Method for forming ceramic microstructures on a substrate using a mold and articles formed by the method

A microstructured assembly including a barrier portions and land portions is described. The microstructures have alternating barrier portions and land portions that have barrier surfaces and land surfaces, respectively. Each barrier surface and land surface is connected by curved surface, which is part of a curved portion. The curved surface and the land surface are substantially continuous.

TECHNICAL FIELD

The present invention generally relates to methods of forming structures on patterned substrates. More specifically, the present invention relates to improved methods of molding ceramic structures that retain a desired shape after thermal processing. The present invention also relates to molding ceramic structures on patterned substrates for display applications, and to displays having molded barrier ribs.

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 the displays for high definition televisions (HDTV) or other digital electronic display devices.

One way in which ceramic barrier ribs can be formed on glass substrates involves laminating a planar rigid mold onto a substrate with a glass- or ceramic-forming composition disposed in the mold. The glass- or ceramic-forming composition is then solidified and the mold is removed. Finally, the barrier ribs are fused or sintered by firing at a temperature of about 550° C. to about 1600° C. The glass- or ceramic-forming composition has micrometer-sized particles of glass frit dispersed in an organic binder. The use of an organic binder allows barrier ribs to be solidified in a green state so that firing fuses the glass particles in position on the substrate. However, in applications such as PDP substrates, highly precise and uniform barrier ribs with few or no defects or fractures are desirable. This can pose challenges, especially during removal of the mold from the green state barriers and during firing of the green state barrier ribs.

Mold removal can damage barriers due to difficulty in mold release. Because barrier ribs tend to shrink during firing, the green state barrier ribs are generally taller than the size desired for the fused barriers. Taller structures can make demolding even more difficult. Mold removal can also damage the mold. When material cannot be completely removed from the mold, the mold is typically discarded. In addition, at temperatures required for firing, the barrier ribs can fracture, delaminate from the substrate, or warp. The substrate also goes through dimensional changes during firing due to thermal expansion and release of internal stresses.

Microstructures, such as the barrier ribs, can also be used in other applications.

SUMMARY OF THE INVENTION

In general, the invention is directed to articles and devices having microstructures disposed on a substrate and methods of making these articles and devices. PDP's and other display devices are examples of such articles and devices. One embodiment is a microstructured assembly. The microstructures have alternating barrier portions and land portions with barrier surfaces and land surfaces, respectively. Each barrier surface and land surface is connected by curved surface, which is part of a curved portion. The curved surface and the land surface are substantially continuous.

Another embodiment of the invention is a microstructured assembly. The assembly includes ceramic microstructures molded and hardened on a glass substrate having a pattern of addressable electrodes. The microstructures have alternating barrier portions and land portions with barrier surfaces and land surfaces, respectively. Each barrier surface and land surface is connected by curved surface which is part of a curved portion. The curved surface and the land surface are substantially continuous. The land portions of the microstructure are also aligned with the pattern of electrodes of the glass substrate.

Another embodiment of the invention is a microstructured assembly. The assembly includes microstructures molded and hardened on a substrate. The microstructures include alternating barrier portions and land portions. The width of the barrier portion at its top is not more than 75 μm.

Another embodiment of the invention is a microstructured assembly. The assembly includes microstructures molded and hardened on a substrate. The microstructures have alternating barrier portions and land portions that have barrier surfaces and land surfaces, respectively. The barrier portions also have ends that are step-shaped.

Another embodiment of the invention is a process for making a microstructured assembly. A curable material is disposed on a patterned substrate. A mold shapes the material into microstructures having alternating barrier portions and land portions that have barrier surfaces and land surfaces, respectively. Each barrier surface and land surface is connected by curved surface, which is part of a curved portion. The curved surface and the land surface are substantially continuous. The mold is removed. Optionally the material is cured or treated to harden the microstructures. Optionally the mold is stretched to align the microstructures with the patterned substrate.

Another embodiment of the invention is a method of making a microstructured assembly. A slurry including ceramic powder and a curable fugitive binder is disposed on a glass substrate patterned with electrodes. The slurry is shaped with a mold into microstructures having alternating barrier portions and land portions that have barrier surfaces and land surfaces, respectively. Each barrier surface and land surface is connected by curved surface, which is part of a curved portion. The curved surface and the land surface are substantially continuous. The binder is cured to harden the slurry and adhere the slurry to the substrate. The mold is removed to leave green state microstructures on the substrate that replicate the pattern of the mold. The green state microstructures are debinded and fired to burn out the binder and sinter the ceramic powder to form ceramic microstructures.

Another embodiment is a process to shape barrier ends of microstructures. A weight is applied to the barrier ends of green state microstructures. A portion of the bottom of the weight contacts the top comer of the barrier ends. The microstructures are fired and the weight is removed.

DETAILED DESCRIPTION

Methods have previously been described that enable accurate molding and formation of microstructures on a patterned substrate. For example, PCT Patent Publication No. WO/0038829 and U.S. patent application Ser. No. 09/219,803, incorporated herein by reference, describe the molding and aligning of ceramic barrier microstructures on an electrode-patterned substrate. PCT Patent Publication No. WO/0038829 and U.S. patent application Ser. No. 09/219,803, describe methods of forming ceramic barrier microstructures that are particularly useful in electronic displays, such as PDPs and PALC displays, in which pixels are addressed or illuminated via plasma generation between opposing substrates. U.S. patent application entitled, “METHOD FOR FORMING MICROSTRUCTURES ON A SUBSTRATE USING A MOLD”, Ser. No. 09/975,385, filed on even date herewith, incorporated herein by reference, describes methods for making ceramic microstructures on a substrate using a mold.

