Patent Application
20060139561
The present invention describes thick film photolithographic molds, methods of making thick film photolithographic molds, and methods of using thick film photolithographic molds to form spacers on a substrate. The thick film photolithographic molds preferably comprise an epoxy bisphenol A novolac resin. The present invention also describes sol gel spacers comprising sodium silicates and potassium silicates. The thick film photolithographic molds and sol gel spacers of the present invention can be used in flat panel displays, such as field emission displays and plasma displays.
Faceplate 104 includes a glass plate 120, a transparent conductor 122, and a phosphor layer 124. Transparent conductor 122 is disposed on one major surface of glass plate 120 and phosphor layer 124 is disposed on transparent conductor 122. The faceplate 104 and baseplate 102 are spaced apart from one another and are disposed so that the phosphor layer 124 is proximal to the grid layer 112.
FED 100 also includes a plurality of spacers 130 disposed between the faceplate 104 and baseplate 102. The spacers 130 maintain the orientation between baseplate 102 and faceplate 104 so that the baseplate and faceplate are substantially parallel to one another. Outer walls (not shown) seal the outer periphery of FED 100 and the space between baseplate 102 and faceplate 104 is substantially evacuated. Since the space between faceplate 104 and baseplate 102 is substantially evacuated, atmospheric pressure tends to press baseplate 102 and faceplate 104 together. However, spacers 130 resist this pressure and maintain the spacing between baseplate 102 and faceplate 104.
FED 100 also includes a power supply 140 for (1) charging the transparent conductor 122 to a highly positive voltage; (2) charging the grid layer 112 to a positive voltage; and (3) selectively charging selective ones of the row and column lines 118 to a negative voltage.
In operation, voltages applied to the row and column lines 118, the grid layer 112, and the transparent conductor 122 cause emitters 108 to emit electrons 150 that travel along path 117 towards, and impact on, phosphor layer 124. Incident electrons 150 on phosphor layer 124 cause phosphor layer 124 to emit photons and thereby generate a visible display on faceplate 104.
The visible display of FED 100 is normally arranged as a matrix of pixels. Each pixel in the display is typically associated with a group of emitters 108, with all the emitters 108 in a group being dedicated to controlling the brightness of their associated pixel. For example,
The top of each spacer 130 contacts a portion of faceplate 104. Electrons emitted by emitters 108 cannot impact phosphor layer 124 in the regions where the spacers 130 touch the phosphor layer 124. So, each spacer 130 creates a black, or dark, region of the display that cannot be illuminated. The presence of dark regions created by spacers 130 does not significantly degrade the quality of the display of FED 100 as long as the area of the dark regions is small compared with the area of the illuminated pixels. Four or more spacers 130 are normally disposed around the periphery of each pixel. It is, therefore, important for the cross-section of the spacers 130 to be relatively small compared with the area of each pixel. Ideally, the spacers 130 are characterized by a relatively high aspect ratio (i.e., the spacer's height is larger than its width). Such a high aspect ratio (1) does not create dark regions large enough to degrade the display quality and (2) provides sufficient spacing between the baseplate 102 and faceplate 104 to permit electrons traveling from emitters 108 towards faceplate 102 to acquire sufficient energy to cause phosphor layer 124 to emit photons. The spacers 130 must also provide sufficient structural strength to resist the atmospheric pressure and thereby maintain the desired spacing between baseplate 102 and faceplate 104. It is also desirable for all spacers 130 to have exactly the same height so they can provide uniform spacing between the baseplate 102 and the faceplate 104. It is also important for the spacers to be properly aligned with respect to the array of pixels so the dark regions created where the spacers 130 contact the faceplate do not interfere with the display (e.g., it is desirable for the bottom of the spacers 130 to contact the grid layer 112 at selected locations that are between the apertures 116 and are equidistant from all the adjacent emitters). Since the spacers 130 are disposed within a vacuum, it is also important for the spacers to be formed from a vacuum-compatible material (e.g., a material that does not outgas significantly).
Prior art spacers 130 are typically constructed using glass posts. After the posts are prepared, they are bonded to the grid layer 112. Following this bonding, the faceplate 104 is fitted onto the posts. Functioning FEDs may be constructed using these techniques, however, these techniques have several disadvantages. For example, when the spacers are fabricated from glass posts, it is difficult to ensure that every spacer has precisely the same height. Variation in spacer height degrades the parallel alignment between the faceplate and baseplate and thereby degrades the quality of the FED. Another problem with prior art spacer manufacturing techniques is that they do not permit precise alignment of the spacers with respect to the pixel array. As stated above, any deviation from the desired alignment can cause the dark regions created where the spacers contact the faceplate 104 to degrade the quality of the display. Ideally, the bottom of each spacer 130 contacts the grid layer 112 at a point that is equidistant from all the adjacent emitters, however, prior art spacer manufacturing techniques make it difficult to achieve this ideal. It would, therefore, be desirable to develop a new technique for fabricating spacers 130 for use in FEDs that provides improved control in manufacturability.
