becomes the cathode. This solution in the electroforming tank can comprise about 60 g/l to about 100 g/l of metal sulfamate (e.g., the metal to be deposited), sufficient acid to attain a pH of less than or equal to about 6, a sufficient amount of surfactant agent to affect wetting of the metallic surface to be coated, and optionally a hardening agent, e.g., to control stress in the deposit. For example, the solution can comprise about 70 g/l to about 90 g/l metal alloy sulfamate (e.g., nickel-cobalt sulfamate, cobalt-tungsten sulfamate, and so forth), about 25 g/l to about 35 g/l boric acid, and sufficient sulfamic acid to attain a pH of about 2 to about 5.0. The anode, as stated above, optionally comprises the metal or metals to be deposited on the sub-master surface.
  • When a current is applied to the system, the anodic metal oxidizes to form metal ions which then flow to the cathode (the surface of the passivated sub-master) and deposit thereon. The cathode then reduces the metal ion into elemental metal. Once the desired thickness of the deposited metal has been achieved, the current is ceased and the plated sub-master is removed from the tank.
  • The thickness of the layer formed on the sub-master, which is a positive of the sub-master, is dependent on its use, for example, to produce further generations of sub-masters or to produce final product. The thickness can be greater than the depth of the features on the sub-master and sufficient to attain the structural integrity for its intended use. The layer thickness can be based upon a desired structural integrity to enable the layer to be removed from the master as well as to be used to produce next generation sub-masters. Thicknesses can be up to and exceeding about 500 micrometers (μm) or so, or, more specifically, about 50 μm to about 400 μm, or, even more specifically, about 100 μm to about 300 μm, and, yet more specifically, about 150 μm to about 250 μm.
  • In some applications it may be desirable to form a multi-layer electroform, e.g., for the final generation tooling (i.e., the electroform employed to make final product such as prismatic films). For example, a single electroform that is produced with layers of different compositions. Each layer can have the same or a different thickness. For example, the surface layer (i.e., the layer the side of the electroform comprising the surface features), can be a material that enhances the release of the final product from the electroform, and/or that inhibits undesired reactions on the surface, while the back layer can be a material that enhances the structural integrity of the electroform. One or more intermediate layers can also be employed, e.g., to enhance the bonding of the other layers. Metals that can be used to form these electroforms include those metals discussed above in relation to the single layer electroforms.
  • For example, a surface layer comprising gold (e.g., gold, silver, chromium, titanium, cobalt, alloys comprising at least one of the foregoing, and combinations comprising at least one of the foregoing) can be plated onto a sub-master, and then backing layer comprising nickel (e.g., nickel, a nickel alloy, or a nickel mixture) can be overplated (plated over the gold). In this process, the sub-master is passivated as described above. The passivated sub-master can then be disposed into an electroplating bath comprising the desired surface-layer material (e.g., gold). The surface layer can then be formed utilizing the sub-master as the cathode in the electroplating process. Once the desired surface layer thickness has been achieved, the sub-master can then be disposed in a second electroplating solution to form the second layer onto the surface layer. Again, the sub-master can be used as the cathode in the electroplating process. The thickness of the second layer is dependent upon a desired overall electroform thickness and the function of this second layer, e.g., as an intermediate layer or as the backing layer. Although passivation of the sub-master between the electroplating solutions is possible, the various layers can be formed without this passivation so that each layer of the multilayer electroform is firmly bonded to the next.
  • The thickness of the surface layer is dependent upon the particular application. The surface layer can have a thickness of millimeters in some applications, while it can be nanometers thick in others. For example, where the surface layer is employed to attain a chemical inertness on the surface of the electroform while controlling the costs of the layer (e.g., while limiting the amount of gold in the layer), the surface layer can have a thickness of up to about 10 μm or so, or, more specifically, about 1 nm to about 1 μm, or, even more specifically, about 5 nm to about 500 nm, and yet more specifically, about 10 nm to about 100 nm. An intermediate layer can have a thickness of up to about 50 micrometers (μm) or so, or, more specifically, about 1 nm to about 25 μm, or, even more specifically, about 10 nm to about 1 μm. The backing layer can have a thickness of up to about 500 micrometers (μm) or so, or, more specifically, about 50 μm to about 400 μm, or, even more specifically, about 100 μm to about 300 μm.