Such plasma displays have various substrate elements, as illustrated inFIG. 1. The back substrate element, oriented away from the viewer, has a back substrate21with independently addressable parallel electrodes23. The back substrate21can be formed from a variety of compositions, for example, glass, ceramic, metal, or plastic. Ceramic microstructures25include barrier portions32that are positioned between back electrodes23and separate areas in which red (R), green (G), and blue (B) phosphors are deposited. The front substrate element includes a glass substrate51and a set of independently addressable parallel electrodes53. The front electrodes53, also called sustain electrodes, are oriented perpendicular to the back electrodes23, 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 sustain53and address electrodes23with enough strength to excite the inert gas atoms therebetween. The excited inert gas atoms emit uv (ultraviolet) radiation which causes the phosphor to emit red, green, or blue visible light.

Back substrate21is preferably a transparent glass substrate. Typically, back substrate21is 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 back substrate21should be able to withstand the temperatures required for sintering, or firing, the ceramic barrier material. Firing temperatures may vary widely from about 400° C. to 1600° C., but typical firing temperatures for PDPs manufactured onto soda lime glass substrates range from about 400° C. to about 600° C., depending on the softening temperature of the ceramic powder in the slurry. Front substrate51is a transparent glass substrate which preferably has the same or about the same coefficient of thermal expansion as that of the back substrate21.

Electrodes are strips of conductive material. Typically, the electrodes are copper, aluminum, or a silver-containing conductive frit. The electrodes can also be made of 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 or in back substrate21. For example, the electrodes can be formed as parallel strips spaced about 120 μm to 360 μm apart, having widths of about 50 μm to 75 μm, thicknesses of about 2 μm to 15 μm, and lengths that span the entire active display area which can range from a few centimeters to several tens of centimeters. In some instances the widths of the back electrodes23can be, for example, narrower than 50 μm or wider than 75 μm, depending on the architecture of the microstructures25. For example, in high definition plasma display panels, it is preferable that the electrodes are less than 50 μm in width.

Material for forming microstructures25typically includes ceramic particles that can be fused or sintered by firing to form rigid, substantially dense, dielectric structures. The ceramic material of the microstructures25is preferably alkali-metal free and can include glass and other non-crystalline oxides. 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 barriers 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 no more than about 600° C., more preferably no more than about 560° C., and most preferably no more than about 500° C. Preferably, the material of the microstructures25has 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 microstructures25and the back substrate21reduces the chances of damaging the microstructures25during processing. Also, differences in coefficients of thermal expansion can cause significant substrate warpage or breakage.

The barrier portions32in PDPs can have heights, for example, of about 100 μm to about 170 μm and widths of about 20 μm to about 80 μm. The pitch (number of barriers per transverse cross-sectional unit length) of the barriers preferably matches the pitch of the electrodes.

PCT Patent Publication No. WO/0038829, U.S. patent application Ser. No. 09/219,803, and U.S. patent application entitled, “METHOD FOR FORMING MICROSTRUCTURES ON A SUBSTRATE USING A MOLD”, Ser. No. 09/975,385, filed on even date herewith, all herein incorporated by reference, describe methods for forming and aligning microstructures on a patterned substrate. One method proceeds by placing a mixture comprising a curable material between a patterned substrate and a patterned surface of a mold.FIG. 2illustrates a transverse cross-section of the mold30, curable material which forms the microstructures25, and a back substrate21with back electrodes23. The patterned surface of the mold30is able to form a plurality of microstructures25from the curable material that is between the mold30and the back substrate21. The mold30can optionally be stretched to align a predetermined portion of the patterned surface of the mold30with a correspondingly predetermined portion of the patterned back substrate21, as defined by the spacing of the back electrodes23.

The material for forming the microstructures25on the patterned back substrate21can be placed between the mold30and the back substrate21in a variety of ways. The material can be placed directly in the pattern of the mold30followed by placing the mold30and material on the back substrate21; the material can be placed on the back substrate21followed by pressing the mold30against the material on the back substrate21; the material can be placed on the back substrate21and then contacted with the mold30; or the material can be introduced into a gap between the mold30and the back substrate21as the mold30and back substrate21are brought together by mechanical or other means. The method used for placing the material between the mold30and the back substrate21depends on, among other things, the aspect ratio of the microstructures25to be formed on the back substrate21, the viscosity of the microstructure-forming material, and the rigidity of the mold30. Generally, microstructures25having heights that are large compared to their widths (high aspect ratio structures) utilize molds30having relatively deep indentations. In these cases, depending on the viscosity of the material, it can be difficult to completely fill the indentations of the mold30unless the material is injected into the indentations of the mold30with some force. Preferably, the indentations of the mold30are completely filled while minimizing the introduction of bubbles or air pockets in the material.

