Method for precise molding and alignment of structures on a substrate using a stretchable mold Number:6,802,754 from the United States Patent and Trademark Office (PTO) owispatent

Title: Method for precise molding and alignment of structures on a substrate using a stretchable mold

Abstract: A method for molding and aligning microstructures on a patterned substrate using a microstructured mold. A slurry containing a mixture of a ceramic powder and a curable fugitive binder is placed between the microstructure of a stretchable mold and a patterned substrate. The mold can be stretched to align the microstructure of the mold with a predetermined portion of the patterned substrate. The slurry is hardened between the mold and the substrate. The mold is then removed to leave microstructures adhered to the substrate and aligned with the pattern of the substrate. The microstructures can be thermally heated to remove the binder and optimally fired to sinter the ceramic powder.

Patent Number: 6,802,754 Issued on 10/12/2004 to Chiu,   et al.

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Inventors:	Chiu; Raymond C. (Woodbury, MN); Hoopman; Timothy Lee (River Falls, WI); Humpal; Paul Edward (Stillwater, MN); King; Vincent Wen-Shiuan (Woodbury, MN); Dillon; Kenneth R. (White Bear Lake, MN)
Assignee:	3M Innovative Properties Company (St. Paul, MN)
Appl. No.:	10/626,285
Filed:	July 24, 2003

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	972655	Oct., 2001	6616887
	779207	Feb., 2001	6325610
	219803	Dec., 1998	6247986

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Current U.S. Class:	445/24
Current International Class:	H01J 9/24 (20060101)
Field of Search:	445/24 313/534,582 264/496

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Primary Examiner: Williams; Joseph
Attorney, Agent or Firm: Fischer; Carolyn A.

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Parent Case Text

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CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 09/972,655, filed Oct. 5, 2001 now U.S. Pat. No. 6,616,887, which is a continuation of application Ser. No. 09/779,207, filed Feb. 8, 2001, now U.S. Pat. No. 6,325,610, which is a divisional of Application Ser. No. 09/219,803, filed Dec. 23, 1998, now U.S. Pat. Ser. No. 6,247,986.

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Claims

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What is claimed is:

1. A method of forming microstructures on a substrate, comprising: providing a substrate comprising a plurality of electrodes patterned on at least one surface of the substrate; placing a slurry comprising a mixture of a ceramic powder containing TiO.sub.2 and a curable fugitive binder between the at least one surface of the substrate and a patterned surface of a mold, wherein the patterned surface of the mold comprises a plurality of microstructures, and further wherein the plurality of microstructures are aligned with the plurality of electrodes patterned on the at least one surface of the substrate; curing the curable binder with blue light to harden the slurry and to adhere the slurry to the substrate; and removing the mold to leave green state microstructures of the slurry adhered to the substrate, wherein the green state microstructures substantially replicate the plurality of microstructures of the patterned surface of the mold.

2. The method of claim 1, wherein the method further comprises debinding the green state microstructures to substantially burn out the fugitive binder, and thereafter firing the green state microstructures at an elevated temperature higher than that used for debinding to sinter the ceramic powder to form ceramic microstructures.

3. The method of claim 2, wherein the slurry further comprises a diluent selected to promote release properties with the mold during the removal step and to facilitate burn out of the binder during the debinding step.

4. The method of claim 2, wherein the positions of the ceramic microstructures on the substrate after the firing step substantially match the positions of the green state microstructures on the substrate before firing.

5. The method of claim 1, wherein curing the curable binder comprises exposing the slurry to ultraviolet or visible light radiation through the substrate, through the mold, or through both the substrate and the mold.

6. The method of claim 1, wherein the mold comprises a thermoplastic material having a smooth surface and an opposing microstructured surface.

7. The method of claim 1, wherein the mold comprises a base film layer and a patterned layer made from a curable polymer, wherein the patterned layer comprises a smooth surface adhered to the base film layer and a microstructured surface opposing the base film layer.

8. The method of claim 1, wherein the slurry further comprises a silane compound selected to promote adhesion with the substrate during curing.

9. The method of claim 1, wherein the plurality of electrodes patterned on the at least one surface of the substrate comprises a series of substantially parallel and independently addressable electrodes spaced a distance apart.

10. A method of forming microstructures on a substrate, comprising: providing a substrate comprising a pattern; providing a mold comprising a microstructured surface, wherein the substrate and the mold further comprise mutual interlocking features such that when the mold is placed over the substrate with the respective mutually interlocking features mated, the microstructured surface of the mold is desirably aligned with the pattern of the substrate; placing a curable material between the substrate and the mold; mating the interlocking features of the substrate and the mold; and curing the curable material.