  • For example, a passivated sub-master can be passivated in an alkaline solution by anodically charging the sub-master. Once a passivation layer has been formed, the passivated electroform can be removed from the alkaline solution and optionally rinsed. The passivated electroform can then be disposed in the surface layer electroplating solution comprising gold cyamide, silver cyamide, and so forth. A current is applied, and, with the passivated electroform acting as the cathode, metal ions (e.g., gold, silver, cobalt, nickel, and so forth, depending on the composition of the solution) are deposited on the surface of the sub-master.
  • Once a desired surface layer thickness has been attained, the coated electroform is moved to the next electroplating solution comprising the metals to be disposed in the subsequent layer (e.g., comprising nickel, cobalt, and so forth). Optionally, the coated electroform can be rinsed (e.g., with deionized water) prior to entering the subsequent electroplating solution. Furthermore, maintenance of the coated electroform in an inert environment can be desirable (i.e., an environment that will not cause a reaction on the surface layer). In the subsequent electroplating solution, the coated sub-master again functions as the cathode and receives metal ions on its surface. This process can be repeated until the desired number of layers and layer thicknesses has been attained.
  • It has been discovered that the formation of an inert coating layer on the surface of the final generation sub-master enhances the life of the sub-master. For example, a surface layer comprising gold (e.g., a gold-cobalt surface layer with a nickel-cobalt backing layer) is particularly useful with acrylate coating material. Not to be bound by theory, where the acrylate coating material would react with the surface of a nickel-cobalt electroform, causing building-up nodules of solid crosslinked coating on the surface of the tool. These nodules grow in size and number over successive copies of the tool on the coater. They are also replicated on the films produced with that electroform. Even when the nodules are less than 100 nm in size (as measured along a major access), they can adversely effect the replication of nanostructures from the electroform. When the grow to about 200 nm to about 400 nm in size, they are large enough to scatter light (preferentially scattering blue light) and give the film product a blue hazy appearance. At this point, for a product requiring nanostructure replication, the product is rejected and the tool is scrapped. Depending on the coating formulation, the failure point can be as little as 100 copies or as many as a few thousand copies, where manufacturing efficiency requires tens of thousands of copies, and even hundreds of thousands of copies from a single electroform to be efficient and production viable. By employing the inert surface layer on the electroform, using the same product coating formulation that caused failure of a single layer electroform after 100 copies, 30,000 copies were produced with no nodule formation (as confirmed with a high resolution scanning electron microscope (SEM)).
  • A further advantage of the multilayer electroform is its surface properties are changes which gives enhanced release of the coating (e.g., plastic replication material) from the electroform in the plastic replication process. For example, a cured acrylate coating peeled off of the multilayer electroform with less force than from a single layer electroform. If the coating sticks to the electroform it can tear, leading to a point defect forming on the tool where a bit of the plastic material stuck to the sub-master. This debris replicates into every subsequently-produced plastic film, causing rejection of all products thus formed and production must be stopped and the tool discarded. When forming single layer electroforms, defects could be developed within the making of 300 to 2,000 plastic films, while, with the use of multilayer electroforms (e.g., with a surface layer comprising gold), at least about 20,000 to about 30,000 plastic films could be made without the formation of point defects.
  • Since the plated sub-master is flat, separation of the 2nd generation sub-master can be accomplished by peeling the electroform (i.e., the plating) from the sub-master. As noted above it is desirable to uniformly remove the sub-master from the substrate without twisting or torquing the sub-master. Twisting, torquing, and other non-uniform removal can damage the pattern on the surface of the next generation sub-master and potentially even damage the surface of the sub-master being replicated.
  • The 1st generation sub-master can be employed to make additional 2nd generation sub-masters by repeating the passivation, plating, and separation. Depending upon the materials used for the mounting and masking, one or several 2nd generation sub-master may be produced prior to replacing those materials.
  • The 2nd generation sub-master can be employed to make 3rd generation sub-masters or to make final product, e.g., film with microstructures having nanoscale resolutions, and in particular, films comprising light-reflecting elements (e.g., retroreflective elements). Possible microstructures include light-reflecting elements such as cube-corners (e.g., triangular pyramid), trihedral, hemispheres, prisms, ellipses, tetragonal, grooves, channels, microlenses, and others, as well as combinations comprising at least one of the foregoing.