The use of porous, low-density xerogel materials for forming spacers in field emission flat panel displays and vacuum microelectronics is described in U.S. Pat. No. 5,658,832. In the methods described therein, a mold is placed on a substrate, such as the baseplate or faceplate, in a field emission flat panel display. A mold release agent, such as glycerol, silicone or wax, is applied to the mold, and then a liquid xerogel precursor is placed in the mold. After the xerogel hardens, the mold is removed, such that a xerogel spacer is formed on the baseplate or faceplate. There are several problems associated with this process. First, the positive mold was created by using a saw to remove material from a four inch by four inch square substrate and thereby form an array of posts. Molds larger than four inches by four inches are generated by aligning an array of four inch by four inch tiles. These methods prevent the posts from being accurately positioned. Second, the posts formed by the techniques disclosed therein tend to be contaminated by air bubbles. That is, air bubbles tend to form near the base of the posts thereby weakening the posts as well as weakening the attachment between the posts and the substrate.
There is a need in the art for new and improved materials for spacers for flat panel displays and for new and improved methods for forming the spacers on the faceplates and baseplates in flat panel displays. The present invention is directed to these, as well as other, important ends.
The present invention comprises methods of making and using molds for forming spacers for flat panel displays. Preferably, photolithography is used to make the molds. Molds are fabricated by depositing a thick film of photoresist, or photolithographic material, onto a support and by using photolithography to pattern the thick film photoresist into a shape for a desired mold. Positive molds define an array of posts, each of the posts corresponding to one spacer of the display that will ultimately be formed by the molding process. Negative molds define a plurality of apertures, each aperture corresponding to one spacer of the display that will ultimately be formed by the molding process. The photoresist can be patterned to form either positive molds (with arrays of posts) or negative molds (defining arrays of apertures).
One preferred thick film photoresist for use in fabricating the molds comprises an epoxy bisphenol A novolac resin, such as EPON® Resin SU-8. Methods are disclosed below for depositing a thick film of an epoxy bisphenol A novolac resin onto a support (e.g., a glass panel), for patterning the epoxy bisphenol A novolac resin layer into a desired shape for a mold, for improving the ability of the epoxy bisphenol A novolac resin to adhere to the support, for making the cured epoxy bisphenol A novolac resin less brittle or more resilient, and for planarizing a thick layer of uncured epoxy bisphenol A novolac resin.
The fabricated molds may be used to form spacers on a display. The mold is filled with a spacer material, brought into contact with a substrate (e.g., a faceplate of an FED), and the spacer material is allowed to cure into hardened spacers. After curing, the mold and substrate are separated leaving cured spacers attached to the substrate. Thereafter, the substrate and spacers may be dried and fired to further harden or cure the spacers.
Preferred materials for use as the spacer material are also disclosed below. Preferably, the spacer material is a liquid or gel that can be used to fill the apertures in a mold and that then cures into a hardened spacer. Preferred spacer materials comprise sodium silicate and potassium silicate. Other preferred materials comprise compositions comprising formamide, potassium silicate and sodium silicate.
Primary positive molds (fabricated using photolithography) may be used to form secondary negative molds. Such a secondary negative mold is preferably made by depositing a resilient material, such as latex, silicone, or plastic, over the primary positive mold so that the material conforms to the positive mold, allowing the material to cure, and then separating the cured resilient material from the positive mold. The resulting secondary negative mold can then be filled with a spacer material and placed in contact with a substrate so as to form spacers on the substrate.
Alignment of the mold and the substrate can be achieved by including an alignment template in the mold. One preferred method of achieving alignment is to provide some alignment marks near the border of a primary positive mold. A thin sheet of perforated metal (e.g., spring steel) that is flexible and resistant to permanent bending is then fabricated that replicates the alignment marks of the positive mold. The metal sheet is then positioned so that the alignment marks of the metal sheet are aligned with the alignment marks of the positive mold. Resilient material used for making the negative mold is then poured over the positive mold so that the resilient material encases the perforated portion of the metal sheet and so that the portion of the metal sheet including the alignment marks protrudes through the exterior of the resilient material. After curing, the resilient material is permanently bonded to the metal sheet. Thereafter, when the negative mold is used to apply spacers onto a substrate, the alignment marks of the metal sheet may be aligned with alignment marks on the substrate.
The molds and spacer materials constructed according to the invention are suitable for forming spacers on large flat panel displays. For example, spacers may be formed on displays with rectangular active areas measuring at least eight inches by ten inches.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description wherein several embodiments are shown and described, simply by way of illustration of the best mode of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims.
For a more complete understanding of the nature and objects of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings in which the same reference numerals are used to indicate the same or similar parts wherein:
FIGS. 13A-B show that mixing propanediol (
In the present invention, molds are used to form spacers in flat panel displays. Thick film photolithographic materials are used to make the molds. A thick film photolithographic material, or photoresist, is a material that can be used to form features that are at least about 400 to about 500 μm thick using standard photolithographic techniques. Preferably, the thick film photolithographic material is an epoxy bisphenol A novolac resin. Epoxy bisphenol A novolac resins include, for example, EPON® Resin SU-2.5 (Shell Chemical), EPON® Resin SU-3 (Shell Chemical), and EPON® Resin SU-8 (Shell Chemical), and the like. Preferably, the epoxy bisphenol A novolac resin is EPON® Resin SU-8 (hereafter “SU-8”). SU-8 has the structure shown in
After the molds have been formed, the molds are filled with a spacer material and then the spacer material is allowed to cure onto a portion of an FED so as to form spacers for the FED. Preferred compositions of the spacer material will be discussed below. Generally, the spacer material is preferably a liquid or gel (1) that may be used to fill apertures in the molds and (2) that hardens or cures to a relatively rigid material.
