HK1220714B - Manufacturing of diffractive pigments by fluidized bed chemical vapor deposition - Google Patents

Description

Diffractive pigments produced by fluidized bed chemical vapor deposition

The present application is a divisional application of the chinese patent application having an application date of 2012/16/8, and an application number of 201210291218.8, and entitled "diffractive pigment produced by fluidized bed chemical vapor deposition".

Technical Field

The present invention relates generally to thin film pigment flakes and more particularly to methods of making microstructured pigment flakes.

Background

Specialty pigments have been developed for use in security applications such as printed security devices on banknotes, packaging of high value items, container seals, and even direct application to commercial items. For example, twenty dollars of U.S. federal reserve banknotes currently use optically variable inks. The number "20" printed in the lower right corner of the front face of the note changes color with changing viewing angle. This is an obvious anti-counterfeiting device. The color-shifting effect is not reproducible by conventional color copiers and the person receiving the note can observe whether the note has color-shifting security characteristics to determine the authenticity of the note.

Other high value documents and items use similar measures. For example, iridescent pigments or diffractive pigments are used in paints and inks that are applied directly to articles, such as stock certificates, passports, original product packaging, or for seals that are applied to articles. As counterfeiters continue to become more sophisticated, there is a need for anti-counterfeiting features that are more difficult to counterfeit.

One method of anti-counterfeiting is to use microscopic symbols on a multi-layer color shifting pigment chip. The symbol is formed on top of at least one layer of the multilayer color shifting pigment flakes by a localized change in an optical property, such as reflectivity. The multilayer color shifting pigment flakes can have an all dielectric design or a metallodielectric design. The symbols may be embossed or etched in the pigment by mechanical means, or formed by laser means.

Microstructured sheets having diffraction gratings or symbols often require additional layers, such as layers that provide a color shifting effect. One conventional method is to use roll-to-roll coating (roll-to-roll coating). A roll of a sheet of polymeric substrate material (also known as a "web") is passed through the deposition zone and coated with one or more film layers. The roll of polymeric substrate may be passed back and forth through the deposition zone a plurality of times. Subsequently, the deposited coating is separated from the polymeric substrate and processed into a sheet. However, the large scale production of such pigments requires very long deposition substrates and in such cases, the roll-to-roll technique is inconvenient.

Accordingly, there is a need to provide a method of making microstructured multilayer pigment flakes that overcomes the limitations of the techniques discussed above.

The all-dielectric interference structure may be formed of dielectric layers having different refractive indices. Various combinations of these layers may be utilized to achieve the desired optically variable effect. All dielectric pigment flakes may be microstructured, they may include indicia for security purposes, or have diffraction gratings that provide optically variable effects.

It is another object of the present invention to provide an efficient method for making a microstructured sheet of all-dielectric.

Disclosure of Invention

The present invention provides a method for forming microstructured pigment flakes. The method includes providing a microstructured dielectric core to a fluidized bed and encapsulating the microstructured dielectric core in the fluidized bed by chemical vapor deposition to form an encapsulation layer encapsulating the microstructured dielectric core.

Another aspect of the invention provides all-dielectric diffractive pigment flakes that include a diffractive core and one or more highly conformal encapsulating layers, wherein the encapsulating layers are provided using chemical vapor deposition while the flakes are in the fluidized bed.

Drawings

The invention will be described in more detail by reference to the appended drawings, which represent preferred embodiments of the invention, and in which:

FIG. 1 is a schematic diagram showing thin film interference;

FIGS. 2 and 3 are schematic diagrams showing diffractive interference;

FIG. 4 is a schematic diagram showing interference in a three layer high/low/high index of refraction dielectric diffractive pigment;

fig. 5A is a flow diagram of a method of making microstructured pigment flakes;

fig. 5B is a schematic diagram showing the fabrication of a pigment flake;

FIG. 6 is a schematic illustration of a fluidized bed used in Fluidized Bed Chemical Vapor Deposition (FBCVD);

fig. 7A-7D are Scanning Electron Microscope (SEM) micrographs of diffractive pigment flakes;

fig. 8A to 8C are Transmission Electron Microscope (TEM) cross-sectional images of a common package sheet;

FIG. 9 is a Scanning Transmission Electron Microscope (STEM) image of a slide edge; and

fig. 10 is a graph plotted from the results of elemental analysis of the spectrum of the region marked in the STEM image shown in fig. 9.

DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION

Multilayer optical sheets can provide diffractive optical effects due to diffractive microstructures formed on the surface of the sheet; and color shift effects can be provided due to optical interference caused by the layered structure. The diffractive microstructure may be formed in a dielectric core, which is subsequently encapsulated with one or more encapsulation layers. So that it is desirable that the coating layer conforms to the microstructure of the diffractive core in its entirety, or at least as much as possible, to produce the desired optical effect based on the thin-film interference combined with the layered coating and the diffractive interference caused by the microstructure.