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Description

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TECHNICAL FIELD

The present invention generally relates to methods of forming and aligning structures on patterned substrates. More specifically, the present invention relates to methods of molding and aligning glass, ceramic, and/or metal structures on patterned substrates for display applications, and to displays having barrier ribs molded and aligned using a stretchable mold.

BACKGROUND

Advancements in display technology, including the development of plasma display panels (PDPs) and plasma addressed liquid crystal (PALC) displays, have led to an interest in forming electrically-insulating ceramic barrier ribs on glass substrates. The ceramic barrier ribs separate cells in which an inert gas can be excited by an electric field applied between opposing electrodes. The gas discharge emits ultraviolet (uv) radiation within the cell. In the case of PDPs, the interior of the cell is coated with a phosphor which gives off red, green, or blue visible light when excited by uv radiation. The size of the cells determines the size of the picture elements (pixels) in the display. PDPs and PALC displays can be used, for example, as display screens in high definition television (HDTV) or other digital electronic displays.

Various methods have been used to fabricate ceramic barrier ribs for PDPs. One method is repeated screen printing. In this method, a screen is aligned on the substrate and used to print a thin layer of barrier rib material. The screen is removed and the material is hardened. Because the amount of material that can be printed with this technique is insufficient to create ribs having the desired height (typically about 100 .mu.m to 200 .mu.m), the screen is then realigned and a second layer of barrier rib material is printed on top of the first layer. The second layer is then hardened. Layers of rib material are repeatedly printed and hardened until the desired barrier height is achieved. The multiple alignment and hardening steps required with this method results in a long processing time and poor control of the overall barrier rib profile shape.

Another method involves masking and sandblasting. In this method, a substrate having electrodes is coated with the barrier rib material which is partially fired. A mask is then applied to the barrier material using conventional lithography techniques. The mask is applied on the areas between the electrodes. The substrate is then sandblasted to remove the barrier rib material exposed by the mask. Finally, the mask is removed and the barrier ribs are fired to completion. This method requires only one alignment step and can therefore be more accurate than the multiple screen printing method. However, because the area of the finished substrate covered by barrier ribs is small, most of the barrier rib material must be removed by sandblasting. This large amount of waste increases the production cost. In addition, because the barrier rib material often includes lead-based glass frit, environmentally-friendly disposal of the removed material is an issue. Also, while the positions of the ribs after sandblasting can be quite accurate, the overall shapes of the ribs, including the height-to-width aspect ratio, can be difficult to control.

Another process utilizes conventional photolithographic techniques to pattern the barrier rib material. In this technique, the barrier rib material includes a photosensitive resist. The barrier rib material is coated onto the substrate over the electrodes, often by laminating the rib material in the form of a tape onto the substrate. A mask is applied over the barrier rib material and the material is exposed by radiation. The mask is removed and the exposed areas of the material are developed. Barrier rib material can then be removed by washing to form the rib structures. This process can give high precision and accuracy. However, as with sandblasting, much material is wasted because the entire substrate is initially coated with the barrier rib material and the ribs are patterned by material removal.

Another process involves using a mold to fabricate barrier ribs. This can be done by direct molding on the substrate or by molding on a transfer sheet and then transferring the ribs to a substrate. Direct molding onto a substrate involves coating either the substrate or the mold with barrier rib material, pressing the mold against the substrate, hardening the material on the substrate, and removing the mold. For example, Japanese Laid-Open Patent Application No. 9-134676 discloses using a metal or glass mold to shape barrier ribs from a glass or ceramic powder dispersed in a binder onto a glass substrate. Japanese Laid-Open Patent Application No. 9-147754 disclosed the same process where electrodes are transferred to the substrate simultaneously with the barrier ribs using a mold. After hardening the barrier rib material and removing the mold, the barrier ribs are fired to remove the binder.

European Patent Application EP 0 836 892 A2 describes printing a mixture of a glass or ceramic powder in a binder onto a transfer sheet. The material is printed using a roll or plate intaglio to form barrier rib shapes on the transfer sheet. A substrate is then pressed against the rib material on the transfer sheet to adhere the material to the substrate. After curing the rib material on the substrate, the ribs are fired. The transfer film can be removed before firing or burned away during firing.

SUMMARY OF THE INVENTION

While direct molding offers less wasted material than sandblasting or lithography and fewer alignment steps than screen printing, it poses challenges such as releasing the mold consistently and repeatedly from the barrier rib material and fabricating a separate mold for each unique display substrate. For example, slight adjustments in barrier rib pitch dimensions are desired to account for variations in shrinkage factors of glass substrates due to, for example, different lots or different suppliers.