Prior to discussing certain aspects of the present invention in detail, the use of molds for forming spacers according to the invention will be briefly reviewed.
As shown in
Negative mold 304 may then be used for forming spacers on an FED as illustrated in
It will be appreciated that the negative mold 304 is reusable and may accordingly be used to form spacers on many substrates. Similarly, the positive mold 302 is also reusable and may be used to form many negative molds.
As discussed above, the posts 320 of positive mold 302 are preferably made by patterning a layer of thick film photoresist material. Instead of patterning thick film photoresist material to form posts, it is also possible to pattern a layer of thick film photoresist material to form apertures and thereby use photoresist material to form a negative mold.
Although negative molds (such as mold 308) made using photolithography are useful, it is currently preferred to use secondary negative molds (such as mold 304) that are made by depositing a material over a positive mold (such as mold 302) that conforms to the positive mold and then cures to a resilient material. This is because negative molds made using photolithography (such as mold 308) tend to be rigid or brittle and it is more convenient to use a resilient mold, such as mold 304.
Now that a brief background on the use of molds for forming spacers in FEDs has been provided, the spacer material that is used to fill the apertures in the molds and that cures or hardens into the spacers for an FED will be discussed. While any spacer material known in the art can be used in the embodiments of the present invention described herein, the spacer material is preferably the novel sol gel of the present invention. A sol gel is a high-density, non-porous structure usually composed of silicates. “High-density” preferably means that the sol gel is about 80 to about 90% SiO2. The common starting materials for forming the sol gels of the present invention are silicates in an aqueous or alcohol solution. The basic starting material is preferably an alkali silicate solution. Alkali silicate solutions include, for example, Kasil 2130 (PQ Corp.) or N-silicate (PQ Corp.).
Silicate solutions are clear and slightly viscous with a pH typically over 11. Formamide (HCONH2), acting as a weak acid and an initiator, converts the alkali hydroxide into a formate while lowering the overall pH of the solution. At a certain critical pH, the silica present as hydrated micelles several nanometers in diameter begin to contact each other and react. Hydroxyl groups on the surface of the micelles are eliminated via hydrolysis, leaving a network of three-dimensionally connected chains of silica. Using formamide as an initiator is described by Ulrich, Chemtech, pages 242-249 (April 1988). Besides formamide, other initiators known in the art, such as propylene carbonate, can be used.
The three-dimensional silica network along with the water in the original solution comprise a gel. Depending on the degree of connectivity, the gel can be soft with long strands of polymer or hard with short, frequently branched strands of polymer. The more frequently connected the polymer strands, the more water is eliminated from the bulk of the gel.
When a large amount of formamide is used (e.g., more than one part formamide to one part silica), gelation begins immediately upon mixing and within minutes, a soft gel is formed. A short time later, the gel shrinks, turns cloudy and eliminates water from the bulk. Within one-half hour after mixing, the gel is a hard white solid. Most of the original liquid in the silicate solution is eliminated from the interior volume of the gel.
When a slightly smaller amount of formamide is used (e.g., less than about one part formamide to one part silica), it leads to a soft gel that eliminates a minor amount of water and dries without cracking. In formamide:silica ratios of less than 1:1 to 1:10, a gel is formed that is soft, clear and only slightly moist to touch. These gels may be air dried, then heat treated to over 500° C. without problems. Weight loss experiments indicate nearly all detectable water is driven off, the unreacted formamide is decomposed as NH4 and CO2, and the alkali formate byproducts are converted to oxalates.
One part formamide in 20 or more parts Kasil lead to gels which require several days to cure at room temperature. Clear, soft and dry to touch, these gels collapse when dried and release all detectable water when baked from 480 to 500° C.
The best formulation for preparing Kasil-based sol gels is ¼ to 1 part formamide to 1 part silica, preferably about ¼ part formamide to 1 part silica. The gelation time is several hours at room temperature. During gelation, the silica network pulls together and shrinks away from the confines of the mold, eliminating water. As a supernatant liquid, this water lubricates the void between the gel surface and the mold allowing easy separation of the two parts.
When propylene carbonate is used as the initiator, it works about ten times faster than formamide. The gels of the present invention can be cast in about six minutes at room temperature when using about 12 grams of silicate to about 1 to 1½ grams of propylene carbonate.
In any of the embodiments of the present invention, drying of the spacers is preferably accomplished in two stages, an air dry which removes a large portion of water, and a firing that may ultimately reach 500° C. A preferred drying rate is described in Example 3.
Drying the gel in air leads to a collapse of the hollow pores that were filled with and supported by water. This volume loss can be related to the degree of connectivity. Simple drying at ambient temperatures will not, in general, make a solid compact body.