When the optical design of the overall dielectric is formed on a grating-like surface, rather than a flat surface, the resulting microstructured slab exhibits a color change with viewing angle change due to both thin film interference and diffractive interference. The combination of interference effects can only occur effectively if all interfaces between the layers of high and low index dielectric material have a grating-like microstructure. In the case of a non-compliant encapsulation layer, the diffractive effect is lost, or at least the effect is severely attenuated, and only or mostly the pigment shows thin-film interference. Therefore, it is important to select an encapsulation technique for a diffractive core piece that can replicate the microstructure in the encapsulation layer. In other words, the deposited layer should be highly conformal to the original microstructure of the core sheet.

Thin film interference occurs when a light wave encounters a boundary between translucent materials having different refractive indices, causing the light wave to separate into a reflected wave and a transmitted wave. When the second material has a higher refractive index than the first material, the reflected beam experiences a phase shift of 180 degrees. A typical example is soap lather. Fig. 1 shows a cross section of a soap bubble (n ═ 1.33) which is filled with air and surrounded by air (n ═ 1). The first transmitted wave is transmitted to the inner bubble/air interface, and is thus again divided into a reflected wave and a transmitted wave. Repeating this process produces an infinite number of reflected and transmitted waves. The constructive and destructive interference conditions are different for different wavelengths of incident white light that produces an attractive color observed in light reflected by soap bubbles.

Diffractive interference or diffraction occurs when a propagating light wave encounters an obstruction of similar size to its wavelength. If the barrier is periodic, some wavelengths of energy are dispersed into different discrete directions (diffraction orders). Such an optical device is called a "diffraction grating".

A diffraction grating is an optical component made up of an assembly of reflective or transmissive elements separated by a distance corresponding to the wavelength of the incident light. When monochromatic light is incident on the grating, it is diffracted in discrete directions. As shown in fig. 2, in the grating, each grating groove serves as a small slit-shaped diffraction light source. The light diffracted by each groove combines to form a diffracted wavefront. As shown in fig. 2, light incident on the grating surface at an angle that is not perpendicular to the grating surface produces zero order or specular reflection. The diffraction grating produces first order diffracted beams (negative to positive orders) on either side of the zero order reflection. Likewise, at higher angles, second and higher order diffracted light may be produced. Diffraction can also occur in transmission, as shown in fig. 3.

The combination of thin films and diffractive interference effects is further discussed with respect to an exemplary three-layer all dielectric pigment of the HLH type surrounded by air, where H represents a high refractive index layer having a refractive index greater than 1.65 and less than 2.7, and L represents a low refractive index layer having a refractive index less than or equal to 1.65 and greater than 1.3. Fig. 4 schematically shows a specular beam and some diffracted beams reflected and/or transmitted at the air/high index and high/low index interface boundaries of the pigment. In practice, the incident beam is reflected according to the law of reflection or diffraction, and the transmitted beam can pass in the specular direction or in the diffractive direction in the layer. Consider only the first transmitted beam, which passes through the high and low index layers and is subsequently reflected by the high/low index interface. The second internal reflection from the low index/high index interface is shown as dashed arrows, regardless of their trajectory. The subscripts "s" and "d" refer to specularly reflected and diffractively reflected or transmitted light beams, respectively. R and T refer to reflected or transmitted light beams, and H and L refer to a high refractive index dielectric layer and a low refractive index dielectric layer. For example, in this term, RLs represents a specular beam reflected by the high/low index interface layer, and THd-represents a diffracted beam transmitted through the air/high index interface.

It is shown that only the first specular reflected beam from the air/high index interface (RHs) and the specular transmission (H)/reflection (L)/transmission (H) labeled as THsRLsTHs contribute to the specular reflection. Note that this optical path corresponds to the path of pure thin film interference.

As for the diffraction aspect, a wave that undergoes one diffraction interaction is considered. The reflected beam, labeled RHd, is the only one from the air/high index interface. The other three waves that pass through the layer and have undergone one diffraction are THdRLsTHs, THsRLdTHs and THsRLsTHd. These three waves interfere with each other and with the waves labeled RHd. Note that the optical path involved in defining the interference is not the same as the optical path of the mirror beam.