If the barrier ribs are initially molded onto a transfer sheet, this method has the same disadvantages as direct molding. In addition, the transfer sheet with the rib material must be aligned with the electrodes on the substrate. This printing method may be used to print a pattern on a flexible film where the pattern on the film can subsequently be used as a mold for direct molding of barrier ribs. One difficulty, however, is that when the mold and rib material are pressed against the substrate to adhere the rib material to the substrate, the mold tends to elongate. This motion of the mold make precise alignment across the substrate very difficult. The method disclosed for solving this problem is to deposit a metal layer on the back of the mold to keep the mold from being able to elongate.

The present invention provides a method for forming and aligning microstructures on patterned substrates. Preferred embodiments of the present invention permit formation and alignment of microstructures on patterned substrates with high precision and accuracy over relatively large distances.

In a first aspect, the method of the present invention is a process for forming and aligning microstructures on a patterned substrate which proceeds by first placing a mixture comprising a curable material between a patterned substrate and a patterned surface of a mold. The patterned surface of the mold has a plurality of microstructures thereon. Microstructure as used in this application refers to indentations or protrusions in the surface of the mold. The mold is stretched to align a predetermined portion of the patterned surface of the mold with a correspondingly predetermined portion of the patterned substrate. The curable material between the mold and the substrate is cured to a rigid state adhered to the substrate. The mold is then removed, leaving hardened structures of the mixture aligned with the pattern of the substrate, the hardened structures replicating the microstructures of the patterned surface of the mold.

In another aspect, the present invention is a process for forming and aligning ceramic microstructures on a patterned substrate. A slurry is provided, the slurry being a mixture of a ceramic powder and a curable fugitive binder. The slurry is placed between a patterned glass substrate and a patterned surface of a mold, the patterned surface of the mold having a plurality of microstructures thereon. The mold is stretched to align a predetermined portion of the patterned surface of the mold with a correspondingly predetermined portion of the patterned substrate. The curable binder of the slurry is cured to harden the slurry and to adhere the slurry to the substrate. Then the mold is removed to leave green state microstructures of the slurry adhered to the substrate, the green state microstructures substantially replicating the microstructures of the patterned surface of the mold. The green state microstructures may be thermally processed to form substantially dense ceramic microstructures.

In another aspect, the present invention is a substrate element for use in an electronic display having microstructured barrier ribs molded and aligned on a patterned portion of a substrate. For example, the present invention provides a high definition television screen assembly including a plasma display panel. The plasma display panel includes a back glass substrate having a plurality of independently addressable electrodes forming a pattern and a plurality of ceramic microstructured barriers molded and aligned with the electrode pattern on the back substrate according to the process of the present invention. Phosphor powder is deposited between the ceramic barriers, and a front glass substrate having a plurality of electrodes is mounted with its electrodes orthogonally facing the electrodes of the back substrate. An inert gas is disposed between the front and back substrates.

In yet another aspect, the present invention provides an apparatus for molding and aligning ceramic microstructures on a patterned substrate. The apparatus stretches a stretchable mold having a microstructure thereon into close proximity with a patterned substrate, registers and aligns the microstructure of the mold with a predetermined portion of the patterned substrate, applies a slurry comprising a ceramic powder dispersed in a curable binder between the microstructure of the mold and the substrate, stretches the mold to align the microstructure of the mold with the predetermined portion of the patterned substrate, and cures the binder of the slurry between the substrate and the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a plasma display panel assembly.

FIG. 2 is a cross-sectional schematic of a slurry disposed between a mold and a patterned substrate.

FIG. 3 is a schematic representation of a method of stretching a structured mold according to the present invention.

FIG. 4 is a schematic representation of a method of removing a mold from green state microstructures.

FIG. 5 is a schematic representation of ceramic microstructures molded and aligned on a patterned substrate.

FIG. 6 is a schematic representation of an apparatus for molding and aligning microstructures.

FIG. 7 is a schematic of a jig used to stretch a mold.

DETAILED DESCRIPTION

The method of the present invention enables accurate molding of microstructures on a patterned substrate. While the method of the present invention can be used to mold and align microstructures made of various curable materials onto various patterned substrates for various applications, it is convenient to describe aspects of the method in terms of a particular application, namely molding and aligning ceramic barrier rib microstructures on an electrode-patterned substrate. Ceramic barrier rib microstructures are particularly useful in electronic displays in which pixels are addressed or illuminated via plasma generation between opposing substrates, such as PDPs and PALC displays. References to ceramic microstructure applications in the description of the method of the present invention that follows serve to illustrate aspects of the present invention and should not be read to limit the scope of the present invention or of the claims recited.