The sol gel is normally fired to create a dense structure. Firing at a temperature of 150 to 200° C. is required to decompose any unreacted formamide and loss experiments carried out on the reactants indicate an oxalate reaction occurs.
While ceramic bodies composed of discrete micron-sized particles require firing at temperatures where the viscosity is about 8 to 8.8 to achieve a dense non-porous structure, sol gels begin to compact and densify at a viscosity of about 11. The densification of gels is described in Ultrastructure Processing of Ceramics, Glasses, and Composites, Hench and Ulrich, Eds., Wiley-Interscience Publication, pages 76-87, the disclosure of which is incorporated by reference herein in its entirety.
A formulation of a sol gel containing 6 parts N-Silicate (sodium silicate) to 10 parts Kasil (potassium silicate) to 0.5 parts Formamide began to densify at 520° C. Larger proportions of sodium silicate will lower the densification temperature.
Densification temperatures of compositions containing varying amounts of Na2O, K2O and SiO2 are shown in Table 1.
The densified material formed two phases: one phase was a glassy material containing mostly silica, potassium and sodium, and the other phase was in the form of long needle-like crystals containing sodium and oxygen, as measured by LDS. In cross-sections of samples run at 480° C. and 500° C., the crystals were present in the pores of the dried gel. In the 520° C. run, the crystals were incorporated into the silica-rich matrix. At 540° C., the sol gel becomes glassy, as evidenced by brittle fracture lines when the bulk gel was split for a cross-sectional view.
Differential scanning calorimetry revealed two peaks in sodium-potassium silicate glasses; the first peak occurred for all compositions at a potassium:sodium silica ratio of about 10:6 to about 10:10 at 450° C. A second peak roughly 20° C. higher shifted lower in temperature as the sodium content was raised. The first peak is narrow and fairly deep indicating some sort of crystallization. It is near the predicated sintering/compaction temperature. Water in the glass precursor is responsible for dropping the sinter temperature to the 450° C. region; normally, for a dry glass of this composition, the sinter temperature is 100° C. higher. The reaction should be irreversible, leading to a glass that sinters and compacts at one temperature then becomes stable afterwards to a higher temperature. This property is engineerable, based on the water content of the sol gel and the degree of ripening.
The spacers can have their surface resistivity adjusted by including a semiconductive material in or on the spacer. Such semiconductive materials include, for example, tin oxide, lead oxide, zinc oxide and/or titanium oxide.
A major difficulty in filling the apertures of the molds of the present invention with the sol gels of the present invention is the CO2 reaction that occurs upon exposure of the silicates in the sol gels to air. CO2 diffuses into the surface where it forms a weak acid, in turn reacting with the alkaline solution. This initiates a hydrolysis reaction that forms a skin over the top of the solution with a film of silica. The silica prevents the liquid from wetting the surface. One way to prevent the skin formation is to perform the mold filling operations described herein in an inert gas, such as argon.
Preferably, each spacer in the present invention can hold about 0.05 pounds or more of pressure, more preferably about 0.07 pounds or more of pressure. Accordingly, about 422 spacers per square inch are preferably used in a flat panel display. Since there are generally about 10,000 spacer bonding sites per square inch in a flat panel display, the yield (i.e., the number of spacers that bond to the substrate) does not have to be particularly high in order to achieve good results.
The spacers and molds described herein can be used in flat panel displays in field emission displays or plasma displays. In field emission display applications, the spacers are generally about 350 to 700 μm tall. In plasma display applications, the spacers are generally about 80 to 100 μm tall.
Now that the spacer material has been discussed, the use of thick film photoresist (e.g., an epoxy bisphenol A novolac resin) for forming the molds will be discussed. An initial step in the formation of such a mold (e.g., primary positive mold 302 as shown in
A novel method for forming layer of photoresist material 404 is by powder coating. In one example of powder coating, dry SU-8 powder is applied to the support 402 and baked at temperatures required for reflow. Specifically, solid SU-8 is mixed with a solvent, preferably a general organic solvent, such as ethyl lactate, and a photoinitiator. Any photoinitiator known in the art can be used, such as, for example, Cyracure 6974 photoinitiator, Cyracure 6990 photoinitiator, and the like. The components are thoroughly mixed until the dry SU-8 powder dissolves. Thereafter, the solvent is removed by heating and the SU-8 powder is finely ground. Conventional powder coating techniques are then used to apply a uniform thickness of dry powder to the support. When heated to about 120 to 130° C., the SU-8 powder melts, reflows, and planarizes during reflow due to its high surface tension to form, for example, a layer such as layer of photoresist material 404 shown in
Meniscus coating can also be used to form a layer of photoresist material 404 of thick film photoresist material that is about 400 to 450 μm thick. Preferably, the thick film photoresist material is an epoxy bisphenol A novolac resin, more preferably SU-8. This is done by using a meniscus coater to form a series of about 80 to 200 μm thick coatings, one on top of another. Preferably, after each meniscus coating is deposited, the support and coatings are baked (e.g., at about 95° C. for about 20 minutes, then at about 110° C. for about 25 minutes) before the next meniscus coating is deposited. This method removes all of the solvents from the thick film photoresist material. The thickness increment is limited by the solids content of the thick film photoresist material, which is, in turn, limited by the specific pumping equipment of the meniscus coater.