The specularly transmitted beam (indicated by the double arrow) from the high/low index interface, which is preceded by specular and diffractive interference at the air/high index interface, is labeled T, respectively_HsTLsAnd T_HdTLs。

The diffracted transmitted beam (indicated by the double arrow) from the high/low index interface, which is preceded by the specular and diffractive interference at the air/high index interface, is labeled T respectively_HsTLdAnd T_HdTLd。

Even though the consideration of these first waves is complex. For example, only the first reflected light beam from the low reflectivity/high reflectivity interface is shown, which is preceded by specular and diffractive transmission from the low and high refractive index layers. When light beams enter and exit the second high index material through transmission and interfaces with air, and/or at low/high index interfaces, each of these light beams will itself follow multiple complex inter-layer and intra-layer reflections and transmissions from specular and diffractive interference.

Fig. 4 shows the light path that occurs if the multilayer coating closely conforms to the rasterized microstructure of the core sheet. In the case where the encapsulating layer does not conform to the diffractive structure of the core, diffractive effects do not occur and the pigment mainly shows thin-film interference.

We have noted conventional encapsulation techniques such as sol-gel or wet-chemical methods, which involve the decomposition of metal alkoxides in the presence of water, followed by drying and annealing; the wet-chemical process is based on precipitation from an aqueous solution of a metal salt, followed by drying and calcination; none of the above methods can produce a sufficiently conformable coating. Our experiments show that Fluidized Bed Chemical Vapor Deposition (FBCVD) techniques improve the conformability of the encapsulation layer of the microstructured dielectric preform (core). The results of the experiments are further discussed with reference to fig. 7A-10.

Referring to fig. 5A, a method of forming microstructured pigment flakes includes a core providing step 210 and an encapsulating step 220, wherein in the core providing step 210 a microstructured dielectric core is provided into a fluidized bed, wherein in the encapsulating step 220 the microstructured dielectric core is encapsulated with one or more encapsulating layers encapsulating the microstructured dielectric core; chemical vapor deposition is used with the microstructured core in a fluidized bed. The core providing step 210 can include the steps of depositing a dielectric coating on a microstructured deposition substrate 212, releasing 214 the dielectric coating, and breaking the dielectric coating into a plurality of cores comprising the microstructured dielectric core 216.

The microstructured pigment flakes can include a dielectric core and one or more encapsulation layers. The tablet core may have microstructures formed therein and is manufactured by a deposition step 212 of depositing one or more dielectric thin film layers (e.g., plastic films) on a microstructured deposition substrate, separating 214 the thin film layers from the substrate, and processing the separated thin film layers, such as a crushing step 216 by milling and sieving into pre-tablets. In an encapsulation step 220, the preform sheet or core is encapsulated with a film layer. Optionally, an additional encapsulation step 222 may be performed to produce more than one encapsulation layer. The resulting pigment flakes are typically about 5-100 microns wide, and typically about 20-100 microns wide.

The core may comprise a single dielectric layer, or a plurality of dielectric layers with diffractive structures formed on the surface of the core. Depending on the desired color and effect of the light, suitable grating microstructures are selected for the production of diffractive flakes having diffractive effects. For example, the pigment flakes can include diffraction grating microstructures having grating frequencies ranging from about 400 grating lines per millimeter (ln/mm) to 4000 ln/mm to produce a wide range of optical effects.

In one embodiment, the preform sheet includes microstructured indicia, such as symbols, typically about 0.5-20 microns in diameter. In one particular embodiment, the symbol is about 700 nanometers in diameter, and in another embodiment, the symbol is about 15 microns in diameter.

Microstructures such as symbols or gratings are typically embossed or cast onto a deposition substrate, and a thin film dielectric layer is deposited on the embossed deposition substrate. The microstructures on the surface of the substrate are replicated in positive or negative relief in at least a first thin film layer deposited on the substrate. The coating of the thin film layer is then separated from the deposition substrate and processed into a microstructured preform for encapsulating the core in step 220.

Preferably, the microstructured core is formed of one or more dielectric materials to produce a translucent bi-color pigment that cannot be achieved when using an opaque metal core.

The microstructured dielectric sheet can be mixed with a carrier, such as an ink vehicle or a coating vehicle, to form a composition, such as an ink or coating or a mixture thereof in a transparent carrier to form a paint. Examples of carriers include polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, poly (ethoxyethylene), poly (methoxyethylene), poly (acrylic) acid, poly (acrylamide), poly (oxyethylene), poly (maleic anhydride), hydroxyethylcellulose, cellulose acetate, poly (polysaccharides) such as gum arabic and pectin, poly (acetals) such as polyvinyl butyral, poly (vinyl halides) such as polyvinyl chloride and polyvinyl chloride (polyvinylene), poly (dienes) such as polybutadiene, poly (olefins) such as polyethylene, poly (acrylates) such as polymethylmethacrylate, poly (methacrylates) such as polymethylmethacrylate, poly (carbonates) such as poly (hydroxycarbonyl) hydroxyhexene (oxyhexamethyylene), poly (esters) such as polyterephthalic acid, poly (urethanes), poly (siloxanes), polythioethers (sulfones), poly (sulfones), polyvinylnitriles (vinylnitriles), polyacrylonitriles (acrylonitriles), poly (styrenes), polyphenylene (phenylenes) such as 2,5 dihydroxy-1, 4-phenylethene (phenyleneethylene), poly (amides), natural rubber, formaldehyde (formaldehyde) resins and other polymers.