  • The particular post-treatment(s) employed prior to using a sub-master in the production of product are dependent upon the particular product to be formed. For example, to produce an acrylate film comprising the desired nanoscale resolution, the surface energy of the sub-master can be reduced (e.g., of a nickel containing sub-master). Desirably, the post-treatment renders the surface of the electroform hydrophobic, and as such, will not attract polar molecules such as organic monomers and in particular acrylate monomers.
  • High surface energy surfaces can attract polar molecules and become wetted with them. In the case of water being the polar species, the surface is wetted with water, there being a low wetting angle, and the surface is said to be “hydrophilic.” Low surface energy surfaces will not attract polar molecules and will not become wetted with them. If the surface is rendered to be of a low surface energy then polar species like water will bead-up on the surface, and the surface is said to be “hydrophobic.”
  • The surface energy of the sub-master surface can be reduced by treating the sub-master with a solution having a pH of less than or equal to 6, or, more specifically, less than or equal to about 5, or, even more specifically, about 2 to about 5. Optionally, a cathodic current can be applied. The cathodic current can have a current density of about 1 ASF to about 60 ASF, or, more specifically, about 2 ASF to about 30 ASF or, even more specifically, about 2 ASF to about 10 ASF.
  • The sub-master can also, optionally, be treated to remove particulate(s) and/or staining (e.g., after the sub-master has been used to make product). This treatment can be before the sub-master has been used to produce product, and/or to refurbish the sub-master. This post-treatment can comprise rinsing the sub-master with water (e.g., deionized water), acidic media, and/or caustic media. For example, the sub-master can be placed in a bath containing deionized water, aqueous acid (e.g., a pH of less than or equal to about 6, or, more specifically, a pH of about 2 to about 5), and/or caustic media (e.g., a pH of greater than or equal to about 8, or, more specifically, a pH of about 8 to about 14).
  • Once removed from the bath, the sub-master can be rinsed with water (e.g., deionized water), for example, to remove particulates and metal salts. The sub-master can then be actively (e.g., contacted the heat, gas, and/or another method of facilitating drying) and/or passively dried. Optionally, the sub-master can be oven dried at a temperature that does not adversely affect the surface features or surface chemistry.
  • When used to make a product (e.g., to mass produce product such as LCD displays to diffuse or collimate light), the electroform (e.g., an electroform that has been post-treated with a cathodic current), can be attached to a calendaring roll. Product material can then be applied to the electroform. For example, a desired film material(s) can be extruded (or co-extruded) such that the material that will comprise the surface feature is disposed in direct physical contact with the electroform. The material can be cured and removed from the electroform to form the product. As an alternative, and/or in addition to the extrusion, preformed film(s) can be employed. Here, the film to be imprinted with the surface features can be sufficiently heated to enable the formation of the surface features into the film surface.
  • Possible product materials include plastics (e.g., thermoplastics and/or thermosets), such as acrylates, polycarbonates, polyesters, terephthalates (e.g., poly(ethylene terephthalate)), polyimides (e.g., polyetherimides), polystyrenes (e.g., ABS, ASA, and so forth), polyolefins, polyacrylonitrile (PAN), polyamide (PA), polyvinyl chloride (PVC), resorcinol, polyarylenes (e.g., polyarylene ether), polyacrylonitrile, polyethers, as well as combinations comprising at least one of the foregoing plastics. Various plastics can be combined in a single layer. These plastics can also be disposed in separate layers to form the product wherein one of the layers comprises the desired surface features. If multilayers are employed, adjacent layers comprise materials that provide sufficient adhesion between the layers for the desired application (e.g., that will not delaminate under use conditions for the product). Optionally, coatings and the like can be applied to the product after the surface features have been disposed in the surface thereof. Possible films that can be produced with the present process include those disclosed in U.S. Patent Publication No. 2003/0214728 A1 to Olczak, U.S. Published Application No. 2004/0109663 A1 to Olczak, and others.
  • In FIG. 2 a cross sectional view of a backlight display device 100 is shown. The backlight display device 100 comprises an optical source 102 for generating light 104. A light guide 106 guides the light 104 therealong by total internal reflection (TIR). The light guide 106 contains disruptive features that cause the light 104 to escape the light guide 106. A reflective substrate 108 positioned along the lower surface of the light guide 106 reflects any light 104 escaping from the lower surface of the light guide 106 back through the light guide 106 and toward an optical substrate 110. At least one optical substrate 110 is receptive of the light 104 from the light guide 106. The optical substrates 110 comprise a three-dimensional surface 112 defined by prismatic structures 116 (FIGS. 3, 4, 5, and 6).