Exemplary methods for forming layer of photoresist material 404 are described in more detail herein. A practical composition comprising SU-8 was described in several papers (e.g., Lorenz et al., “EPON SU-8: A Low Cost Negative Resist For MEMS,” Sensors & Activators, A64:33-39 (1998)), the disclosure of which is incorporated by reference herein in its entirety), although a more preferred composition is described herein. Preferably, the composition comprises about 200 to 600 parts by weight solvent (e.g., ethyl lactate thinner), about 200 to 600 parts by weight SU-8, and about 1 to 100 parts by weight photoinitiator (e.g., Cyracure 6974 or 6990 photoinitiator). More preferably, the composition comprises about 300 to 500 parts by weight solvent (e.g., ethyl lactate thinner), about 300 to 500 parts by weight SU-8, and about 10 to 70 parts by weight photoinitiator (e.g., Cyracure 6974 or 6990 photoinitiator). Most preferably, the composition comprises about 400 parts by weight solvent (e.g., ethyl lactate thinner), about 400 parts by weight SU-8, and about 40 parts by weight photoinitiator (e.g., Cyracure 6974 or 6990 photoinitiator).
After the solvent and photoinitiator are mixed together, SU-8 is added. After all the SU-8 dissolves, ultrasonic agitation can be used to de-gas the material. A meniscus coater is then preferably used to successively deposit several layers of the material over a support 402 to form a thick layer 404 of photolithographic material. The support speed in the meniscus coater can be about 0.1 to 1.0 cm/second, preferably about 0.55 cm/second. Any soft-bake temperatures and time that slowly drive off the solvent can be used, such as, for example, about 95° C. for 30 minutes followed by 130° C. for 30 minutes.
After layer of photoresist material 404 has been formed (e.g., by meniscus coating, powder coating, or spin coating), the upper or “contact” surface 406 of layer of photoresist material 404 is sometimes “wavy” or non-planar. As shown in
Since developed SU-8 is brittle, when negative mold 308 is used to apply spacers to a substrate (as discussed above in connection with
One problem with thick film photoresists is that they are sometimes characterized by poor adhesion to a support. It can, therefore, be advantageous to prime the support 402 prior to forming the thick film photoresist layer 404 to improve adhesion between the support 402 and the thick film photoresist layer 404. One reason for desiring good adhesion between the photoresist layer and the support can be illustrated with reference to
Conventional primers that have been used to improve adhesion of photoresist materials include silanes, such as HMDS, which have silanol groups at one end and organic methyl groups at the other. These priming materials generally work for novolac resins, but are generally ineffective for SU-8 because SU-8 is an epoxy, and the methyl groups do not participate in bonding. As described below in Example 1, making the support 402 as polar as possible and terminating it in hydroxyl groups or amine groups provides the best possible surface for wetting and adhesion to the photolithographic material. For example, triethanol amine and/or ethylene glycol can be applied to the upper surface of support 402 as a priming agent before the photolithographic material is deposited onto the upper surface of the support 402. Adhesion of the layer of photoresist material 404 to the support 402 can be greatly improved by, for example, mixing triethanol amine as a 1% solution in IPA, coating the substrate with the solution for three minutes, and then rinsing off the solution.
Alternatively, a silane bonding agent can be used to improve adhesion between the support 402 and the thick film photoresist layer 404. A silane bonding agent preferably comprises silane, cyclohexane and an epoxy group, such as 2(3,4-epoxycyclohexyl)ethyl-trimethoxysilane (manufactured by Gelest). This material, used as a primer from a deionized water/alcohol bath at a pH of 4.5-5.5, forms a monomolecular film that hydrolyzes to the support 402 via the silane groups and permanently bonds to the layer of photoresist material 404 with a ring opening polymerization reaction. The general reaction scheme for bonding a silane to a glass surface is shown in
After a thick film layer of photoresist has been formed over a support, conventional photolithographic processing (e.g., exposure followed by developing or etching) can be used according to the invention to pattern the layer 404 of photoresist material and thereby convert layer of photoresist material 404 into a mold (e.g., such as positive mold 302 as shown in
PGMEA (propylene glycol monomethyl ether acetate) can be used to develop the exposed photoresist material. For example, two baths of PGMEA can be used, where the first bath is used to remove all of the undeveloped SU-8, and the second bath is used as a rinse to remove the SU-8-loaded developer from the first bath. A final rinse in IPA can then be performed. Going from PGMEA developer to IPA without the intermediate rinse in clean PGMEA results in scumming, as the dissolved resist coagulates in the incompatible IPA solvent.
A post-development firing may also be performed to increase adhesion and dry out any remaining developer or solvent (e.g., about 85 to 95° C. for about 10 to 20 minutes).