In one embodiment, the deposition substrate is embossed with a diffraction grating pattern. Thus, a flake core formed by depositing a dielectric thin film layer onto a rastered surface also has a grating pattern on one or both sides thereof. Alternatively, the dielectric core may comprise more than one dielectric layer formed by depositing a thin film layer on the deposition substrate before releasing the coating and separating it into individual pre-fabricated pieces.

Microstructured deposition substrates comprising microstructures such as diffraction gratings and/or symbols can be made from plastic materials such as polyvinyl chloride, polycarbonate, polypropylene and polyester G. Methods that can be used to form a surface relief pattern in a deposition substrate are well known to those skilled in the art. For example, the surface of the substrate may be embossed by pressing it into contact with a heated nickel embossing shim under high pressure. Other methods include photolithography and molding of plastic substrates against patterned surfaces.

The layer of microstructured lamellar core can be deposited using a variety of conventional techniques such as physical vapor deposition using electron beam or resistive heating evaporation, reactive direct current magnetron sputtering, Radio Frequency (RF) magnetron sputtering, Chemical Vapor Deposition (CVD), Plasma Enhanced Chemical Vapor Deposition (PECVD) and the like.

Suitable materials for the dielectric flake core include those having a "high" index of refraction (defined herein as greater than about 1.65), as well as those having a "low" index of refraction (defined herein as about 1.65 or less).

Examples of suitable high refractive index materials for the dielectric core include zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide (ZrO)₂) Titanium dioxide (TiO)₂) Carbon (C), indium oxide (In)₂O₃) Indium Tin Oxide (ITO), tantalum pentoxide (Ta)₂O₅) Cerium oxide (CeO)₂) Yttrium oxide (Y)₂O₃) Europium oxide (Eu)₂O₃) For example, iron (II) and iron (III) oxides₃O₄) And (III) valent iron (Fe) oxide₂O₃) Iron oxide of (2), hafnium nitride (HfN), hafnium carbide (HfC), hafnium oxide (HfO)₂) Lanthanum oxide (La)₂O₃) Magnesium oxide (MgO), neodymium oxide (Nd)₂O₃) Praseodymium oxide (Pr)₆O₁₁) Samarium oxide (Sm)₂O₃) Antimony trioxide (Sb)₂O₃) Silicon carbide (SiC), silicon nitride (Si)₃N₄) Silicon oxide ((SiO), selenium arsenic trioxide (Se)₂O₃) Tin oxide (SnO)₂) Tungsten trioxide (WO)₃) And combinations thereof, and the like.

The low refractive index material for the dielectric core comprises silicon dioxide (SiO)₂) Aluminum oxide (Al)₂O₃) For example magnesium fluoride (MgF)₂) Aluminum fluoride (AlF)₃) Cerium fluoride (CeF)₃) Lanthanum fluoride (LaF)₃) Metal fluoride of (2), sodium aluminum fluoride (e.g., Na)₃AlF₆Or Na₅Al₃F₁₄) Neodymium fluoride (NdF)₃) Samarium fluoride (SmF)₃) Barium fluoride ((BaF)₂) Calcium fluoride (CaF)₂) Lithium fluoride (LiF) and combinations thereof, or any other low index material having a refractive index of about 1.65 or less.

Fig. 5B shows the fabrication of microstructured pigment flakes formed of high and low index materials with a symmetric design obtained by encapsulating core diffractive flakes. Preferably, the core sheet is made of a single layer of dielectric material, which may have a high or low refractive index. In the particular example shown in fig. 5B, a low index dielectric core formed of silicon dioxide is encapsulated with a high index material, titanium dioxide, resulting in a 3-layer (HLH) design. The encapsulation layer of titanium dioxide is obtained by a chemical vapor deposition process using titanium tetrachloride as precursor, which, in the presence of water, in the form of vapor at 200 ℃ for about 1 hour, undergoes the following reaction:

titanium tetrachloride (TiCl)₄) (gas) +2H₂O (gas) → titanium dioxide (TiO)₂) +4 hydrochloric acid (HCl)

Suitable materials for the encapsulation layer include the same dielectric materials that can be used for the core. By way of example, a 5-layer (hlhlhlh) design may be obtained by encapsulating a titanium dioxide core preform slab with a layer of silicon dioxide deposited, followed by a layer of titanium dioxide. In another example, a 7-layer design (hlhlhlhlhlhlhlhh) is obtained by encapsulating a low index of refraction silica diffractive core piece with a continuous layer of titania, silica and titania.

