Method of fabricating crystalline island on substrate

Abstract

Certain electronic applications, such as OLED display back panels, require small islands of high-quality semiconductor material distributed over a large area. This area can exceed the areas of crystalline semiconductor wafers that can be fabricated using the traditional boule-based techniques. This specification provides a method of fabricating a crystalline island of an island material, the method comprising depositing particles of the island material abutting a substrate, heating the substrate and the particles of the island material to melt and fuse the particles to form a molten globule, and cooling the substrate and the molten globule to crystallize the molten globule, thereby securing the crystalline island of the island material to the substrate. The method can also be used to fabricate arrays of crystalline islands, distributed over a large area, potentially exceeding the areas of crystalline semiconductor wafers that can be fabricated using boule-based techniques.

Claims

1. A method of fabricating a crystalline island of an island material, the method comprising: depositing particles of the island material abutting a substrate; heating the substrate and the particles of the island material to melt and fuse the particles to form a molten disk; cooling the substrate and the molten disk to crystallize the molten disk, thereby securing the crystalline island of the island material to the substrate; and planarizing at least a portion of the crystalline island to expose a cross-section of the crystalline island.

2. A method of fabricating a crystalline island of an island material, the method comprising: depositing the island material on a substrate; heating the substrate and the island material, the heating melting the island material to form a first molten disk, the heating also forming a second molten disk comprising oxygen and the island material, the second molten disk disposed between the first molten disk and the substrate; cooling the substrate, the first molten disk, and the second molten disk to crystallize the first molten disk, thereby forming the crystalline island of the island material; and planarizing at least a portion of the crystalline island to expose a cross-section of the crystalline island.

3. The method of claim 2, further comprising, after the cooling: over-coating the crystalline island and the substrate with an over-coating layer to form a stack; and planarizing the stack to expose a cross-section of the crystalline island.

4. The method of claim 2, further comprising, before the depositing: forming an oxide layer on the substrate; and wherein: the depositing comprises depositing the island material on the oxide layer; and the second molten disk comprises the oxide layer in a molten state.

5. The method of claim 4, wherein the forming the oxide layer comprises depositing on the substrate the oxide layer comprising an oxide of the island material.

6. The method of claim 5, wherein the depositing comprises depositing the oxide layer according to a predetermined pattern.

7. The method of claim 4, wherein the forming the oxide layer comprises: depositing the island material on the substrate according to a predetermined pattern; and oxidizing the island material.

8. The method of claim 7, wherein the depositing the island material comprises one or more of: depositing particles of the island material; and depositing a layer of the island material.

9. The method of claim 4, wherein the heating comprises heating the substrate, the island material, and the oxide layer in a non-oxidizing atmosphere.

10. The method of claim 2, further comprising one or more of, before the depositing: polishing the substrate according to a predetermined pattern; and roughening the substrate according to the predetermined pattern.

11. The method of claim 2, wherein the depositing comprises depositing the island material on the substrate in a shape of a plurality of interconnected nodes, each node connected to one or more other nodes.

12. The method of claim 2, wherein the island material comprises silicon.

13. The method of claim 12, wherein the substrate comprises alumina.

14. The method of claim 13, wherein the second molten disk further comprises aluminum originating from the substrate.

15. The method of claim 12, wherein the heating comprises heating the substrate and the island material to at least about 1500 C.

16. The method of claim 2, wherein the first molten disk has a maximum thickness that is at least about ten times smaller than the smaller of its maximum length and maximum width.

Description

DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the various implementations described herein and to show more clearly how they may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings in which:

(2) FIG. 1 depicts a method of fabricating a crystalline island abutting a substrate, according to non-limiting implementations.

(3) FIG. 2 depicts a top perspective view of crystalline islands on a substrate, according to non-limiting implementations.

(4) FIG. 3 depicts a top perspective view of a substrate having a depression, according to non-limiting implementations.

(5) FIG. 4 depicts a collection of possible shapes for a depression, according to non-limiting implementations.

(6) FIG. 5 depicts a top perspective view of a substrate having a depression within a depression, according to non-limiting implementations.