  • The optical substrates 110 may be positioned, one above the other, in a crossed configuration wherein the prismatic structures 116 are positioned at an angle with respect to one another (e.g., 90 degrees). The prisms 116 have a prescribed peak angle, α, a height, h, a length, l, and a pitch, p and one or both of the prismatic surfaces 112 may be randomized in their peak angle, α, height, h, length, l, and pitch, p. Yet further, one or both sides of the substrates 110 may have the prisms 116. In FIGS. 3, 4, and 5, the sidewall or facets 132 of the prisms 116 which comprise the surface 112 are curved. The curvature can be described as a segment of a parabola, or more generally as a polynomial surface given by the sag equation: z = cr 2 1 + 1 - ( 1 + k ) c 2 r 2 + dr 2 + er 4 + fr 6 + Higher order terms in r ( 2 )
    where z is the perpendicular deviation (or “sag”) in microns of the sidewall or facet 132 of the prisms 116 from a straight reference line 128, originating at a first reference point (b) at a base of the prism and terminating at a second reference point (a) near the peak of the prism and c−1 is the radius of curvature of the facet. Here the coefficients of the polynomial may have the following approximate ranges: −20<c<20, −10<d<10, −10<e<10, −10<f<10, and −1<k or less than or equal to zero, wherein r is a radial coordinate or distance from an optical axis in microns. It is noted that c2r2 is greater than or equal to zero and less than or equal to 1. Odd order terms in r (e.g., r1, r3, r5, r7, etc.) with appropriately chosen coefficients may also be used as in Eq. 2. The higher order terms for the even and odd order terms have appropriately chosen coefficients. Terms other than the first r2 term may be written as: i = 1 N a i r i .
  • Linear segments or other approximations to the polynomial described by Eq. 2 may also be used. Linear segments result in a compound angle prism having a first facet 126 at an angle of θ and a second facet 124 at an angle of β. As best understood from FIG. 6, the curvature of the curved sidewall or facet 132 of the prisms 116 can be either convex or concave. In FIG. 6, the side facets of the prism are positioned so as to form one or more compound facets 124, 126, respectively subtending an angle of β or θ with the base of the prism.
  • The light-redirecting structure can be created, for example, by applying the curable coating to the base film and casting the desired light-redirecting structure in the curable coating, by hot-embossing the structure directly onto the base film, or the like. While the base film material can vary depending on the application, suitable materials include those base film materials discussed in published U.S. Patent Application No. 2003/0108710 to Coyle et al. More specifically, the base film material of the brightness enhancement film can comprise metal, paper, acrylics, polycarbonates, phenolics, cellulose acetate butyrate, cellulose acetate propionate, poly(ether sulfone), poly(methyl methacrylate), polyurethane, polyester, poly(vinylchloride), polyethylene terephthalate, and the like, as well as blends copolymers, reaction productions, and combinations comprising at least one of the foregoing.
  • The following examples are provided merely to further illustrate the electroforms and the methods described herein, and are not intended to limit the scope hereof.
  • EXAMPLES Example 1
  • Passivation in a caustic solution with anodic current.
  • Sample 1, a nickel sub-master electroform having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 4 minutes at 25° C. while applying an anodic current density of 4 ASF. The aqueous solution comprised 20 g/l potassium hydroxide and 0.5 g/l sodium lauryl sulfate and had a pH of 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The sub-master was then plated with a nickel-cobalt (NiCo) alloy by electroforming a layer that was about 100 μm in thickness. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was readily peeled from the sub-master, showing complete removal and no visible damage to the microstructures when examined under a microscope at up to 40×. It is also noted that samples have been examined to a magnification of 100× without visible damages, and even passed SEM (scanning electron microscope) review at 100K× (100,000×).
  • Example 2
  • Sample 2, a nickel sub-master electroform having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 4 minutes at 35° C. while applying an anodic current density of 4 ASF. The aqueous solution comprised 20 g/l of StamperPrep (a high alkalinity (pH greater than 13), low foaming, cleaning agent comprising sodium hydroxide commercially available from DisChem, Inc., Ridgway, Pa.), and had a pH of greater than 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The sub-master was then plated with a nickel-cobalt alloy by electroforming a layer that was about 100 μm in thickness under the same conditions as in Example 1. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was readily peeled from the sub-master, showing complete removal and no visible damage to the microstructures when examined under a microscope.