As used herein, the term ceramic refers generally to ceramic materials or glass materials. Thus, in the slurry used in one aspect of the method of the present invention, the included ceramic powder can be glass or ceramic particles, or mixtures thereof. Also, the terms fused microstructures, fired microstructures, and ceramic microstructures refer to microstructures formed using the method of the present invention which have been fired at an elevated temperature to fuse or sinter the ceramic particles included therein.

In an illustrative aspect, the method of the present invention includes using a slurry which contains a ceramic powder, a curable organic binder, and a diluent. The slurry is described in co-related U.S. Pat. No. 6,352,763, which is incorporated herein by reference. When the binder is in its initial uncured state, the slurry can be shaped and aligned on a substrate using a mold. After curing the binder, the slurry is in at least a semi-rigid state which can retain the shape in which it was molded. This cured, rigid state is referred to as the green state, just as shaped ceramic materials are called "green" before they are sintered. When the slurry is cured, the mold can be removed from the green state microstructures. The green state material can subsequently be debinded and/or fired. Debinding, or burn out, occurs when the green state material is heated to a temperature at which the binder can diffuse to a surface of the material and volatilize. Debinding is usually followed by increasing the temperature to a predetermined firing temperature to sinter or fuse the particles of the ceramic powder. After firing, the material can be referred to as fired material. Fired microstructures are referred to herein as ceramic microstructures.

FIG. 1 shows the substrate elements of a plasma display panel. The back substrate element, oriented away from the viewer, has a glass substrate 10 with independently addressable parallel electrodes 12. Ceramic barrier ribs 14 are positioned between electrodes and separate areas in which red (R), green (G), and blue (B) phosphors are deposited. The front substrate element includes a glass substrate 100 and a set of independently addressable parallel electrodes 102. The front electrodes 102, also called sustain electrodes, are oriented perpendicular to the back electrodes 12, also referred to as address electrodes. In a completed display, the area between the front and back substrate elements is filled with an inert gas. To light up a pixel, an electric field is applied between crossed sustain and address electrodes with enough strength to excite the inert gas atoms therebetween. The excited inert gas atoms emit uv radiation which causes the phosphor to emit red, green, or blue visible light.

Back substrate 10 is preferably a transparent glass substrate. Typically, substrate 10 is made of soda lime glass which can optionally be substantially free of alkali metals. The temperatures reached during processing can cause migration of the electrode material in the presence of alkali metal in the substrate. This migration can result in conductive pathways between electrodes, thereby shorting out adjacent electrodes or causing undesirable electrical interference between electrodes known as "crosstalk." The substrate should be able to withstand the temperatures required for sintering, or firing, the ceramic barrier rib material. Firing temperatures may vary widely from about 400.degree. C. to 1600.degree. C., but typical firing temperatures for PDP manufacture onto soda lime glass substrates range from about 400.degree. C. to about 600.degree. C., depending on the softening temperature of the ceramic powder in the slurry. Front substrate 100 is a transparent glass substrate which preferably has the same or about the same coefficient of thermal expansion as that of the back substrate.

Electrodes 12 are strips of conductive material. Typically, the electrodes are Cu, Al, or a silver-containing conductive frit. The electrodes can also be a transparent conductive oxide material, such as indium tin oxide, especially in cases where it is desirable to have a transparent display panel. The electrodes are patterned on back substrate 10, usually forming parallel strips spaced about 120 .mu.m to 360 .mu.m apart, having widths of about 50 .mu.m to 75 .mu.m, thicknesses of about 2 .mu.m to 15 .mu.m, and lengths that span the entire active display area which can range from a few centimeters to several tens of centimeters.

Barrier ribs 14 contain ceramic particles which have been fused or sintered by firing to form rigid, substantially dense, dielectric barrier ribs. The ceramic material of the barrier ribs is preferably alkali-metal free. The presence of alkali metals in the glass frit or ceramic powder can lead to undesirable migration of conductive material from the electrodes on the substrate. The ceramic material forming the barrier ribs has a softening temperature lower than the softening temperature of the substrate. The softening temperature is the lowest temperature at which a glass or ceramic material can be fused to a relatively dense structure having little or no surface-connect porosity. Preferably, the softening temperature of the ceramic material of the slurry is less than about 600.degree. C., more preferably less than about 560.degree. C., and most preferably less than about 500.degree. C. Preferably, the material of the barrier ribs has a coefficient of thermal expansion that is within 10% of the coefficient of expansion of the glass substrates. Close matching of the coefficients of expansion of the barrier ribs and the substrates reduces the chances of damaging the ribs during processing. Also, differences in coefficients of thermal expansion can cause significant substrate warpage or breakage. Barrier ribs in PDPs typically have heights of about 120 .mu.m to 140 .mu.m and widths of about 20 .mu.m to 75 .mu.m. The pitch (number per unit length) of the barrier ribs preferably matches the pitch of the electrodes.