While placing the curable material between the mold30and the back substrate21, pressure can be applied between the back substrate21and the mold30to set a thickness of the land portion34, as inFIG. 2. The land portion34is generally the portion of the microstructure25between the barrier portions32, and which partially surrounds or is positioned above the back electrodes23. If a zero thickness of the land portion34is desired, it may be preferable to fill the mold30with the material and then remove any excess material from the mold30using a blade or squeegee before contacting the back substrate21. For other applications, it may be desirable to have a non-zero thickness of the land portion34. In the case of PDPs, the material forming the microstructures25is generally a dielectric, and the thickness of the land portion34determines the thickness of dielectric material positioned on back electrodes23. Thus, for PDPs, the thickness of the land portion34can be important for determining what voltage is applied between the back electrodes23and sustain electrodes53to generate a plasma and to activate a picture element.

After alignment of the pattern of the mold30with the pattern of the substrate, the material between the mold30and the back substrate21is cured to form green state microstructures45adhered to the surface of the back substrate21. Prior to debinding, microstructures can be referred to as green state microstructures. 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 using visible light, ultraviolet light, e-beam radiation, other forms of radiation, heat curing, or cooling to solidification from a melted state. When curing by radiation, radiation can be propagated through the back substrate21, through the mold30, or through the back substrate21and the mold30. Preferably, the cure system chosen facilitates adhesion of the cured material to the back substrate21. As such, in cases where material is used which tends to shrink during hardening and radiation curing, the material is preferably cured by irradiating through the back substrate21. If the material is cured only through the mold30, the material might pull away from the back substrate21via shrinkage during curing, thereby adversely affecting adhesion to the back substrate21. In the present application, curable refers to a material that can be cured as described above.

After curing the material to form green state microstructures45adhered to the back substrate21surface and aligned to the pattern of the back substrate21, the mold30can be removed. Providing a stretchable and flexible mold30can aid in mold30removal because the mold30can be peeled back so that the demolding force can be focused on a smaller surface area. As shown inFIG. 3, when green state microstructures45having barrier portions32are molded, the mold30is preferably removed by peeling back along a direction parallel with barrier portions32and the pattern of the mold30. This reduces the pressure applied perpendicular to the barrier portions32during mold removal, thereby reducing the possibility of damaging the barrier portions32. Preferably, a mold release is included either as a coating on the patterned surface of the mold30or in the material that is used to form the microstructure25itself. A mold release material can become more important as higher aspect ratio structures are formed. Higher aspect ratio structures make demolding more difficult, and can lead to damage to the microstructures25. As discussed above, curing the material from the back substrate21side not only helps improve adhesion of the hardened green state microstructures45to the back substrate21, but can allow the green state microstructures45to shrink toward the back substrate21during curing, thereby pulling away from the mold30to permit easier demolding.

After the mold30is removed, what remains is the patterned back substrate21having a plurality of hardened green state microstructures45adhered thereon and aligned with the pattern of the back substrate21. Depending on the application, this can be the finished product. In other applications the hardened material contains a binder that is preferably removed by debinding at elevated temperature. After debinding, or burning out of the binder, firing of the green state microstructures is performed to fuse the glass particles or sinter the ceramic particles in the microstructure material. This increases the strength and rigidity of the microstructures25. Shrinkage can also occur during firing as the microstructures25densify.FIG. 4illustrates ceramic microstructures25after firing on a back substrate21having patterned back electrodes23. Firing densifies microstructures25so that their profile shrinks somewhat from the profile of the green state microstructure45as indicated. As shown, fired microstructures25maintain their positions and their pitch according to the back substrate21pattern.

For PDP display applications, phosphor material is applied to the channels16of the microstructures25. The back substrate21with fired microstructures25then can be installed into a display assembly. This can involve aligning a front substrate51having sustain electrodes53with the back substrate21having back electrodes23, microstructures25, and phosphor such that the sustain electrodes53are perpendicular with the back electrodes23, as shown inFIG. 1. The 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.

The thickness profile of the land portion34of the microstructure25, including dielectric thickness, can be an important aspect of a plasma display panel. The thickness of the land portion34can affect the electrical performance of the plasma display panel. The microstructures25can be molded or otherwise formed to create a thickness profile of the land portion34. The thickness profile can be designed to provide a thickness that is constant over the width of the land portion34. In other cases, the thickness profile of the land portion34can be designed to provide a thickness that is variable over the width of the land portion34. A variable thickness profile can be compatible with other aspects of the PDP, for example, the placement and dimensions of the back electrodes23or the architecture of the barrier portions32. During processing, however, changes may occur in the material of the microstructure25that have an undesirable effect on the electrical performance of the PDP.

Substantial differences between individual land portions31, for example, different thicknesses or different thickness profiles of the land portions34, can result in undesirable light emission patterns (e.g., unequal emissions of the phosphors.) This can be the result of, for example, substantial differences in the switching voltage for individual pixels during operation of the plasma display panel due to substantial differences between individual land portions. These undesirable light emission patterns may be manifested by variations in pixel-to-pixel brightness or a difficulty in lighting up some pixels.

Electrical performance can also be compromised by defects introduced into the microstructure25following manufacturing steps (such as curing or thermal processing steps). Microstructures25can suffer from defects such as, for example, fracturing, splintering, breakage, unequal shrinkage, splitting, and bulging. Fractures33or other defects in the microstructure25, as illustrated inFIG. 4, can expose portions of the back electrode23, back substrate21, or both. These defects can also cause undesirable electrical performance of the plasma display by creating substantial differences in the switching voltage during operation of the plasma display panel. In addition, fractures can trap gas species that, over time, diffuse into adjacent cells during operation. This degrades performance of the plasma display panel during use and ultimately shortens its lifetime.