  • Example 3
  • Passivation of a nickel containing sub-master.
  • Sample 3, a nickel sub-master electroform having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 30 seconds at 35° C. while applying an anodic current density of 35 ASF. The aqueous solution comprised 90 g/l of StamperPrep, and had a pH of greater than 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The sub-master was then plated with a nickel-cobalt alloy by electroforming a layer that was about 100 μm in thickness under the same conditions as in Example 1. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was readily peeled from the sub-master, showing complete removal and no visible damage to the microstructures when examined under a microscope.
  • Example 4
  • Passivation of a nickel containing sub-master.
  • Sample 4, a production-size nickel-cobalt sub-master electroform (a nominal size of 40 centimeters (cm) by 65 cm), having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 4 minutes at 35° C. while applying an anodic current density of 4 ASF. The aqueous solution comprised 20 g/l of StamperPrep, and had a pH of greater than 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The sub-master was then plated with a nickel-cobalt alloy by electroforming a layer that was about 200 μm in thickness under the same conditions as in Example 1. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was readily peeled from the sub-master, showing complete removal and no visible damage to the microstructures when examined under a microscope.
  • Sample 4 was then recycled through the passivation step and electroforming step a total of 57 times, thus generating 57 electroforms all of which separated cleanly and wholly, showing complete removal and no visible damage to the microstructures when examined under a microscope.
  • Example 5
  • Passivation of a nickel containing sub-master.
  • Sample 5, a production-size nickel-cobalt sub-master electroform (a nominal size 40 cm by 65 cm), having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 4 minutes at 35° C. while applying an anodic current density of 4 ASF. The aqueous solution comprised 20 g/l of StamperPrep, and had a pH of greater than 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The sub-master was then plated with a nickel-cobalt alloy by electroforming a layer that was about 200 μm in thickness under the same conditions as in Example 1. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was readily peeled from the sub-master, showing complete removal and no visible damage to the microstructures when examined under a microscope.
  • Sample 5 was then recycled through the passivation step and electroforming step a total of 65 times, thus generating 65 electroforms all of which separated cleaning and wholly showing complete removing and with no visible damage to the microstructures when examined under a microscope.
  • Example 6
  • Passivation of a nickel containing sub-master with a solution comprising dichromate.
  • A nickel-cobalt sub-master electroform having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth was passivated by immersion into an aqueous solution comprising 5 g/l potassium dichromate for 5 minute at 25° C. while agitating the solution. The sub-master was removed and rinsed with deionized water. The sub-master was then plated with a nickel-cobalt alloy by electroforming a layer that was about 100 μm in thickness. After electroforming, the plated sub-master was rinsed and dried, and the nickel-cobalt electroform was peeled from the sub-master. Examination under a microscope showed the microstructures to be partially damaged, whereas the very finest structures at the sharpest corners of the peaks were torn from the electroform and remained on the sub-master, thus causing visible defects and loss of optical performance. The dichromate passivation process was inadequate to passivate the nickel containing sub-master in that not all areas (in particular, the deepest valleys on the sub-master), were passivated sufficiently to allow complete removal of the entire electroform copy.
  • Example 7
  • Post-Treatment by Immersion in a Caustic Solution, Followed By Rinsing and Drying
  • An electroform, such as Sample 1, can be immersed in a caustic solution having a pH of about 8 to 14, at 40° C., for 1 to 5 minutes, rinsed with deionized water, and dried. The electroform can then be placed on a roll for use in the formation of acrylate films. A liquid coating mixture, comprising UV-curable acrylate monomer(s), oligomer, photoinitiator, and non-reactive additive(s), can be pressed into the electroform surface by a backing film, (e.g., a plastic film, such as polycarbonate, polyester, and so forth, as well as reaction products comprising at least one of the foregoing, and combinations comprising at least one of the foregoing), and can be cured to fix the microstructures into the surface. The film, with the cured acrylate microstructures, can then be separated from the roll. It has been observed that electroforms, such as Sample 1, (that have only been rinsed, but not post-treated to reduce the surface energy), i.e., with a high surface energy, cause permanent sticking of minute domains of the acrylate coating during the production of the transparent film. These minute domains accumulate on the electroform surface, ultimately changing the surface features, and thereby effectively causing the loss of the desired nanoscale resolution.
  • Example 8
  • Post-treatment by immersion in an acidic solution and application of a reverse current (cathodic current).