(7) FIGS. 6a-d depict a cross-section of a substrate having a depression within a depression at different stages of forming a crystalline island in the depressions, according to non-limiting implementations.

(8) FIG. 7 depicts a cross-section of a substrate having a depression within a depression, according to non-limiting implementations.

(9) FIG. 8 depicts a cross-section of a substrate having a depression within a depression, according to non-limiting implementations.

(10) FIGS. 9a-c depict a cross-section of a substrate having a through hole, at different stages of forming a crystalline island in the through hole, according to non-limiting implementations.

(11) FIG. 10 depicts a method of depositing particles of an island material on a substrate, according to non-limiting implementations.

(12) FIG. 11 depicts a cross-section of a crystalline island on a substrate, both over-coated, according to non-limiting implementations.

(13) FIG. 12 depicts a cross-section of a crystalline island on a substrate, both over-coated, according to non-limiting implementations.

(14) FIG. 13 depicts a method of fabricating a crystalline island sandwiched between two substrates, according to non-limiting implementations.

(15) FIG. 14 depicts a cross-section of a molten globule sandwiched between two substrates, according to non-limiting implementations.

(16) FIG. 15 depicts a top plan view of an array of pyramids and abutting crystal grains on a substrate, according to non-limiting implementations.

(17) FIG. 16 depicts a cross-section of a molten globule sandwiched between two substrates, according to non-limiting implementations.

(18) FIGS. 17a-d depict in cross-section various stages of fabricating a crystalline island on a substrate, according to non-limiting implementations.

(19) FIG. 18 depicts a photomicrograph of a crystalline island on a substrate, according to non-limiting implementations.

DETAILED DESCRIPTION

(20) An implementation of the present invention is reflected in method 100 shown in FIG. 1. Method 100 can be used to fabricate a crystalline island abutting a substrate, on top of or inside a substrate. First, as shown in box 105, island material can be deposited abutting the substrate, on or in the substrate. The island material can be in particulate form. Particles of the island material can be deposited at a predetermined location relative to the substrate, which can include, but is not limited to, deposition on all or a portion of the surface of the substrate. Particles of the island material can be deposited as a substantially pure powder, a powder with some additives or impurities, or as particles of the island material suspended in a carrier medium. Additives can be used to dope, alloy, or otherwise compound the island material.

(21) Subsequently, as shown in box 110, the substrate and the particles of the island material can be heated. The heating can be by conduction, convection, and/or radiative heating, and can be performed in a furnace, kiln, or other suitable heating apparatus known to the skilled person. The heating melts and fuses the particles of the island material to form a molten globule of the island material. When a carrier medium is used to deposit the island material, the heating can evaporate, burn off, or otherwise eliminate the carrier medium before the melting and fusing of the island material particles. The island and substrate materials can be chosen so that the substrate does not melt at the temperature required to melt and fuse the particles of the island material.

(22) Subsequently, as shown in box 115, the substrate and the molten globule are cooled to crystallize the molten globule, thereby securing the resulting crystalline island to the substrate. In some implementations, the islands are secured strongly enough to allow mechanical polishing, or other abrasive processing, of the island to expose a cross-section of the island, without dislodging the island from the substrate.

(23) FIG. 2 shows an array of crystalline islands 210 formed on substrate 205. Method 100 can be used to fabricate one or any number of crystalline islands. When a plurality of islands is formed, they can be arranged in an ordered array, or can be distributed on the substrate without perceptible periodic ordering. In some implementations, the position of each island 210 relative to substrate 205 is known to allow for subsequent processing of the islands.

(24) Islands 210 can be single-crystalline or poly-crystalline. Nano-crystalline and amorphous islands can also be formed. In some implementations after islands 210 are formed, they are planarized, abraded, or otherwise operated upon so that some material is removed from the surfaces thereof to expose a cross-section of each island. This exposed cross-section can then be used to fabricate electronic devices, for example when the islands are formed from a semiconductor such as silicon.