In addition to requiring fewer steps for formation, microstructures molded with a uniform land portions34and barrier portions32can have desirable physical properties. The presence of a land portion34can provide overall structural stability to the molded microstructures25. However, fractures introduced in or near the land portion34during debinding and sintering can potentially compromise the attachment of the barrier portion32to the back substrate21.

Shrinkage occurs during firing as the microstructure25densifies.FIG. 4illustrates a transverse cross-sectional view of microstructures25on a back substrate21after firing andFIG. 5illustrates a lateral cross sectional view of an end of a microstructure25on a back substrate21after firing. Firing densifies microstructures25so that their profile shrinks somewhat from their green state profile45as indicated. As shown, most portions of the fired microstructures25generally maintain their relative shape according to the shape of the green state microstructure45.FIG. 4also illustrates that the fired microstructures25generally maintain their position and pitch relative to the back substrate21and back electrode23patterned on the back substrate21. However, shrinkage of the microstructure25during the firing can cause increased stress in the fired material. This stress is released during the firing or cooling process and can result in cracks, or fractures, in the microstructure25.

Fractures33can be attributed, at least partially, to the shape of the green state microstructures45prior to thermal processing. Green state microstructures45which are molded to form a shape similar to that illustrated inFIG. 4are particularly likely to suffer from fractures33after firing. This is especially true when transverse cross-sectional profile of the green state microstructures45contains a surface discontinuity43near the land portion34. As illustrated inFIG. 4the microstructure25contains a channel16that has a surface61, the surface61includes a barrier surface52and a land surface54. A surface discontinuity43is a point where two parts of the surface61meet (as illustrated inFIG. 4it is the point where the barrier surface52meets the land surface54) and there is a substantial discontinuity in slope, for example, a substantial discontinuity in the slope of the barrier surface52relative to the slope of land surface54.

Another example is shown inFIG. 6, which illustrates microstructures25having a curved portion36. In this example, a surface discontinuity at point43may exist on a surface61between a curved surface56and a land surface54when the slope of the land surface54is not substantially the same as the slope of the curved surface56at that point43. A surface discontinuity can be visualized as a break in the smoothness of the surface61. Surface discontinuities also can exist at points, for example, where a curved surface56meets a barrier surface52, however, fractures typically occur at or near a surface discontinuity that is proximal to the land portion34.

A surface61is considered discontinuous at a point43if there is a substantial difference between the slope102at that point43when the point is approached from one direction and the slope104at that point43when approached from the opposite direction, as illustrated inFIG. 14. In other words, as used herein, a surface61is continuous at a point100if the instantaneous linear slopes102,104derived by approaching the point from two directions along the surface61differ in angle106by no more than about 5 degrees, and, preferably, no more than about 3 degrees, when extended linearly, as illustrated inFIG. 14.

As another source of microstructure difficulties, the shrinkage that occurs during firing of the microstructures can affect the ends of the barrier portions. As illustrated inFIG. 5, a lateral cross section shows deformations (for example, deformation37) that appear at the barrier portion end29of the microstructure25after thermal processing. Firing densifies microstructures25so that their profile shrinks from their green state profile45as indicated. In at least some instances, this shrinkage ranges from 30% to 40% after firing.

As shown, the top48of the barrier portion over the majority of the length of the barrier portion32maintains a relatively flat surface. However, the barrier portion ends29generally do not shrink uniformly with the rest of the barrier portion32and a slight curling of the barrier portion ends29typically occurs, resulting in a deformation37. This deformation37can create multiple problems in the assembly and the functioning of the plasma display panel or other device. First, during sealing and handling of a display, mechanical forces can cause the deformations37to break off. End pieces that have broken off can be detrimental to PDP function and life. Second, if deformations37stay intact in a display, the deformations37will provide an area of lift to the front substrate51. The front substrate51will not be flush with the tops48of the barrier portions along the length of the barrier portions32and a gap is created between the tops48of the barrier portions and the surface of the front substrate51. This can lead to cross talk between excited gas species in adjacent cells as well as large differences in switching voltage during operation.

Microstructures incorporating novel shapes have been developed. The current invention can be used to, if desired, overcome one or more problems that are associated with the thermal processing of materials, for example, fracturing and deformation of that material. This can be particularly useful for the preparation of microstructures that include a land portion and a barrier portion. In one embodiment, the microstructures are provided having a curved surface of a curved portion that is continuous with a land surface of a land portion. In another embodiment, microstructures are provided that have a thin barrier width profile. In these embodiments the shape or dimensions of the microstructures typically provide increased fracture-resistance. In another embodiment of the invention, the microstructures include barrier portions with shaped ends, in particular, step-shaped ends. In yet another embodiment a method to shape barrier ends of microstructures by weighting the ends of the barrier portions is provided. In addition, techniques allowing for the molding and forming of microstructures are also embodiments of the inventions.

The shape of the microstructures25is formed by a patterned mold30that is generally fabricated to be a reverse image of the green state microstructures formed on the back substrate21. The microstructures25are generally formed by placing the material between the back substrate21and the patterned surface of the mold30. In one embodiment, as illustrated inFIG. 6, the patterned mold30shapes the material into a plurality of repeating microstructure units15, each repeating microstructure unit15having three primary portions: a barrier portion32, a land portion34, and a curved portion36. The repeating microstructure units15form a plurality of channels16in the material, the channels16having a surface61, a portion of which is curved and defined by the shape of the barrier portion32, land portion24, and curved portions36. The surface61of the channel16can include a barrier surface52, a land surface54, and a curved surface56, corresponding to the surface of the respective portions.