It has been discovered that exposed and developed SU-8 is a hard and brittle plastic that cracks under stress and is not particularly compliant. The brittle nature of the cured SU-8 can impose restrictions on the flatness of an SU-8 mold's contact surface. For example, if SU-8 is used as the photolithographic layer 309 in negative mold 308 (as shown in
Accordingly, it is desired to modify the properties of cured SU-8 so that the mold can better contact the substrate. As shown in Example 2, it has been unexpectedly discovered that cross-linking agents, such as polyols, can be used to successfully modify the properties of cured SU-8. For example, polyols added to uncured, undeveloped SU-8 increase flexibility, photospeed, and adhesive strength. The polyol is preferably a triol or a diol, more preferably a diol. Preferred diols include propanediol, hexanediol, ethylene glycol and the like. The base ratio of SU-8 terminal groups to diol terminal groups is preferably 1.5 to 1. The quantity of the diol can be increased or decreased, depending on the desired properties of the resin.
Without intending to be bound by any theory of the invention, the property of the diols that make them an effective additive is the increase in cross-linking speed that occurs with their use. For best adhesion, the post-exposure bake (PEB) temperature of the SU-8 resist was about 115° C. to 125° C. This exceeds the melting point of the unexposed SU-8 resin, which is 83° C. This creates a problem in properly baking microstructures that do not bend or otherwise deform under the stress of the unexposed resist expanding and melting. The diols rapidly cross-link the exposed resist during the initial part of the bake, making it strong enough to prevent warping under stress. The increase in viscosity during PEB also affects the resolution of the resist. A fast rise in viscosity prevents diffusion of the acid groups from the exposed to the unexposed regions, which can lead to blurring. This is seen in developing small holes and bars. With base resin, the diffusion of the acid generator into the unexposed regions creates a viscous layer around the features which does not develop away. This leads to stringers, at best, and filled holes and gaps, at worst. The polyol, such as propanediol and hexanediol, appears to have rapidly allowed the SU-8 to polymerize. This would have the effect of blocking diffusion quickly so that blurring does not occur.
It has also been discovered that developed SU-8 can be further treated so that it reacts better with the spacer material that is used to fill apertures in the molds (e.g., apertures 326 in cured photoresist layer 309 of negative mold 308 as shown in
Preferred spacer materials for use with the invention and preferred methods of treating photoresist according to the invention have been discussed above. Additional factors concerning how to form spacers with molds according to the invention will now be discussed.
Molds, such as mold 740, can be formed by applying and imaging successive layers of photoresist (e.g., SU-8). For example, a positive mold for forming negative mold 740 may be constructed by applying, imaging, developing, and curing a first layer of photoresist material to define a plurality of shallow trenches. These trenches in the positive mold will later correspond to the lips 744 in the negative mold. Then, a second layer of photoresist may be applied over the first layer and then imaged, developed, and cured to define a plurality of posts, each of the posts being surrounded by the previously formed trenches. The posts correspond to the apertures 742 in mold 740. Such a positive mold may then be used to form negative molds, such as mold 740.
As an example of the above methods used in development work, liquid spacer material (e.g., silicate solution) was filled into all the apertures of a negative mold constructed according to the invention such that an excess of the spacer material protruded from the mold apertures to a depth of 1/16 to 3/32 of an inch. The wet mold was placed in a vacuum chamber and a moderate vacuum was pulled on it, whereby the air bubbles were removed from the mold volume and, at the same time, the solution was degassed. The vacuum prevents the above-discussed “skinning over” effect. After the solution was fully degassed and the holes filled, the spacer material-filled mold was removed from the vacuum chamber and excess spacer material was squeezed off. An excess of spacer material (e.g., silicate solution) is preferably used to prevent the skin that forms on the spacer material (e.g., silicate) during vacuum exposure from covering the mold openings and trapping the air bubbles. Degassing the spacer material solution has the unexpected effect of delaying the gelation time. This is mainly due to the loss of CO2 incorporated in the solution during mixing.
In a second implementation, the mold was placed in a vacuum chamber in a vertical position (i.e., in a position that is rotated clockwise ninety degrees from the orientation shown in
Referring again to
One preferred method for using secondary molds, such as mold 304, for producing spacers is as follows. First, the secondary mold is preferably washed to remove any release agents that were used to separate the primary positive and secondary negative molds. The secondary mold is then placed in a vacuum chamber and the apertures of the mold are filled with a spacer material while the mold is under a vacuum. After filling, the surface of the mold is cleaned with a scalpel to remove any excess spacer material from the mold. The mold is then placed in contact with a substrate (e.g., a faceplate of an FED) with the apertures facing down and the substrate below the mold as depicted in
Another variation of this technique is to use a relatively thin ledge (e.g., several millimeters thick) to separate the substrate and a negative mold that has been filled with spacer material. The ledge preferably extends around the outer periphery of the negative mold. When pressure is applied to the top of the mold, the pressure initially pushes the center of the mold into contact with the substrate, but the ledge prevents the outer periphery of the mold from contacting the substrate. As the pressure increases, more and more of the mold is pushed into contact with the substrate and the area of contact gradually extends from the center of the mold out towards the outer periphery. The ledge is preferably placed far enough away from the active area of the mold (i.e., the portion of the mold that includes apertures that have been filled with spacer material) so that the entire active area may be brought into contact with the substrate. The mold is then held in place until the spacer material cures. This technique minimizes the amount of air that is sometimes trapped at the interface of the mold and the substrate and thereby facilitates placing the spacer material into good contact with the substrate.