It is known in the art that all-dielectric designs, such as alternating high and low refractive index layers, can provide an optically variable effect that depends on the thickness of the layers.

Alternatively, the encapsulation layer may include one or more metal absorber layers, forming a multi-layered metal dielectric design that may provide a color shifting effect due to interference of light. By way of example, a dielectric core formed of silicon dioxide and encapsulated with a metal absorber such as W, Ti, Cr, Mo forms a microstructured pigment that can provide a color shifting effect due to the metal dielectric stack. Optical designs using a high index dielectric core encapsulated with a metal absorber produce pigments that also have very vivid colors but less color shifting effects. Multilayer designs of alternating low and high index layers, such as HLH, LHL, hlhlhlh, lhlhlhl, may also be encapsulated with a metal absorber. If the microstructure is a diffraction grating, the pigment may provide an optically variable effect due to diffractive interference. Where the microstructures comprise indicia, the pigments may be used for security purposes because the indicia provides covert security characteristics and color change-overt security characteristics.

In diffractive flakes, the encapsulation layer conforms as much as possible to the microstructure of the core to produce the desired visual optical effect based on diffraction, so we use a Fluidized Bed Chemical Vapor Deposition (FBCVD) technique. Chemical Vapor Deposition (CVD) such that a monolayer or multilayer coating is deposited on the surface of the core particle; the deposited material is formed from gaseous, liquid or solid chemical precursors. CVD techniques result in a conformal encapsulating film that replicates the surface microstructure of the core sheet. The efficiency of this process depends on the contact between the particles and the surface of the film precursor. A technical solution allowing good contact between the particles and the gaseous precursor is to use fluidized bed technology.

The FBCVD process is based on a chemical reaction between a precursor and a reactant. In most cases, the precursor is oxidized with the aid of the reactants, so that an oxidized coating is obtained on the particles. The reactants providing nitrogen and carbon may form respective nitride and carbide coatings. Mixtures of reactants that can be used for the deposition of compounds such as carbonitrides, oxycarbides, oxynitrides and oxycarbonitrides. The precursors and reactants may be in the form of gaseous, liquid or solid materials. Preferably, the reactants are provided in a direction opposite to the precursor flow. Preferably, the method comprises the use of an inert fluidising gas for mixing the particles. Advantageously, the chemical vapor deposition process may be carried out at atmospheric pressure. However, depending on the material of the core sheet and the film to be deposited, low pressure or plasma activation may be used.

A variety of geometries can be used for the fluidized bed reactor; the fluidized bed should meet the limitations due to particle fluidization and chemical vapor deposition processes. The fluidized bed may be operated under thermal or plasma activated conditions suitable for the type of particle reactive gas precursor to be treated.

The optical effect of special effect pigments with a combination of thin films and diffractive interference is very dependent on the smooth replication of the diffractive microstructure of the core diffractive pigment pre-sheet, making FBCVD technology a perfect solution for the production of special effect pigments with optical diffractive properties.

Referring to fig. 6, the fluidized bed reactor may be a cylindrical vessel 100 having a perforated or perforated floor 170 for holding particles 180 and uniformly distributing the gas flow in the lateral regions of the vessel, thereby obtaining uniform suspension in the particles of the fluidized bed. In operation, the gas provided by the fluidized bed moves and partially supports the particles, causing the particles to expand in volume and diffuse throughout the vessel as a fluid. In the fluidized bed, the gas flow is turbulent and good quality and heat transfer between the particles is obtained, which is very important for uniform encapsulation of the chemical vapor deposition of the pigment flakes.

FBCVD methods offer several advantages over other deposition techniques such as Physical Vapor Deposition (PVD). PVD deposits the coating material primarily to the surface of the particles that are facing the vapor flow, while FBCVD provides uniform coverage of the particles. Compared to PVD, FBCVD not only provides higher growth rates due to its tri-vitamin nature, but also provides better uniformity and conformability of the encapsulation layer on the microstructured core particles. This is very important in the packaging of diffractive pigment flakes because the surface of the flakes has symmetrically aligned grooves with a pitch as low as 250 nm, which can range from a few microns for a low frequency grating to a high frequency grating, i.e., the grating has a frequency range from 400 to 4000 lines/mm.