Microstructures25can be shaped, if desired, to reduce the probability that fractures33will develop near the region of the microstructure25where the barrier portion32meets the land portion34, as illustrated inFIG. 4. In one embodiment, an example of which is illustrated inFIG. 6, a substantially continuous surface61from the curved portion36to the land portion24is provided. As discussed herein, the current invention describes microstructures25, and techniques for making microstructures25, that include a curved portion36having a curved surface56that is continuous with the land surface54. Examples of parameters that describe the overall shape of the microstructures25, including the surface61, are described below.

For a typical plasma display panel (FIG. 1), the patterned mold30can form 1000 to 5000 or more repeating microstructure units15on the surface of the back substrate21. The surface of the back substrate21is typically patterned with parallel address electrodes23and, when the microstructures25are formed, the microstructures25are aligned with the back electrodes23. Typically the land portions34are aligned with the back electrodes23.

The barrier portion32forms a barrier structure that physically contains the inert gasses of the plasma display panel. Although the material of the barrier portion32is physically continuous with material of the land portions34and curved portions36, it is convenient to describe details of the current invention by defining artificial boundaries of the barrier portion32. Each side of the barrier portion32is bounded by a barrier line42. The barrier line42runs from the barrier portion top48to a point on the microstructure/substrate interface41. The barrier line42follows the slope of the vertical surface of the barrier portion32near the barrier portion top48. A barrier line angle49is formed by the barrier line42and the microstructure/substrate interface41. The barrier line angle49is generally in the range of 130° to 90°, typically in the range of 115° to 90°, and can be in the range of 95° to 90°.

One example of a plasma display panel includes barrier portions32having heights (hBP) in the range of 80 to 200 μm or in the range of 100 to 170 μm, as measured from the microstructure/substrate interface41to the barrier portion top48. At the barrier portion top48, the width of the barrier portion32is typically, for example, in the range of 20 to 80 μm. At microstructure/substrate interface41the width of the barrier portion32is typically, for example, in the range of 20 to 120 μm.

In some cases the land portion34can form a dielectric layer that encompasses the tops and the sides of the back electrode23on the surface of the back substrate21. For example, when the back electrode23is formed on the surface of the back substrate21(e.g., above the microstructure/substrate interface41), the material of the microstructures25is in contact with the top and sides of the back electrodes23. In other cases the back electrode23can be formed in the back substrate21so the material of the microstructure25is only in contact with the top of the back electrode23or not in contact with the back electrode23at all.

The material of the land portion34is contiguous with material of the barrier portion32and curved portions36. Each side of the land portion34is bounded by the barrier lines42of the adjacent barrier portions32; the barrier lines42therefore, can define the width of the land portion34. The bottom of the land portion34is bounded by the microstructure/substrate interface41and the top of the land portion34is bounded by the land line44, which is a horizontal line that runs along the land surface54. The land line44deviates from the land surface54when the surface61curves away from the land portion34.

In one example of a plasma display panel the land portions have a thickness in the range of 8 to 25 μm, as measured from the microstructure/substrate interface41to the land surface54. The width of the land portion, for example, is in the range of 100 to 400 μm, measured as the distance between barrier lines42of adjacent barrier portions32. Since a part of the material of the land portion34forms a dielectric layer above the back electrode23, in some instances it is desirable to keep the thickness of this layer constant above at least a portion of the width of the back electrode23. For example, the thickness is constant over at least 75%, 85%, 95%, or, preferably, over 100% of the electrode.

In one embodiment of the invention, as illustrated inFIG. 6, the surface61is substantially continuous from the curved surface56to the land surface54. The surface61may optionally include a surface discontinuity between the curved surface56and the barrier surface52. Therefore, the curved surface56may not be continuous with the barrier surface52. It is convenient to describe details of this embodiment by defining the curved surface56as originating from the barrier surface52and terminating on the land surface54. In one embodiment, the curved surface56preferably originates on the barrier line42closer to the microstructure/substrate interface41than the barrier portion top48. The curved surface56preferably terminates on the land line44closer to the barrier line42than to the back electrode23.

In another embodiment, as shown inFIG. 7, the surface61can be substantially continuous between the barrier surface52and the land surface54. The continuity along the surface61does not provide a surface discontinuity within the channel16. In one embodiment, the curved surface56preferably originates on the barrier line42closer to the microstructure/substrate interface41than the barrier portion top48. In one embodiment, the curved surface56preferably terminates at a point on the land line44that is closer to the barrier line42than to the back electrode23.

In another embodiment of the invention, as illustrated inFIG. 8, the curved surface56originates at the barrier top comer63and terminates horizontally on the land surface54. Since the curved surface56originates at the barrier top corner63the side of the barrier portion32generally has curvature. In one embodiment, the curved surface56preferably terminates at a point on the land line44that is closer to the side of the barrier line42than to the back electrode23.

In some instances, it is useful to define the surface61or the curved surface56by a radius of curvature R. The radius of curvature R and the curvature κ, are inversely proportional to each other and can be represented by the equation:
R=1/κ
As the radius of curvature R increases, the curvature κ, decreases. The radius of curvature R for a curved surface can be described relative to other dimensions of the microstructure25, for example, the barrier portion height hBP, the barrier portion width wBP, or the land portion thickness hLP.