(25) When the starting particulate island material has impurities, the process of melting and crystallizing set out in method 100 can reduce the impurities inside the crystalline islands by pushing the impurities towards the surface of each molten globule as a crystalline lattice begins to form inside the molten globule, a process known as gettering. The lattice then tends to exclude any impurities that would interfere with its ordered arrangement of atoms, thereby excluding at least a portion of the impurities from the inside of the crystal. When the crystalline islands are poly-crystalline, the impurities are pushed to the grain boundaries between the crystals.

(26) In some implementations, particles of the island material are deposited onto the substrate by transferring the island material into one or more depressions defined in the substrate surface. FIG. 3 shows depression 310 in substrate 305. In some implementations a plurality of depressions can be defined, and they can be arranged into an ordered array. The depressions can be defined lithographically, or using other means known to the skilled person, including but not limited to laser ablation or localized etching using a photolithographically defined mask.

(27) The depression 310 contains the molten globule, and can serve to more precisely locate the molten globule, and the resulting crystalline island, relative to the substrate 305. In addition, the shape of the depression can guide the crystallization process by providing nucleation sites for initiating crystallization. In some implementations the depression can have one or more vertexes 425 as shown in FIG. 4 in relation to depressions 405, 410, 415, and 420. The vertex 425 can be at the end of a taper such as in the case of depression 420, or part of a reverse taper, as shown in depressions 405 and 415. A depression can have multiple vertexes as shown in depressions 410 and 415. Having multiple nucleation sites in the form of vertexes 425 increases the likelihood for the island being poly-crystalline and decreases the likelihood of formation of a single-crystalline island. Nucleation sites can also be three-dimensional smaller depressions into or protrusions from the surface of depression 310. Such smaller depressions can have one or more vertexes (not shown). These vertexes can in turn serve as a nucleation site for crystallization of the molten globule.

(28) As shown in FIG. 5, in some implementations, substrate 505 can have a larger and shallower depression 510, within which there can be a smaller and deeper depression 515.

(29) FIGS. 6a-d show a cross-section of a substrate 605, with a larger depression 610 and a smaller and deeper depression 615 within depression 610. As shown in FIG. 6b, particles of the island material 620 can be deposited to fill both the larger and the smaller depressions 610 and 615. As shown in FIG. 6c, when the particles are melted, they fuse to form molten globule 625. The surface tension of the molten globule can pull it into an approximately spherical shape, which is then cooled and crystallizes to form a crystalline island. As shown in FIG. 6d, the crystalline island and/or the substrate 605 can be planarized to yield a crystalline island 630 in substrate 605. Depression 610 serves as a reservoir for particles of the island material which eventually form the molten globule 625. Depression 615, in turn, serves to locate the molten globule 625 and initiate its crystallization using vertexes discussed above in relation to FIG. 5.

(30) The relative volume of depressions 610 and 615 can determine the size of the molten globule. This, combined with the depth of depressions 610 and 615 can determine the size of cross-section of crystalline island 630 available at different depths of substrate 605. When particles of island material 620 fuse to form molten globule 625 leaving at least a portion of the volume of depression 610 empty, this empty space can be back-filled after molten globule 625 crystallizes. This back-filling can help to create a planar surface.

(31) FIG. 7 shows a side elevation cross-section of a substrate 705 having large depression 710, and small depression 715 within large depression 710. As shown in the cross-section, the surface of small depression 715 envelops more than half of molten globule 725 (shown in dotted line) thereby physically securing the crystallized molten globule 725 to the substrate. Even in implementations where the surface of small depression 715 envelops less than half of the molten globule 725, the enveloping can still contribute to physically securing the crystalline island to the substrate. The shape of depression 715 is not limited to a portion of a sphere. Any suitable shape can be used. When it is desirable to physically secure the crystalline island to the substrate, the shape of depression 715 can be used whereby the molten globule can flow into the shape, but the solid crystalline island cannot be physically removed from depression 715. An example of such shapes for depression 715 can be any shape where the opening of depression 715 is smaller than the largest dimension of the crystalline island that must pass through the opening in order for the crystalline island to be removed from depression 715.