It is important that PDP barrier ribs be positioned on the substrate between electrode positions. In other words, the pitch, or the periodicity, of the barrier ribs should closely match the pitch of the electrodes across the entire width of the display area. Misalignment adversely affects the functionality of the display. The spacing between the peaks of adjacent barrier ribs is preferably held to a tolerance of tens of parts per million (ppm) of the electrode pitch over the entire width of the display. Because the larger displays can have widths of 100 cm or more with an electrode pitch of about 200 .mu.m, the barrier ribs are preferably patterned to hold their alignment with the electrodes to within 10 .mu.m to 40 .mu.m over nearly 100 cm.

While it is the phosphors and not the barrier ribs that give off visible light in an active display, the optical properties of the ribs can enhance or detract from the display characteristics. Preferably, the sides of the barrier ribs are white and highly reflective so that light which does not directly exit an activated cell is not lost to absorption in significant amounts.

The barrier ribs also preferably have a low porosity. Highly porous ribs have large surface areas that can trap molecules which may contaminate the display and decrease the life of the display. When the display substrates are put together and sealed, the air between the substrate elements is replaced with an inert gas mixture for plasma generation. Molecules adsorbed in porous ribs can remain inside the display and desorb over time, leading to contamination and reducing the lifetime of the display.

After forming and firing the barrier rib materials, the phosphor materials are deposited between the barrier ribs, typically by screen printing. For linear barrier ribs, one type of phosphor material is deposited along the entire length of each channel defined by an adjacent pair of barrier ribs. The type of phosphor is alternated for adjacent channels to form a repeating pattern such as red, green, blue, red, green, blue, and so on.

The process of the present invention permits forming and aligning microstructures on a patterned substrate. The process of the present invention involves providing a mold, providing a material which can be cured or hardened to form microstructures, placing the material between the mold and a patterned substrate, aligning the mold with the pattern of the substrate, hardening the material between the mold and the substrate, and removing the mold. The mold has two opposing major surfaces, a generally flat surface and a patterned, or structured, surface. The patterned surface of the mold has a plurality of microstructures which represent the negative image of the microstructures to be formed and aligned on the patterned substrate. As described in further detail below, the pattern of the mold is designed such that matching between the pattern of the mold and the pattern of the substrate can be achieved by stretching the mold in at least one direction. By so stretching the mold for alignment, the mold can be corrected for mold or substrate variations due to variations in processing conditions, variations in the environment (such as temperature and humidity changes), and aging which can cause slight shifting, elongation, or shrinking of the pattern of the mold. If the position of the mold shifts in any manner during processing, the microstructures being formed on the substrate can become damaged and/or misaligned.

In many applications, the microstructures to be formed on the substrate are to be aligned with a patterned portion of the substrate in such a manner that each microstructure is positioned in a precise location relative to the pattern of the substrate. For example, on PDP substrates having a plurality of parallel electrodes, it is desirable to form uniformly-sized ceramic barriers positioned between each electrode. PDP substrates can have 1000 to 5000 or more parallel address electrodes that must each be separated by barrier ribs. Each of these barrier ribs must be placed with a certain precision, and this precision must be held across the width of the substrate. The process of the present invention allows for accurate and precise alignment of the mold pattern with the substrate pattern to form microstructures on the substrate with accurate and precise alignment which is consistently held across the substrate.

The material for forming the microstructures on the patterned substrate can be placed between the mold and the substrate in a variety of ways. The material can be placed directly in the pattern of the mold followed by placing the mold and material on the substrate, the material can be placed on the substrate followed by pressing the mold against the material on the substrate, or the material can be introduced into a gap between the mold and the substrate as the mold and substrate are brought together by mechanical or other means. The method used for placing the material between the mold and the substrate depends on, among other things, the aspect ratio of the structures to be formed on the substrate, the viscosity of the microstructure-forming material, and the rigidity of the mold. Structures having heights that are large compared to their widths (high aspect ratio structures) require molds having relatively deep indentations. In these cases, depending on the viscosity of the material, it can be difficult to completely fill the indentations of the mold unless the material is injected into the indentations of the mold with some force. In addition, care should be taken to fill the indentations of the mold while minimizing the introduction of bubbles or air pockets in the material.