In one embodiment of the invention, the curved surface56of the microstructure25has a single radius of curvature. This indicates that the curvature κ does not change at any point along the curved surface56. The shape of the curved surface56can be identical to the shape of an arc of a circle, wherein the radius of the circle is equal to the radius of curvature R of the curved surface56. The radius of curvature R can be selected based on other dimensions of the microstructure25. For example, the radius of curvature R can be a fraction of the barrier portion height hBP. In a useful embodiment of the invention where the microstructure25has a curved surface56and the curved surface56is defined by a single radius of curvature R, the radius of curvature R is in the range of 5% to 80% of the barrier portion height hBP, in the range of 10% to 50% of the barrier portion height hBP, or in the range of 12% to 25% of the barrier portion height hBP.

In another embodiment of the invention, the curved surface56is defined by more than one radius of curvature. In one example of this embodiment, as illustrated inFIG. 6, two radii of curvature, R1and R2define the curved surface56where the land surface54meets the curved surface56and the curved surface56meets the barrier surface52, respectively. More than two radii of curvature can be used. In some embodiments, a curved surface56that includes more than one radius of curvature is substantially continuous (i.e., contains no surface discontinuities). For example, the curved surface includes radii of curvature that are between the values of R1and R2for individual points on the curved surface56. The change in the radii of curvature for points along the curved surface follows the function of the curved surface56. It is understood that variations in the radius of curvature can be used in combination with the any of the shapes of the curved surfaces56of the microstructures25as described for any of the embodiments depicted inFIGS. 6,7, and8.

Another aspect of the invention relates to reducing or preventing fractures by modifying the dimensions of the microstructures25. It has been discovered that reducing the width of the barrier portion also reduces or prevents stress-associated fractures that occur upon debinding and sintering the microstructure material. Therefore, in another embodiment, as illustrated inFIG. 9, the invention includes microstructures25with barrier portions32having a reduced barrier portion width wBP. In this embodiment the barrier portion width wBP, as measured at the microstructure/substrate interface41, is preferably in the range of 25–75 μm, more preferably in the range of 50–75 μm, and most preferably in the range of 65–75 μm. The barrier portion heights hBP are typically in the range of 100–170 μm.

In general, the methods and structures described herein can be used to form articles and devices having microstructures with reduced fracturing. For example, articles and devices can be formed with microstructures on a substrate where at least 99% of the microstructures, and preferably 100% of the microstructures, do not have cracks that have a depth equal to 25% or more of the land thickness as measured between the microstructure/substrate interface41and the land line44.

Following debinding and sintering, it is typically desirable that the tops48of the barrier portions are flat and substantially free of physical irregularities. This flatness promotes contact of the facing glass substrate51with the tops48of the barrier portions along their entire length. This complete contact also “seals” the channels15formed by the barrier portions32and prevents or substantially hinders gas species in adjacent channels15from escaping via gaps between the tops of the barrier portions and the facing glass substrate51.

During debinding and sintering, the ends of the barrier portions32of the microstructures25experience shrinkage and suffer from unequal stress release. As illustrated inFIG. 5, a lateral cross section shows deformations in the end of the barrier portion32of the microstructure25after debinding and sintering. Firing densifies microstructures25so that their profile shrinks from their green state profile45as indicated. As shown, the tops48of the barrier portions, between the barrier portions ends29, and over the majority of the length of the barrier portions32, maintain a flat surface according to the shape of the green state microstructure45. However, the barrier portions ends29do not shrink uniformly with the rest of the barrier portion32and a slight curling of the barrier portion end29occurs, resulting in a deformation37. The presence of the deformation37can create an area of lift on the tops48of the barrier portions near the barrier portion ends29. A deformation37can create multiple problems in the assembly and the functioning of the plasma display panel. First, during sealing and handling of a display, mechanical forces can cause the deformations37to break off. Barrier end pieces that have cracked off can be detrimental to PDP function and life. Second, as previously indicated, deformations37can prevent complete contact of the facing glass substrate51with the tops48of the barrier portions. In the absence of complete contact gaps can exist between the tops48of the barrier portions and the surface of the front substrate51. This can lead to cross talk between excited gas species in adjacent cells as well as differences in switching voltage during operation.

Therefore, it is desirable to shape the barrier ends of the green state microstructures45in such a way to prevent deformations from interfering with proper assembly or function of the PDP. As illustrated inFIG. 10, one embodiment of the invention provides green state microstructures45molded to have step-shaped ends47that, in particular, overcome the problems associated with deformations37which occur upon debinding and sintering the microstructures.

As illustrated inFIG. 10, the step-shaped ends47of the barrier portion have a first step58, a second step68, and a third step78. Preferably, the step-shaped ends47have at least two steps. Each step of the step-shaped end47has a step height hS, a step width wS, and a step angle67. Each step of the step-shaped ends47can, respectively, have a different step height hS, a different step width wS, and a different step angle67. Preferably, the step height hS of each step is at least 20 μm and, preferably, the step width wS is equal to or greater than the step height hS. The step angle is generally in the range of 90 to 175°, typically in the range of 90 to 145°, and can be in the range of 90 to 125° or 90 to 110°. The shape of the microstructure25after firing can mimic the shape of the green state microstructure45over the entire lateral cross-sectional profile of the barrier portion32, including the stepped-shaped end47. At a location on the stepped-shaped end47, usually on a step that is adjacent to the back substrate21, for example on the third step78, the stepped-shaped end47can display a deformation37which appears after debinding and sintering. However, in a step-shaped end47this deformation37is not likely to be detrimental in the assembly or the functioning of the PDP.