  • Sample 7, a NiCo sub-master such as Sample 1, can be post-treated, e.g., to reduce the surface energy. Sample 7 can be immersed in an acidic solution for 1 to 5 min at 40° C., while applying a cathodic current density of 4 ASF. The acidic solution can comprise Citranox™, and can have a pH of about 4. The electroform can then be placed on a roll for use in the formation of acrylate films. A liquid coating mixture, comprising UV-curable acrylate monomer(s), oligomer, photoinitiator, and non-reactive additive(s), can be pressed into the electroform surface by a backing film, (such as polycarbonate, polyester, and so forth), and can be cured to fix the microstructures into the surface. The film, with the cured acrylate microstructures, can then be separated from the roll. This acrylate film has been observed to have a nanoscale resolution.
  • Example 9
  • Treatment by immersion in a caustic solution (after production of product with the electroform).
  • Sample 8, a NiCo sub-master such as Sample 1, can be post-treated, e.g., to reduce the surface energy. Sample 8 can be immersed in a caustic media for 1 to 5 min at 40° C., while applying a cathodic current density of 4 ASF. The caustic solution can comprise StamperPrep™, sodium hydroxide, etc., and can have a pH of about 8 to about 14. The electroform can then be placed on a roll for use in the formation of acrylate films. A liquid coating mixture, comprising UV-curable acrylate monomer(s), oligomer, photoinitiator, and non-reactive additive(s), can be pressed into the electroform surface by a backing film, (such as polycarbonate, polyester, and so forth), and can be cured to fix the microstructures into the surface. The film, with the cured acrylate microstructures, can then be separated from the roll. It was further observed that the caustic soak and reverse current, when applied to the nickel or nickel-cobalt electroforms, also eliminated particulate and staining defects from the microstructured electroform surface, which resulted in tool yield improvements.
  • Example 10
  • Multilayer electroform.
  • Sample 9, a nickel sub-master electroform having a microstructure comprising a plurality of channels and grooves of about 1 μm to about 37 μm in depth, was passivated by immersion into an aqueous solution for 4 minutes at 25° C. while applying an anodic current density of 4 ASF. The aqueous solution comprised 20 g/l potassium hydroxide and 0.5 g/l sodium lauryl sulfate and had a pH of 13.5. The sub-master was removed, rinsed with deionized water, and then dried. The submaster was then plated with gold-cobalt (AuCo) alloy by electroforming a layer that was about 1 μm or less in thickness by submersion in a solution of potassium aurocyanide and cobalt complex (Orosene 999 gold salts, commercially available from Technic Inc., Cranston R.I.), and applying a current density of 5 ASF. After gold plating the submaster was removed, rinsed, and dried. The sub-master was then plated with a nickel-cobalt (NiCo) alloy by electroforming a layer that was about 200 μm in thickness. After electroforming, the plated sub-master was rinsed and dried, and the multilayer electroform was readily peeled from the sub-master. For experimental comparison purposes, only about one half of the submaster was plated with gold-cobalt, so that the experimental tool was ½ multilayer with a gold-cobalt surface and the other ½ was single-layer NiCo (e.g., see FIG. 9). Both halves showed nanoscale replication of the submaster surface.
  • Example 11
  • Plastic replication from multilayer electroform.
  • The electroform produced in Example 10 was used to make light-collimating films comprising microfeatures (prisms) with nanoscale resolution. The electroform was used on a coater to make plastic films via a UV cast and cure process with an acrylate coating material. Film 726 (that is, the 726th film produced using the multilayer electroform) was visually inspected. The film showed a visible blue haze on the portion of the film formed from the single layer electroform (e.g., areas 3-5 (see FIG. 9)). However, the portion of the film formed from the multilayer electroform (areas 1 and 2) showed no visible haze.
  • The electroform was used to make a total of 10,015 films. Micrographs of the electroform were again taken. At a resolution of 20,000×, the multilayer portion of the electroform (areas 2), as shown in FIG. 7, showed no nodule growth, and the films showed no visible haze. The single layer portion of the electroform (areas 5), however, as shown in FIG. 8, showed substantial nodule growth on the surface of the electroform, and the film showed visible haze. From this data, it is clear that the multilayer electroform is capable of making hundreds of thousands films without being replaced. The choice of the surface layer of the multilayer electroform depends on the precise application. As discussed above, various metals can be employed such as those used for single layer electroforms. Here it was chosen as gold-cobalt for its chemical inertness and hardness. In some applications, metals can be gold, silver, chromium, titanium, cobalt, alloys thereof, as well as combinations thereof.