While placing the material between the mold and the substrate, pressure can be applied between the substrate and the mold to set a land thickness, L, as in FIG. 2. The land is the material between the substrate and the base of the microstructures formed on the substrate. The land thickness can vary depending on the application. If zero land thickness is desired, it may be preferable to fill the mold with the material and then remove any excess material from the mold using a blade or squeegee before contacting the substrate. For other applications, it may be desirable to have a non-zero land thickness. In the case of PDPs, the material forming the microstructured barrier ribs is a dielectric, and the land thickness determines the thickness of dielectric material positioned on substrate electrodes 12. Thus, for PDPs, the land thickness can be important for determining what voltage must be applied between electrodes to generate a plasma and to activate a picture element.

The next step is to align the pattern of the mold with the pattern of the substrate. Under ideal conditions, the pattern of the mold as fabricated and the pattern of the substrate as fabricated would perfectly match. However, in practice this is rarely, if ever, the case. Processing steps can cause the dimensions of the substrate and the mold to change. While these dimensional changes might be slight, they can adversely affect the precise placement of microstructures aligned with the substrate pattern using a mold. For example, a PDP substrate having a width of 100 cm and an electrode pitch of 200 .mu.m requires that each of 5000 barrier ribs be placed precisely between adjacent electrodes. A difference between the pitch of the electrodes and the pitch of the mold of only 0.1 .mu.m (or 0.05%) means that the pattern of the barrier ribs and the electrode pattern on the substrate will be misaligned, and be 180.degree.out of phase in at least two regions across the substrate. This is fatal for display device functionality. For such a PDP substrate, the pitch of the mold and the pitch of the electrodes should have a mismatch of 0.01% or less.

The process of the present invention employs a mold capable of being stretched to facilitate precise alignment of the pattern of the mold with the pattern of the substrate. First, the mold is rough aligned by placing the pattern of the mold in the same orientation as the pattern of the substrate. The mold and substrate are checked for registry of their respective patterns. The mold is stretched in one or more directions parallel to the plane of the substrate until the desired registry is achieved. In the case of substrates having a pattern of parallel lines, such as electrodes on a PDP substrate, the mold is preferably stretched in one direction, either parallel to the substrate pattern or perpendicular to the substrate pattern, depending on whether the pitch of the mold is greater than or less than the pitch of the substrate pattern. FIG. 3 shows the case where mold 30 is stretched in a direction parallel to the parallel line pattern of the substrate 34. In this case, the pitch of the pattern of the mold is reduced during stretching to conform it to the pitch of the pattern of the substrate. To expand the pitch of the mold, the mold is stretched in the perpendicular direction.

Stretching can take place using a variety of known techniques. For example, the edges of the mold can be attached to adjustable rollers which can increase or decrease the tension on the mold until alignment is achieved. In cases where it is desirable to stretch the mold in more than one direction simultaneously, the mold can be heated to thermally expand the mold until alignment is achieved.

After alignment of the pattern of the mold with the pattern of the substrate, the material between the mold and the substrate is cured to form microstructures adhered to the surface of the substrate. Curing of the material can take place in a variety of ways depending on the binder resin used. For example, the material can be cured by curing using visible light, ultraviolet light, e-beam radiation, or other forms of radiation, by heat curing, or by cooling to solidification from a melted state. When radiation curing, radiation can be propagated through the substrate, through the mold, or through the substrate and the mold. Preferably, the cure system chosen optimizes adhesion of the cured material to the substrate. As such, in cases where material is used which tends to shrink during hardening and radiation curing is used, the material is preferably cured by irradiating through the substrate. If the material is cured only through the mold, the material might pull away from the substrate via shrinkage during curing, thereby adversely affecting adhesion to the substrate. In the present application, curable refers to a material that may be cured as described above.

After curing the material to form microstructures adhered to the substrate surface and aligned to the pattern of the substrate, the mold can be removed. Providing a stretchable and flexible mold can aid in mold removal because the mold can be peeled back so that the demolding force can be focused on a smaller surface area. As shown in FIG. 4, when linear rib-like microstructures are molded such as barrier ribs 24, mold 30 is preferably removed by peeling back along a direction parallel with ribs 24 and mold pattern 34. This minimizes the pressure applied perpendicular to the ribs during mold removal, thereby reducing the possibility of damaging the ribs. Preferably, a mold release is included either as a coating on the patterned surface of the mold or in the material that is hardened to form the microstructure itself. The advantages of including a mold release composition in the hardenable material is described in more detail below with respect to a moldable slurry used to form ceramic barrier ribs on a PDP substrate. A mold release material becomes more important as higher aspect ratio structures are formed. Higher aspect ratio structures make demolding more difficult, and can lead to damage to the microstructures. As discussed above, curing the material from the substrate side not only helps improve adhesion of the hardened microstructures to the substrate, but can allow the structures to shrink toward the substrate during curing, thereby pulling away from the mold to permit easier demolding.