In another variation of this embodiment, as illustrated inFIG. 11, the step that is adjacent to the back substrate21, for example the third step78, is elongated. Preferably the ratio of the step height hS to the step width wS (hS:wS) for the step adjacent to the back substrate21, for example the third step78, is in the range of 1:1 to 1:10, more preferably in the range of 1:1.5 to 1:8, and most preferably in the range of 1:2 to 1:6.

In another embodiment of the invention, as illustrated inFIG. 12, the ends47of the barrier portions32taper from the barrier portion top48to the back substrate21surface. The tapered ends47of the barrier portions32can be of various shapes or geometries and provide a green structure shape47, that, when thermally processed, are structurally sound and do not form substantial deformations which rise above the barrier portion top48. Preferably, the tapered end angle57of the green state tapered end47is no more than 60° and no less than 15°.

As illustrated inFIG. 12, a suitable shape of a tapered end47of a barrier portion32in the green structure states includes a straight line that runs from the top of the green state microstructure45to the surface of the back substrate21. Following thermal processing, the microstructure shrinks from its green state shape45to the processed state25. However, due to the tapered end47, any change in shape at the end of the barrier portion32that occurs during thermal processing does not substantially affect the flatness of the barrier portion top48or the integrity of the end of the barrier portion.

Another embodiment is a process to prevent or reduce the likelihood or amount that the ends of the barrier portions32curl during debinding and sintering the microstructures25. As shown inFIG. 13a weight19is placed in contact with the top comer77of the barrier portion. Typically a weight will contact at least one top comer77of the barrier portion. Multiple weights can be present along each edge of the assembly that present barrier ends. In a preferred embodiment, a weight is placed along each assembly edge that presents barrier ends, and the weight contacts most or all of the top comers of the barrier portions.

Preferably, the pressure exerted by the weight on the top comer77of the barrier portion is sufficient to prevent the emergence of a deformation37(for example, as seen inFIG. 5) during debinding and sintering. The pressure exerted by the weight during debinding and sintering can create an angled barrier end comer87. The pressure should not too great as to flatten the end of the barrier portion32to the surface of the back substrate21. Typically a sufficient pressure is preferably in the range of 0.0001 to 0.002 N (Newtons) per barrier end, more preferably in the range of 0.0001 to 0.001 N per barrier end, and most preferably in the range of 0.0002 to 0.0005 N per barrier end. The weight19can be of various shapes, for example a rectangular shape, a triangular shape, a trapezoidal shape, or a rhomboidal shape. Preferably, the bottom75of the weight19is flat, however a bottom75that is somewhat curved or angled can also be used provided that the weight bottom75of the weight19distinctly contacts the top comer77of the barrier portion.

In one embodiment, as illustrated inFIG. 13, contact is made between the top comer77of the barrier portion and a point on the weight bottom75, and contact is also made between the outside bottom comer71and the back substrate21at a point on the surface of the back substrate21. However, the outside bottom comer71can alternatively be in contact with another surface, for example a surface of an object not associated with the assembly. Contact of the outside bottom comer71and the back substrate21at a point on the surface of the back substrate21can create a weight/substrate angle85. The weight/substrate angle85is generally between 0.5 to 2.5°, typically between 0.5 to 1°, and can be between 0.5 to 0.8°.

The weight19is typically composed of materials that can withstand temperatures reached during debinding and sintering of the ceramic material, for example, glass or metal. Preferably these materials do not bond to the or chemically react with the ceramic material during debinding and sintering. Examples of suitable materials include aluminum oxide, soda-lime glass, and zirconia. One preferred material is zirconia. Unprimed soda-lime glass does stick to rib formulation slightly during sintering. Alumina and zirconia did not. Zirconia is least reactive.

It will be recognized that other articles can also be formed using a substrate with the molded microstructures. For example, the molded microstructures can be used to form capillary channels for applications such as electrophoresis plates. In addition, the molded microstructures could be used for plasma or other applications that produce light.

EXAMPLES

Barrier ribs were formed on a substrate using a mold and a photocurable glass frit slurry. A glass frit slurry was prepared. The glass frit slurry formulation used in these examples included 80 parts by weight RFW030 glass powder (Asahi Glass Co., Tokyo, Japan) which contains lead borosilicate glass frit with refractory fillers such as TiO2and Al2O3. To the glass powder was added 8.034 parts by weight BisGMA (bisphenol-a diglycidyl ether dimethacrylate), available form Sartomer Company, Inc., Exton, Pa., and 4.326 parts by weight TEGDMA (triethylene glycol dimethacrylate), available from Kyoeisha Chemical Co., Ltd., Japan, to form the curable fugitive binder. As a diluent, 7 parts by weight of 1,3 butanediol (Aldrich Chemical Co., Milwaukee, Wis.) was used. In addition, 0.12 parts by weight POCAII (phosphate polyoxyalkyl polyol), available from 3M Company, St. Paul, Minn. (other phosphate polyoxyalkyl polyols can be used and are available from other manufacturers) was added as a dispersant, 0.16 parts by weight A174 Silane (Aldrich Chemical Co., Milwaukee, Wis.) was added as a silane coupling agent, and 0.16 parts by weight Irgacur™ 819 (Ciba Specialty Chemicals, Basel, Switzerland) was added as the cure initiator. In additional, 0.20 parts by weight BYK A555 from BYK Chemie USA, Wallingford, Conn. was added as a de-airing agent.