  • The electroforms prepared as described herein can be use to produce acrylate films, e.g., display films with microscale features. For such brightness enhancement films, the key optical property to be measured is the on-axis luminance of these films in an LCD backlight assembly, which was measured with using the following protocol. A Teijin D120 (commercially available from Tsujiden Co., Ltd., Japan) bottom diffuser was placed on backlight (i.e., LG Phillips LP121X1 single CCFL notebook backlight), and a vertical prism film was placed over the bottom diffuser (i.e., the lower prism film was oriented with the prisms running vertically) while the horizontal prism film was placed over the vertical prism film (i.e., the upper film was placed with prisms running horizontally). The inverter was a Taiyo Yuden LS 390 (commercially available from Taiyo Yuden (U.S.A.) Inc., Schaumburg, Ill.). A thermocouple was used to monitor the temperature of the active backlight in real-time while letting the system equilibrate (until the average temperature did not change more than 0.1° F. over a 5 minute duration). Once equilibrium was attained, the Microvision SS220 display analysis system (commercially available from Microvision, Auburn, Calif.) output the luminance in units of candelas per square meter (also known as “nits”). These units were converted to “relative luminance units” compared with a BEF-II film standard (commercially available from Minnesota Mining and Manufacturing Co., St. Paul, Minn.). It is noted that the Microvision software included: luminance uniformity—measured on-axis luminance across 13 points of the backlight; and view angle—measured luminance as a function of angle at the center point of the backlight.
  • A 3rd-generation electroform designated M-1-1-1 (i.e. the 1st 3rd-geneartion copy of the 1st 2nd-generation copy of the 1st 1st-generation copy of the master “M”) was prepared according to the process described above to produce 67 4th-generation copies of itself. Copies number 2 and 67 were used to produce display films that were tested for luminance. The films from the 2nd copy had an average normalized luminance of 103.5% with a standard deviation of 0.19%, while the films from the 67th copy had an average normalized luminance of 103.2% with a standard deviation of 0.22%, which makes them statistically equal at a 95% confidence limit. In other words, even after 67 production cycles, there was no decrease in quality of the film produced.
  • The present electroplating process is more effective than prior art processes, e.g., the replication accuracy has been maintained for greater than or equal to about 100 replicas. For example, due to the passivation process, the replicas (i.e., next generation sub-masters) readily separate from the sub-master after the plating process without loss of surface features.
  • Additionally, the post-treatment process for treating the sub-master prior to using the sub-master in production of a product (such as a display film), enables the reproducible production of a more regular geometric pattern than when the sub-master has been post-treated with deionized water and a caustic media (pH of about 8 to about 14). This post-treatment, which uses a cathodic current and acidic media, is particularly useful on electroforms used in the production of articles from a polar material. The caustic media can be employed with the electroform between product production runs, to reduce, and possibly eliminated particulate and staining defects from the microstructured electroform surface; resulting in tool yield improvements.
  • It is noted that the mounting techniques, and/or passivation techniques disclosed above can be used with various processes for producing an electroform (e.g. a sub-master), and are not limited to the electroplating technique discussed herein. Other possible processes for depositing the metal material onto the master or sub-master to produce the next generation sub-master include plasma spraying, vapor deposition (e.g., chemical vapor deposition), electroless plating.
  • While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (31)

  1. 1. A method for making an electroform, comprising:
    passivating a sub-master to form a passivation layer;
    disposing the passivated sub-master into an initial electroplating solution;
    applying a first current, wherein the passivated sub-master is one of two initial electrodes enabling the application of the current;
    depositing a first metal onto the passivated sub-master to form an initial coating on the sub-master;
    moving the initially coated sub-master from the initial electroplating solution to a subsequent plating solution;
    applying a second current, wherein the initially coated sub-master is one of two second electrodes enabling the application of the current;
    depositing a subsequent layer of a second metal onto the initial coating to form a multilayer; and
    removing the multilayer to form the electroform.
  2. 2. The method of claim 1, wherein the first metal is selected from the group consisting of gold, silver, chromium, titanium, cobalt, alloys comprising at least one of the foregoing, and combinations comprising at least one of the foregoing.