After the mold is removed, what remains is the patterned substrate having a plurality of hardened microstructures adhered thereon and aligned with the pattern of the substrate. Depending on the application, this can be the finished product. In other applications such as substrates that will have a plurality of ceramic microstructures, the hardened material contains a binder which is preferably removed by debinding at elevated temperatures. After debinding, or burning out of the binder, firing of the green state ceramic microstructures is performed to fuse the glass particles or sinter the ceramic particles in the material of the microstructures. This increases the strength and rigidity of the microstructures. Shrinkage also occurs during firing as the microstructure densifies. FIG. 5 shows ceramic microstructures 14 after firing on a substrate 10 having patterned electrodes 12. Firing densifies microstructures 14 so that their profile shrinks somewhat from their green state profile 24 as indicated. As shown, fired microstructures 14 maintain their positions and their pitch according to the substrate pattern.

For PDP display applications, phosphor material is applied to fired barrier ribs, and the substrate can then be installed into a display assembly. This involves aligning a front substrate having sustain electrodes with the back substrate having address electrodes, barrier ribs, and phosphor such that the sustain electrodes are perpendicular with the address electrodes. Tile areas through which the opposing electrodes cross define the pixels of the display. The space between the substrates is then evacuated and filled with an inert gas as the substrates are bonded together and sealed at their edges.

It should be noted that the process of the present invention can lend itself well to automation to take advantage of the efficiencies offered by continuous processing. For example, the patterned substrate can be conveyed by a belt or other mechanisms to an area where the mold can be brought into close proximity with the substrate by, for example, a rotating drum. As the mold is brought close to the substrate, an extrusion die or other means can be used to apply the curable slurry between the patterned surface of the mold and the patterned surface of the substrate. The conveyer means for the substrate and the conveyer means for the mold are positioned such that rough positioning of the pattern of the mold with the pattern of the substrate occurs as the two are brought together and as the material is placed therebetween. After placing the hardenable material between the substrate and the mold, alignment between the pattern of the mold and the pattern of the substrate can be automatically checked, for example by optical detectors. The optical detectors can look for alignment fiducials or check for a moire interference pattern due to misalignment of the pattern of the mold and the pattern of the substrate. The mold can then be stretched by, for example, gripping a pair of opposing edges of the mold and pulling until the optical detectors confirm alignment. At this point, the material between the mold and substrate can be cured by irradiating the material through the substrate, through the mold, or both. After a predetermined curing time, the substrate and mold can be advanced as the rotating drum peels the mold away from the cured microstructures formed and aligned on the patterned substrate.

FIG. 6 shows an apparatus for molding, aligning, and curing microstructures on a patterned substrate using a microstructured mold. Substrate 84 resides on mechanical stage 92 which preferably has the ability of x-motion (motion from left to right in the figure), y-motion (motion in and out of the page of the figure), and .theta.-motion (rotational motion in the x-y plane). Such motion allows substrate 84 to be moved into position for alignment and curing, to be rough aligned with the mold, and to be moved out of position for removal of the mold after curing. Rolls 90a and 90b are wind up and unwind rolls, respectively, for moving flexible, stretchable mold 80 in line with substrate 84. To introduce the curable material between substrate 84 and mold 80, substrate 84 and mold 80 are moved in concert as the curable material is injected by injection means 98 into a gap between mold 80 and substrate 84 near roll 88a. Substrate 84 and mold 80 are moved in unison as the material is applied therebetween until the desired amount of material is applied between the pattern of the substrate and the pattern of the mold. FIG. 6 shows the substrate 84 and mold 80, having curable material 82 disposed between, moved into an area where optical detectors 96a and 96b check for alignment. Depending on the pattern of the microstructures, two or more detectors may be required. Rollers 88a and 88b can then be oppositely rotated to stretch the mold until the pattern of the mold and the pattern of the substrate are aligned with the desired precision. At this point, radiation source 94 is used to irradiate curable material 82 through substrate 84. After material 82 is cured, the substrate and mold are moved in unison as roller 88b acts to peel the mold away from the cured microstructures which have been molded in alignment with the pattern of the substrate.