(32) FIG. 8 shows a side elevation cross-section of a substrate 805 having a large depression 810 and a small depression 815 within large depression 810. Small depression 815 has protrusion 820 extending into the space to be occupied by molten globule 840. Protrusion 820 can have at least one vertex 825. Instead of, or in addition to, protrusion 820, small depression 815 can have a further depression 830 in the surface of small depression 815. Further depression 830 may have at least one vertex 835. Further depression 830 can also be described as a protrusion of the space to be occupied by molten globule 840 into small depression 815. Depression 815 may have any number or combination of protrusions 820 and further depressions 830. Vertexes 825 and 835 can form a nucleation site for initiating the crystallization of molten globule 840. Once molten globule 840 is crystallized, protrusion 820 and further depression 830 can contribute to physically securing the crystallized island into depression 815, and in turn securing the crystalline island to substrate 805.

(33) The securing means discussed in relation to FIGS. 7 and 8 can also be used in a single-stage depression such as depression 310 shown in FIG. 3. In addition to these means of physically securing the crystalline island to the substrate, surface adhesion can also be used to secure the crystalline island to the substrate. For example, if the molten globule has a wetting angle with the substrate of less than about 90, the molten globule sufficiently wets the substrate surface and contributes to the adhesion of the crystallized island to the substrate surface.

(34) When the particulate island material is in the form of a loose powder, it can be transferred into the depression in the substrate using means including but not limited to: 1) doctor-blading the powder into the depression; and 2) electrostatically depositing the powder into the depression using charged pins to pick and then deposit the powder into the depression.

(35) When the particulate island material is in the form of a suspension of the particles in a carrier medium, the suspension can be flowed onto the substrate to fill the depression and then squeegeeing the excess suspension located outside the depression from the surface of the substrate. When such a carrier medium is used, during the heating step it can be evaporated, burnt off, or otherwise eliminated before the melting and fusing of the particulate island material.

(36) In the cooling stage, cooling alone can be sufficient to initiate the crystallization of the molten globule. Other techniques can be used to facilitate or more finely control the initiation and progress of the crystallization. For example, the molten globule can be super-cooled below its melting point. Super-cooling can take the form of cooling the molten globule to less than around 300 C. below its melting point before the crystallization starts. Applying a physical impact or shock to the substrate bearing the molten globule can also set off crystallization. This can also be used when the molten globule is super-cooled. In addition, the surface of the molten globule can be exposed to different chemical reactants, such as oxygen, to further guide the crystallization process. The oxygen can form a thin layer or skin on the surface of the molten globule which serves to isolate the molten silicon from the substrate and can serve to increase the surface tension of the globule.

(37) FIG. 9a shows an implementation where substrate 905 has a through hole 910 filled with particulate island material 915. Hole 910 has a first end 940 and a second end 945. Hole 910 can be filled with particulate island material 915 using doctor-blading, Hole 910 can also be filled with a liquid suspension comprising the particulate island material dispersed in a carrier medium. The suspension can be flowed onto the substrate and into hole 910, and then the excess suspension located outside hole 910 can be squeegeed off the surface of the substrate. After filling hole 910, substrate 905 and island material 915 can be heated to melt and fuse the island material into molten globule 935. When such a carrier medium is used, during the heating step it can be evaporated, burnt off, or otherwise eliminated before the melting and fusing of the particulate island material. Optionally, substrate 905 can be flipped before the heating. For example, the flipping can be used when hole 910 has a closed end, to point the open end of hole 910 towards the earth's gravitational force.

(38) FIG. 9b shows molten globule 935 forming a convex meniscus 920 extending out of the first end 940 of hole 910. Meniscus 920 can form under the force of gravity. In addition, if surface 925 of substrate 905 adjacent first end 940 of hole 910 has a low wetting angle with the molten globule 935, this can encourage the molten globule 935 to wet the surface 925 of substrate 905 and for the meniscus 920 to form and extend out of first end 940 of hole 910. Furthermore, molten globule 930 can be encouraged to extend out of hole 910 and form meniscus 920 if pressure is applied against the molten globule 935 through the second end 945 of hole 910 to push molten globule 935 out of first end 940 of hole 910. As shown in FIG. 9c, once molten globule 935 crystallizes, meniscus 920 can be polished and/or planarized to expose a cross-section of crystalline island 950 and to form a crystalline island 950 in substrate 905.