  3. 3. The method of claim 2, wherein the first metal is selected from the group consisting of gold, silver, chromium, titanium, cobalt, alloys thereof, as well as combinations thereof.
  4. 4. The method of claim 2, wherein the first metal is selected from the group consisting of gold, silver, alloys comprising at least one of the foregoing, and combinations comprising at least one of the foregoing.
  5. 5. The method of claim 4, wherein the first metal comprises gold and cobalt.
  6. 6. The method of claim 1, wherein the initial coating has a thickness of about 1 nm to about 1 μm.
  7. 7. The method of claim 6, wherein the thickness is about 10 nm to about 100 nm.
  8. 8. The method of claim 1, wherein the subsequent layer comprises a thickness of about 50 μm to about 500 μm.
  9. 9. The method of claim 8, wherein the subsequent layer comprises a thickness of about 100 μm to about 300 μm.
  10. 10. The method of claim 1, wherein, prior to removing the multilayer, further comprising:
    moving the multilayer from the subsequent electroplating solution to an additional plating solution;
    applying another current, wherein the multilayer is one of two second electrodes enabling the application of the current; and
    depositing an additional layer of an additional metal onto the multilayer.
  11. 11. The method of claim 10, wherein the subsequent layer has a subsequent layer thickness of about 1 nm to about 50 μm, and the additional layer has an additional layer thickness of about 50 μm to about 500 μm.
  12. 12. The method of claim 10, wherein the subsequent layer has a subsequent layer thickness of about 1 nm to about 25 μm, and the additional layer has an additional layer thickness of about 100 μm to about 300 μm.
  13. 13. The method of claim 10, wherein passivating comprises contacting at least a surface of the sub-master to be passivated with a solution comprising an oxidizing agent, and applying an anodic current to the sub-master.
  14. 14. The method of claim 10, wherein the initially coated sub-master is not passivated before being moved to the subsequent plating solution.
  15. 15. A multilayer electroform formed from the method of claim 1.
  16. 16. A multilayer electroform formed from the method of claim 10.
  17. 17. An electroform, comprising:
    a surface layer comprising a surface layer composition; and
    a backing layer comprising a different composition than the surface layer composition.
  18. 18. The electroform of claim 17, wherein the initial coating has a thickness of about 1 nm to about 1 μm.
  19. 19. The electroform of claim 18, wherein the thickness is about 10 nm to about 100 nm.
  20. 20. The electroform of claim 17, wherein the subsequent layer comprises a thickness of about 50 μm to about 500 μm.
  21. 21. The electroform of claim 20, wherein the subsequent layer comprises a thickness of about 100 μm to about 300 μm.
  22. 22. The electroform of claim 17, wherein the first metal is selected from the group consisting of gold, silver, chromium, titanium, cobalt, alloys comprising at least one of the foregoing, and combinations comprising at least one of the foregoing.
  23. 23. The electroform of claim 22, wherein the surface layer comprises gold.
  24. 24. The electroform of claim 23, wherein the backing layer comprises nickel.
  25. 25. A method for making an electroform, consisting essentially of:
    passivating a sub-master to form a passivation layer;
    disposing the passivated sub-master into an initial electroplating solution;
    applying a first current, wherein the passivated sub-master is one of two initial electrodes enabling the application of the current;
    depositing a first metal onto the passivated sub-master to form an initial coating on the sub-master;
    moving the initially coated sub-master from the initial electroplating solution to a subsequent plating solution;
    applying a second current, wherein the initially coated sub-master is one of two second electrodes enabling the application of the current;
    depositing a subsequent layer of a second metal onto the initial coating to form a multilayer; and
    removing the multilayer to form the electroform.
  26. 26. The method of claim 25, wherein passivating comprises contacting at least a surface of the sub-master to be passivated with a solution comprising an oxidizing agent, and applying an anodic current to the sub-master.
  27. 27. A method for making a film, comprising:
    applying a coating to a surface of an electroform, wherein the electroform comprises multiple layers, and wherein microfeatures having nanoscale resolution are disposed in the surface of the electroform;
    curing the coating; and
    removing the coating from the electroform to form the film.
  28. 28. The method of claim 27, wherein a surface layer of the electroform comprises a chemically inert material that resists chemical reaction with the coating.
  29. 29. A film formed from the method of claim 27.
  30. 30. A light management article comprising the film of claim 29.
  31. 31. The article of claim 30, wherein the article is a backlight computer display.
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