One problem that has been encountered in the use of negative molds, such as mold 304, is that cured spacers may break off of the substrate when the mold and substrate are separated. If the mold is hard or brittle, then the mold fails to deform around the spacers as the mold is pulled away from the substrate and tends to break the spacers off. Conversely, if the mold is soft enough to deform around the spacers, then the mold tends to stretch while it is being separated from the substrate and this stretching also tends to break off the spacers. One solution to this problem is to construct a negative mold by bonding two layers of rubber together, each layer being characterized by a different durometer.
Molds, such as positive mold 902, can be formed by applying and imaging successive layers of photoresist (e.g., an epoxy bisphenol A novolac resin, such as SU-8). In one preferred method, a layer of SU-8 that is fifty microns thick is deposited over the positive mold 902 and developed to establish a base comprising support 910. Then, a second layer of SU-8 is deposited over the first layer and the second layer is then imaged to form the row and column lines 922. Then, a third, thick layer of SU-8 is deposited over the second layer and the third layer is then imaged to form the posts 920. In general, multiple layers of photoresist can be successively applied and imaged to generate complex shapes, such as that of positive mold 902.
One problem with negative molds, such as mold 304 (
In operation, after the clamp ring has been applied to hold the mold onto the substrate, the vacuum pump-out channels 972 can be used to remove any air that has been trapped between the mold and the substrate. For example, the entire assembly including substrate 306, negative mold 960, and clamp ring 980 may be placed in a vacuum chamber. Evacuating the chamber evacuates all the gas from the environment surrounding the assembly, and the vacuum in the surrounding environment causes any gas trapped between the mold and substrate to be removed via the vacuum pump-out channels. Thereafter, the spacer material can be injected into the top of injection port 986. The injected spacer material travels down through the mold towards the vacuum pump-out channels 972 (i.e., towards the interface of the substrate and the mold) and suction from the vacuum pump-out channels causes the spacer material to spread across the entire mold and to fill all the apertures of the mold. After all apertures have been filled, the spacer material is then allowed to cure. After the spacer material has cured, the negative mold 960 and substrate 306 are separated leaving the cured spacers attached to substrate 306. Air can be injected into the vacuum pump-out channels to facilitate separation of the mold and substrate. During the molding process, the substrate and mold are normally supported by some kind of surface or table. It may also be useful to use a flange to push the outer perimeter of the mold down onto the table during the molding process.
Negative mold 950 may be used in a similar fashion as negative mold 960. However, rather than injecting the spacer material into the mold, the spacer material is filled into the mold apertures through the top of the mold (i.e., at the top of the through-hole) and a doctor blade can be run over the top of the mold to remove excess spacer material. Although negative mold 950 is useful, negative mold 960 is currently preferred because separation of the mold and substrate tends to break off more spacers when negative mold 950 is used.
When using a mold to form spacers on a display, it is very desirable to be able to accurately align the mold with the substrate so that the spacers are located at desirable positions within the display. One method according to the invention for improving the alignment of the mold with respect to the substrate of the display shall now be described. Initially, a primary positive mold (e.g., such as mold 302 of
The mold 1002 is then used to prepare a secondary negative mold. The first step in preparing the secondary negative mold is to prepare a sheet of perforated metal (e.g., spring steel that is 0.001 inches thick) that has alignment apertures that match the marks of positive mold 1002.
After the mold 1050 is separated from positive mold 1002, the sheet 1030 comprises an alignment template for the mold 1050. The sheet 1030 is flexible and permits the mold to flex or bend, but the sheet 1030 will not stretch or otherwise permanently deform. Mold 1050 may then be used with substrates that include alignment marks matching the alignment apertures of the sheet 1030. The alignment apertures of the mold 1050 are aligned with the alignment marks of the substrate and this alignment is maintained as the substrate and mold are brought into contact. In this way, the accuracy of the photolithography process (used to create the positive mold 1002) is transferred to the substrate and the spacers are placed at desired locations on the substrate.
In use, the alignment apertures of the alignment sheet 1130 of mold 1102 are first aligned with the alignment marks of a substrate. Then, the mold 1102 and substrate are brought into contact while maintaining alignment of the alignment marks of the substrate and the alignment apertures of the sheet 1130. Then, a clamping ring (similar to the clamp ring 980 shown in
The following examples are for purposes of illustration only and are not intended to limit the scope of the invention or the claims.
A mold fabricated from SU-8 can be formed as described herein. Briefly, a support, such as soda lime glass, is cleaned and coated with a thin film of SU-8 resist (e.g., about 50 μm), which is then flood-exposed to completely cure the SU-8 resist. A thicker film of the SU-8 resist is then applied (e.g., 200 to 600 μm), exposed with the desired photopattern, baked, and developed in a PGMEA solvent. Negative resist allows the process of coating, exposing and developing to be repeated, such that a complex 3D shape can be formed.