The reactor used in our experiments was made of quartz. An example of a fluidized bed reactor is shown in fig. 6. The reactor has a removable top surface (not shown) with access ports for instrumentation, gas and liquid introduction and additional vibration. Arrows 110 show the path of introduction of reactant gases such as nitrogen, ammonia, water, carbon dioxide, hydrogen and/or liquid precursors for the injected gases. Alternatively, argon, helium or other inert gas is introduced to dilute or transport the reactants. The precursors and/or reactants may be in liquid or solid form. The liquid or solid may be heated in the container (referred to as bubbles for liquid precursor); an inert gas is introduced into the vessel to displace the vapor of the precursor. Typically, solids have a low vapor pressure and must be heated using a furnace at a higher temperature. The fluidization conditions can be varied by adjusting the flow rate using a flow controller based on visual observation of the fluidized bed. The top surface may also have a vibration device 160 such as a mechanical vibrator and an exhaust device such as an exhaust filter or scrubber; arrow 120 shows the flow of exhaust gas from the exhaust. The instrument 130 may include a thermocouple, and an extraction system and other sensor devices for extracting particles to control their optical properties.

Depending on the physical properties (density, vapor pressure, etc.) of the coating precursor, the precursor 150 may be introduced upstream with a fluidizing gas (argon, nitrogen, helium, etc.) at the bottom of the reactor through a sintered alumina grid 170. The separate control of the fluidizing gas and the precursor gas before entering the reactor makes it possible to control the fluidization conditions of the fluidized bed. The precursors may be from gas bubbles, for example silicon tetrachloride, titanium tetrachloride, trichlorosilane (SiHCl)₃) Precursors, or from different precursor vapor sources, e.g. tungsten hexacarbonyl (W (CO)₆) Nickel hexacarbonyl (Ni (CO))₆) For low vapor pressure solid precursors of W and Ni metals or compounds, heating at high temperatures can be performed using closed furnaces, respectively. The fluidizing gas may be provided from a flow controller for the fluidizing gas. In the case of precursors having a low vapor pressure, such as some organometallics including (tetraethylorthosilicate) TEOS and 2,3, 5-triiodobenzoic acid (TIBA), it can be introduced directly through the top of the reactor using a metered liquid injector; in this case, the reactants and fluidizing gas are supplied through the bottom of the reactor.

FBCVD reactors can operate under thermal or plasma activated conditions. For thermal activation, external heating may be performed by a cylindrical resistance furnace 150, or internal heating may be performed using a graphite susceptor (susceptor) and an external Radio Frequency (RF) induction coil. An external Rf coil may also be used for activation of the plasma for plasma assisted chemical vapor deposition, which has the advantage of an unbalanced plasma that can activate gaseous species at lower temperatures. In some cases, the FB reactor can have a vacuum pump for Low Pressure Chemical Vapor Deposition (LPCVD); the vacuum pump may be part of or be part of the exhaust; filters may be used to avoid damage to the vacuum pump.

Possible precursors include metal halides (chloride, iodide and bromide). Hydrogen halide gas is a by-product of the hydrolysis process.

FBCVD may be based on the following chemical reactions:

titanium tetrachloride (TiCl)₄) + water (H)₂O) → titanium dioxide (TiO)₂) + hydrochloric acid (HCl)

For the deposition of titanium dioxide, and

silicon tetrachloride (SiCl)₄) + water (H)₂O) → silicon dioxide (SiO)₂) + hydrochloric acid (HCl)

For the deposition of silicon dioxide.

Oxygen or ozone can be used in place of water to form the oxide. To change the oxidation conditions and avoid homogeneous nucleation in the gas phase rather than on the surface of the sheet, H may be used₂And CO₂Instead of water or oxygen. In this case, the chemical reaction of the gas would be:

H₂+CO→H₂O+CO

other possible precursors are those which can be used for SiO₂Encapsulated, e.g. trichlorosilane (SiHCl)₃) The alkylsilane of (a). In addition, in some cases, the original precursor in the gas phase is reacted to form other chemical gases, such as titanium trichloride, while reactingWhen titanium tetrachloride is used as a precursor for the deposition of titanium dioxide, titanium trichloride is generally observed in the vapor phase.

For example AlCl₃And other chlorides of zirconium tetrachloride may be used to deposit their respective oxides.

And N₂Or NH₃The reaction of the reactants produces the corresponding metal nitride. The reaction with the reactant gas that provides the carbon (e.g., methane) for the reaction results in the formation of metal carbides.

Halides may be used in combination with reactant gases that provide oxygen, nitrogen, and/or carbon to deposit oxides, nitrides, carbides, or combinations of compounds such as oxynitrides, carbonitrides, oxycarbides (oxycarbonitrides) and oxycarbonitrides (oxycarbonitrides).