All liquid ingredients and the photo-initiator were combined in a stainless steel mixing container. The ingredients were blended using a cowles blade (VWR Scientific Products, West Chester, Pa.) driven by a pneumatic motor. With the mixing blade running, the solid ingredients were slowly added. After all the ingredients were incorporated, the mixture was blended for an additional 5 minutes. The slurry was transferred to a high-density polyethylene container charged with½ inch cylindrical high density aluminum oxide milling media. Milling was performed using a paint conditioner (Red Devil Model5100, Union, N.J.) for 30 minutes. The slurry was then drained from the ball mill. Finally, the slurry was milled using a 3-roll mill (Model 2.5×5 TRM, Charles Ross & Son Company, Haupauge, N.Y.) at 60° C.

A knife coater was used to coat the slurry on 2.3 mm thick soda-lime glass substrates (Libbey Owen Ford Glass Co., Charleston, W. Va.). The knife gap was set at 75 micrometers for all of the samples.

After coating, a mold having barrier rib features was laminated onto the coated substrate. Lamination pressure was nominally 0.68 kg/cm and lamination speed was nominally 3 cm/sec. The molds used were polycarbonate or photo-curable acrylate material which was cast and cured onto a high stiffness backing material such as 125 μm thick PET (E. I. Du Pont De Nemours and Company, Wilmington, Del.). The mold was produced by casting and curing of an acrylate resin against a metal tool. Molds having different types of barrier rib microstructures were evaluated.

After molding, the coated substrate was exposed to a blue light source to harden the glass frit slurry. Curing was performed using a blue light source at 1.5 inch (about 3.8 cm) sample surface. The light source is constructed from 10 super-actinic fluorescent lamps (Model TLDK 30W/03, Philips Electronics N.V., Einhoven, Netherlands) spaced at 2 inches (about 5.1 cm) apart. These superactinic lamps provide light in a wavelength range of about 400 to 500 nm. Curing time was typically 30 seconds.

The mold was removed and the samples were sintered in air according to the following thermal cycle: 3° C./min to 300° C., 5° C./min to 560° C., soak for 20 minutes, and cooled at 2–3° C./min to ambient.

During sintering, the barrier ribs were constrained to the rigid glass substrate. Due to this constraint, in-plane stresses were developed as barrier ribs densified and shrunk during sintering. Furthermore, because of the large difference between the feature thickness between a barrier rib and adjacent continuous land regions, a large differential stress could develop during sintering. Hence, a sharp corner at the base of barrier ribs showed a high tendency to cracking during sintering. The result was no different by putting chamfer in this area. To alleviate this cracking, the transition from barrier rib to land was done in a relatively smooth manner. Mathematically, if one were to represent the transition from barrier rib side-wall to land as a continuous line, then the derivative of this function was preferably continuous to avoid developing large stress concentration. In Examples 4–8 and 10, barrier ribs having various rib base radii of curvature were tested. All produced cracked free parts. In the cases of Examples 3 and 9, the radius blends were not completely tangent to the land layer and cracks were observed.

Rib cracks were evaluated using light microscopy (through transmitted light) (Leitz DMRBE, Leica Mikroskopie & System GmbH, Wetzlar, Germany) and scanning electron microscope (AMPAX model 1920, Bedford, Mass.). All cracks were observed at the rib base. The following Table provides information on the products produced in each Example. All dimensions are for the green state prior to sintering. The draft angle refers to the angle of the barrier line relative to vertical.

Examples 11–14 were made in the same manner as Examples 1–10 except the coating gap was adjusted using metal feeler gauges. Barrier rib dimensions for these molds were 360 μm pitch, 213 μm high, 37 μm rib top width, 8° draft angle, and 50 μm smooth radius blend.

This indicates that the land thickness can be controlled by choice of the coating thickness.

Microstructured barrier ribs were formed on a substrate as described for Examples 1 to 10. During the debinding and sintering process, the barrier rib weighted to prevent deformations. Three different strips of material as weights: 1) 98% aluminum, 2) yttrium stabilized zirconia, and 3) glass. The alumina pieces were 102 cm×25.4 cm×0.060 cm, 6.0 grams, covering approximately 282 ribs at 360 um pitch. The glass pieces were 14.2×2×0.28 cm, 19.8 g covering approximately 394 ribs at 360 μm pitch. The zirconia pieces were 5.8×2×0.5 cm, 34.8 g, covering approximately 161 ribs. Different loads were imposed on the rib edges, as listed in the following Table. Rib heights were 202 um and 360 um pitch for all the samples. Rib number=length/pitch. Based on angle, weight, and width of weights, one can calculate rib load in Newton/rib.

In all cases, following debinding and sintering, the rib ends did not substantially lift during sintering. When the zirconia weights were used during the process, the rib ends were 10–20 micrometers shorter. After debinding and sintering the zirconia weight demonstrated the smallest amount of adhesion to the glass frit and the soda-lime glass demonstrated the greatest amount of adhesion. There was no residual glass frit observed on the zirconia weights after sintering. Small fragments of glass frit were bonded to the soda-lime glass strips after sintering.