As described herein, adhesion of the SU-8 resist to a glass support can be poor, such that delamination occurs when the SU-8 resist exceeds a thickness of 200 μm. A search for possible priming methods was carried out in three broad categories: (1) cleaning processes that leave the surface hydrophilic; (2) cleaning processes that leave the surface hydrophobic; and (3) depositing several monolayers of molecules to promote adhesion. Table 2 shows the wetting behavior of the support for different surface treatments. Treatments that left the surface hydrophilic promoted wetting of the SU-8 resist to the substrate. On the other hand, adhesion promoters, such as HMDS, surfasil, FRSiCl3 and octadecyltrichlorosilane, failed to promote adhesion between the glass support and the SU-8 resist.
Taking the surface conditions into account along with the solubility of the monomer in polar organic solvents, it was decided that a better adhesion promoter would be terminated in hydroxyl groups. Accordingly, triethanol amine was dissolved in methanol and glass supports were soaked for three to five minutes therein, then air dried. A two-level Toguchi test was performed with factor one being the presence of the priming agent and factor two being the temperature of the post-exposure bake. Temperature was the primary factor that effected adhesion with 56% probability, and the primer was the secondary factor that effected adhesion with 24% probability comparing 95° C. PEB with 115° C. PEB.
Surface treatments with triethanol amine significantly increased the adhesive strength. This was demonstrated by a major reduction in undercutting of small features during development. Environmental testing of the resist in the presence of water, however, showed the same failure rates as unprimed material.
Surface treatments with 2(3,4-epoxycyclohexyl)ethyltrimethoxysilane significantly increased the adhesive strength between the glass support and the SU-8 resist, as shown in the crack test graph in
Propanediol and hexanediol were added to an SU-8 resist to determine the effects on the properties of the SU-8. As shown in FIGS. 13A-B, the photospeed of the propanediol/SU-8 mix and the hexanediol/SU-8 mix was faster than the photospeed of the SU-8 alone, and the adhesive strength of the propanediol/SU-8 mix with the support and the hexanediol/SU-8 mix with the support was better than the adhesive strength of the SU-8 alone.
After the sol gel spacers are formed, they must be washed and dried, as described herein.
Washing the wet gels in deionized water can lead to swelling and cracking if care is not taken to balance the osmotic pressure between the solvent and the gel network. Sol gels will match the dilution of the ionic species it is immersed in by swelling or shrinking. Since the sol gels of the present invention contain a large amount of sodium and potassium, they will swell and rupture when placed in deionized water. When the sol gels of the present invention are washed in a solution of deionized water saturated with NaCl, they tend to shrink and expel water because the ion concentration gradient is higher outside the gel than inside. Thus, it has been found that a 1 to 10% solution of HCl in deionized water can effectively wash the sol gels of the present invention without swelling.
After washing, the gels must be dried. An increasing temperature cycle produces dense sol gel spacers that are well-bonded to the substrate.
First, a temperature of 110° C. is required to remove bulk water from the gel. This may be performed in air or a vacuum. Initially, a 40 to 70° C. pre-bake is performed to remove the bulk water by evaporation. This was shown to close off the pores and seal the interior of the gel. When heated to about 100° C., the gel would swell from steam build-up, creating bubbles that led to rupture. Going directly from the acid wash to 110° C. has been shown to be an effective means of drying. The time required for this step is about 24 hours.
The next ramp temperature is 120° C. There is a transition between 110° C. and 120° C. in the behavior of the drying process. Below 120° C., the bulk water is evaporated from the gel. At 120° C. and above, water adsorbed on the surface of the silica begins to evaporate. The time required for this step is about 4 hours.
At 150° C., the hydroxyl groups on the surface of the silica begin to condense in the reaction: —Si—OH+HO—Si—→Si—O—Si—+H2O. This process is responsible for turning the gel into an insoluble porous glass.
The process continues at 170 to 180° C., where the silanol groups quickly condense. At this stage, the mechanical properties of the glass improve as the bonds between the particles of silica in the network get stronger.
Organic burnout is accomplished at 270° C., followed by a ramp to 430 to 460° C. where the porous gel consolidates.
Each of the patents and publications cited herein is incorporated by reference herein in its entirety.
Although the invention has been set forth in detail and with reference to examples, one skilled in the art will appreciate that changes and modifications can be made to the invention without departing from the spirit and scope of the invention.
The data shown in Table 1 were taken from page 30 of the Handbook of Glass Data, Part C, Ternary Silicate Glasses, by Mazurin, Streltsina and Shvaiko-Shvaikovskaya.
This application is a divisional of application Ser. No. 10/978,323, filed Nov. 1, 2004, pending, which is a divisional of application Ser. No. 09/560,260, filed Apr. 26, 2000, now U.S. Pat. No. 6,812,990, issued Nov. 2, 2004.
The present invention was made with Government support under Contract No. DABT63-97-C-0001 awarded by the Advanced Research Projects Agency (ARPA). The Government may have certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10978323 | Nov 2004 | US |
| Child | 11356907 | Feb 2006 | US |
| Parent | 09560260 | Apr 2000 | US |
| Child | 10978323 | Nov 2004 | US |