The alkoxide precursor may contain sufficient oxygen to form an oxide without the need for additional oxygen. However, O is usually used₂To minimize the possibility of carbon being incorporated into the deposited layer. Water may be used as a reactant instead of oxygen to lower the reaction temperature.

Examples of organometallic precursors for silicon dioxide deposition include Tetraethylorthosilicate (TEOS) silicic acid ([ Si (OC2H5)4]), dimethyldiethoxysilane (DMDEOS) dimethyldiethoxysilane ([ (CH3)2SI (OC2H5)2]), Hexamethyldisiloxane (HMDSO) hexamethyldisiloxane ([ (CH3) 3SiOSi (CH3)3]), Tetramethyldisiloxane (TMDSO), 1,1,3, 3-tetramethyldisiloxane ([ (CH3)2HSiOSiH (CH3)2) ]), trichloroethylsilane (ETEOS) ethyltriethoxysilane ([ C2H5Si (OC2H5)3]), Trimethylethoxysilane (TMEOS) trimethylethoxysilane ([ (CH3)3SiOC2H5) ].

Examples of organometallic precursors for depositing titanium dioxide include ethyl titanate, isopropanol, and tert-butanol.

Decomposition of titanium isopropoxide titanium tetraisopropoxide (Ti iso-pro oxide Ti { OCH (CH3) 2}4) can also be used in FBCVD:

titanium tetraisopropoxide Ti { OCH: (CH3)2 → 4 → TiO (TiO)₂) + propane (C)₃H₈) + propanol (C)₃H₇OH) + water, above 450 ℃.

Tantalum ethoxide (Ta (OC2H5)5) can be used with oxygen for another high index material, Ta₂O₅The growth of (2).

Triisobutylaluminum (TIBA) is a pyrophoric liquid that decomposes on aluminum and isobutylene at temperatures above 200 ℃ and can be used to deposit aluminum trioxide layers of medium refractive index (refractive index n is about 1.65).

Notably, the deposited encapsulation layer may not be fully oxidized (e.g., metal hydroxide), and thus may require a high temperature anneal in the range of 400 ℃ to 900 ℃ to achieve the desired stoichiometry.

Other precursors, such as carbonyl groups, decompose at relatively low temperatures and deposit oxides. By way of example, iron carbonyl Fe (CO)₅To deposit iron oxide:

2Fe(CO)₅+O₂→Fe₂O₃+5CO₂

figures 7A-10 show a dielectric diffraction sheet formed by providing a microstructured silica core to a fluidized bed and encapsulating the microstructured dielectric core in the fluidized bed by chemical vapor deposition using a titanium tetrachloride precursor reacted with water vapor to form a titanium dioxide encapsulation layer encapsulating the microstructured dielectric core.

FIGS. 7A-7D show micrographs of diffractive pigment flakes obtained using a Scanning Electron Microscope (SEM) at various magnifications from 250 (FIG. 7A) to 25000 (FIG. 7D); the micrographs in fig. 7A-7C show small rectangles that are further magnified in the next picture. The sheet has three layers of symmetrical TiO₂/SiO₂/TiO₂Structures formed by encapsulating a microstructured single layer core formed of silicon dioxide with a titanium dioxide encapsulation layerAnd (5) forming. The core was a preformed sheet of 25X 25 microns shaped with a diffraction grating having a frequency of 1400 l/mm.

Fig. 8A-8C show cross-sectional images of a typical encapsulating sheet such as that shown in fig. 7A-7D for analysis by microtomy using a Transmission Electron Microscope (TEM) with a magnification of 25000 times. The thickness of the silica core was about 120 nm; the encapsulated titanium dioxide layer has a thickness of about 30 nanometers and preferably conforms well to the microstructure of the core sheet.

FIG. 9 shows an image of the edge of a slide obtained with a Scanning Transmission Electron Microscope (STEM); and fig. 10 shows energy dispersive X-ray spectrometer (EDS) elemental analysis of spectra from marker regions 1,2,3 and 4 in the STEM image. Table 1 shows the corresponding quantitative elemental analysis in atomic percent. Chlorine in TiO_xThe layer is detected. The V signal is found in most Ti films. Ti and O₂The presence of (a) confirms the nature of the encapsulating titanium dioxide layer.