An alternative method of molding and aligning microstructures on a patterned substrate according to the present invention involves a static stretching method. For example, a patterned substrate can be provided which has protrusions or indentions located outside of the pattern of the substrate and on opposing ends of the substrate. The stretchable mold also has protrusions or indentions located outside of the microstructured pattern of the mold which align and interlock with those provided on the substrate when the mold is slightly stretched. These added interlocking features on the substrate and the mold hold the pattern of the mold in alignment with the pattern of the substrate without the need for other machinery.

The method of the present invention preferably uses a mold capable of being stretched in at least one direction to align the pattern of the mold to a predetermined portion of the patterned substrate. The mold is preferably a flexible polymer sheet having a smooth surface and an opposing microstructured surface. The mold can be made by compression molding of a thermoplastic material using a master tool which has a microstructured pattern. The mold can also be made of a curable material which is cast and cured onto a thin, flexible polymer film.

The microstructured mold of the present invention is preferably formed according to a process similar to the processes disclosed in U.S. Pat. No. 5,175,030 (Lu et al.) and U.S. Pat. No. 5,183,597 (Lu). The formation process preferably includes the following steps: (a) preparing an oligomeric resin composition; (b) depositing the oligomeric resin composition onto a master negative microstructured tooling surface in an amount barely sufficient to fill the cavities of the master; (c) filling the cavities by moving a bead of the composition between a preformed substrate and the master, at least one of which is flexible; and (d) curing the oligomeric composition.

The oligomeric resin composition of step (a) preferably is a one-part, solvent-free, radiation-polymerizable, crosslinkable, organic oligomeric composition. The oligomeric composition is preferably one which is curable to form a flexible and dimensionally-stable cured polymer. The curing of the oligomeric resin should occur with low shrinkage. One preferred suitable oligomeric composition is an aliphatic urethane acrylate such as one sold by the Henkel Corporation, Ambler, Pa., under the trade designation Photomer 6010, although similar compounds are available from other suppliers.

Acrylate functional monomers and oligomers are preferred because they polymerize more quickly under normal curing conditions. Further, a large variety of acrylate esters are commercially available. However, methacrylate, acrylamide and methacrylamide functional ingredients can also be used without restriction. Herein, where acrylate is used, methacrylate is understood as being acceptable.

Polymerization can be accomplished by usual means, such as heating in the presence of free radical initiators, irradiation with ultraviolet or visible light in the presence of suitable photoinitiators, and by irradiation with electron beam. For reasons of convenience, low capital investment, and production speed, the preferred method of polymerization is by irradiation with ultraviolet or visible light in the presence of photoinitiator at a concentration of about 0.1 percent to about 1.0 percent by weight of the oligomeric composition. Higher concentrations can be used but are not normally needed to obtain the desired cured resin properties.

The viscosity of the oligomeric composition deposited in step (b) is preferably between 500 and 5000 centipoise (500 and 5000.times.10.sup.-3 Pascal-seconds). If the oligomeric composition has a viscosity above this range, air bubbles might become entrapped in the composition. Additionally, the composition might not completely fill the cavities in the master tooling. For this reason, the resin can be heated to lower the viscosity into the desired range. When an oligomeric composition with a viscosity below that range is used, the oligomeric composition usually experiences shrinkage upon curing that prevents the oligomeric composition from accurately replicating the master.

Almost any material can be used for the base (substrate) of the patterned mold, as long as that material is substantially optically clear to the curing radiation and has enough strength to allow handling during casting of the microstructure. In addition, the material used for the base should be chosen so that it has sufficient thermal stability during processing and use of the mold. Polyethylene terephthalate or polycarbonate films are preferable for use as a substrate in step (c) because the materials are economical, optically transparent to curing radiation, and have good tensile strength. Substrate thicknesses of 0.025 millimeters to 0.5 millimeters are preferred and thicknesses of 0.075 millimeters to 0.175 millimeters are especially preferred. Other useful substrates for the microstructured mold include cellulose acetate butyrate, cellulose acetate propionate, polyether sulfone, polymethyl methacrylate, polyurethane, polyester, and polyvinyl chloride. The surface of the substrate may also be treated to promote adhesion to the oligomeric composition.

Examples of such polyethylene terephthalate based materials include: photograde polyethylene terephthalate; and polyethylene terephthalate (PET) having a surface that is formed according to the method described in U.S. Pat. No. 4,340,276.

A preferred master for use with the above-described method is a metallic tool. If the temperature of the curing and optionally simultaneous heat treating step is not too great, the master can also be constructed from a thermoplastic material, such as a laminate of polyethylene and polypropylene.

After the oligomeric resin fills the cavities between the