TABLE 1

Optical spectrum	O	Si	Cl	Ti	V
						Spectrum 1	23.45		7.69	67.13	1.72
Spectrum 2	30.00		7.34	60.97	1.69

Spectrum 3	53.20	40.37		6.44
						Spectrum 4	57.08	37.50		5.42

The encapsulation is carried out using a fluidized bed chemical vapor deposition process in the presence of a titanium tetrachloride precursor. However, other precursors may be used. By way of example, an organometallic such as titanium-isopropyl alcohol (TI (OC3H7)4) may be used for the deposition of the titanium dioxide encapsulation layer.

In another embodiment, the all-dielectric sheet may have more than one encapsulation layer. For example, a symmetric design of 7 layers is obtained by alternately depositing titania, silica and titania to the microstructured core of silica. Silicon halide precursors such as SiCl4 may be used₄Or using, for example, tetraethoxysilane TEOS (Si (OC2H5)4 or hexamethyldisiloxane HMDSO (Si)₂O(C₂H₃)₃) To deposit SiO₂A layer; the titanium dioxide layer and core may be formed as described above.

Another possible design could start with a high refractive index layer (e.g. titanium dioxide) and then encapsulate the deposited silicon dioxide and titanium dioxide. Silica and titania materials have the advantage of being compatible with the cosmetic industry. However, other materials with high and low refractive indices may be used in the all dielectric design.

Fluidized Bed Chemical Vapor Deposition (FBCVD) and chemical precipitation are two more suitable techniques for coating core particles. However, wet chemistry processes such as that disclosed in U.S. patent No. 6,241,858 require extensive separation between solid core sheets so that the reaction solution can contact as much of the sheet surface as possible. Furthermore, it requires drying and disintegration (disintegration) of the sheet after the drying step. As a similar technique, FBCVD avoids these additional steps. The FBCVD technique avoids the agglomeration problem associated with wet chemical processes due to mass and heat transfer and solid mixing. Advantageously, the layer applied using FBCVD techniques conforms fully to the surface microstructure of the diffractive core sheet, thereby giving the sheet higher performance. When conventional methods are used to coat microstructured sheets, the lower conformability of the encapsulation layer results in a deterioration or even a complete loss of the diffractive effect brought about by the microstructure of the diffractive pigment. Conformal layers are very difficult to obtain by chemical precipitation or any other form of wet chemistry, but instead tend to produce non-conformal, planarized coating layers. Furthermore, FBCVD techniques can be used for the deposition of high metal absorber layers, i.e., containing a high percentage of metal layers, whereas typical chemical deposition cannot produce a high percentage of metal layers.

Claims (17)

1. A method of forming a microstructured pigment flake, the method comprising:

depositing a single dielectric layer on the microstructured substrate to form a single microstructured dielectric layer;

separating the single microstructured dielectric layer from the microstructured substrate;

breaking the single microstructured dielectric layer to form a plurality of microstructured dielectric cores;

encapsulating the plurality of microstructured dielectric cores in a fluidized bed with one or more additional dielectric layers by chemical vapor deposition.

2. The method of claim 1, wherein the chemical vapor deposition comprises a thermally activated reaction.

3. The method of claim 1, wherein the chemical vapor deposition comprises plasma activation.

4. The method of claim 1, wherein encapsulating the plurality of microstructured dielectric cores comprises providing a fluidizing gas through a bottom of the fluidized bed.

5. The method of claim 1, wherein encapsulating the plurality of microstructured dielectric cores comprises providing a precursor to the fluidized bed from above.

6. The method of claim 1, wherein encapsulating the plurality of microstructured dielectric cores comprises providing a precursor through a bottom of the fluidized bed.

7. The method of claim 1, wherein encapsulating the plurality of microstructured dielectric cores comprises providing a reactant in a direction opposite to a flow direction of the precursor.

8. The method of claim 1, wherein the chemical vapor deposition comprises using an organometallic precursor.

9. The method of claim 1, wherein the fluidized bed comprises a perforated floor or a perforated floor.

10. The method of claim 1, wherein the reactant is water.

11. The method of claim 1, wherein each of the plurality of microstructured dielectric cores comprises a grating in a surface thereof.

12. The method of claim 11, wherein each of the one or more additional dielectric layers conforms to a grating located in a surface of each of the plurality of microstructured dielectric cores.

13. The method of claim 11, wherein each of the one or more additional dielectric layers replicates the grating in the surface of each of the plurality of microstructured dielectric cores.

14. The method of claim 1 wherein the microstructured pigment flake exhibits a diffractive effect provided by a grating and one or more additional dielectric layers in at least one surface of the plurality of microstructured dielectric cores.

15. The method of claim 1, wherein the microstructured dielectric core is formed from a low refractive index material.

16. The method of claim 1, wherein the one or more additional dielectric layers are formed of a high index of refraction material.

17. The method of claim 1, further comprising depositing an absorber layer on the one or more additional dielectric layers.