(39) FIGS. 9a-c show one hole 910, but a plurality of holes can be used. The holes can be arranged in an ordered array. Crystalline islands can be secured to substrate 905 by respective holes, such as hole 910, enveloping and physically securing the respective island 950 and/or the surface adhesion of the crystalline island 950 to the surface of hole 910. Adhesion can be stronger when the wetting angle between the molten globule 935 and the surface of hole 910 is smaller than about 90.

(40) In another implementation (not shown), particles of island material can be dispersed in a carrier medium to form a suspension. The suspension can then be transferred onto the substrate. Next, the substrate and the suspension can be heated, which can evaporate, burn off, or otherwise eliminate the carrier medium. The heating can also melt and fuse the particles of the island material to form a molten globule. The cooling and securing can be carried on as previously described. A wetting angle of less than about 90 between the molten globule and the substrate can contribute to stronger adhesion between the substrate and crystalline island and to securing the crystalline island to the substrate.

(41) The suspension can be transferred to the substrate using techniques including, but not limited to, one or more of stamping, screen printing, or inkjet printing of the suspension onto the substrate following procedures known in the art. The suspension can also be spin-coated to form a layer on the substrate. This layer can then be lithographically patterned to define one or more regions on the substrate where particles of the island material are present, and other regions where island material particles are absent.

(42) There may not be any depressions in this implementation. However, the substrate surface can be patterned to have areas of higher wetting angle and other areas of lower wetting angle with the molten globule. The molten globule will tend to form on the areas of lower wetting angle. The patterning of areas with low wetting angle can serve as a means of further locating the molten globule, and thus the crystalline island on the substrate. This can be applied to one crystalline island or a plurality of crystalline islands. Methods for patterning a substrate to have high and low wetting angle areas are well known in the art, and can include applying a patterned mask to the surface followed by subjecting the unmasked areas to chemical modification or deposition of other materials, such as SiO.sub.2, on the unmasked areas.

(43) FIG. 10 shows a further implementation of the present invention reflected in method 1000 for depositing particles of the island material on the substrate. First, as shown in box 1005, the particles can be dispersed in a carrier medium to create a suspension. Next, as shown in box 1010, the suspension can be formed into a sheet by spreading or spin coating the suspension or using other methods known in the art. Next, as shown in box 1015, the sheet of the suspension can be transformed into a solid sheet. This can be accomplished by drying, baking, cross-linking, or otherwise solidifying the suspension. Next, as shown in box 1020, the solid sheet can be patterned by cutting away or removing one or more portions of the sheet to form a patterned sheet. The pattern can be applied using a mechanical punch, lithographically, or using other means known in the art. Next, as shown in box 1025, the patterned sheet can be overlaid on the substrate.

(44) At this stage the remaining steps of heating and cooling-and-securing can be applied as previously described. During heating, the carrier medium can be evaporated, burnt off, or otherwise eliminated before melting and fusing of particles of the island material. A wetting angle of less than 90 between the molten globule and the substrate can contribute to adhesion of the crystalline island to the substrate. This implementation can be used to make a single island or a plurality of crystalline islands on the substrate, which can be arranged in an ordered array.

(45) In the implementations where there is no depression, initiation of the crystallization of the molten globule can still be guided and controlled. One or more guiding depressions into and/or guiding protrusion from the substrate surface coming into contact with the molten globule can initiate crystallization. These guiding depressions and protrusions can have at least one vertex to provide an initiation point for the crystallization process.

(46) Alternatively, the shape of the contact area of the molten globule with the substrate can be controlled to provide an initiation point for crystallization. By patterning the relatively low and high wetting angle areas on the substrate, the molten globule can be made to wet or contact the substrate along a patterned lower wetting angle shape while avoiding the higher wetting angle areas of the substrate. The shape of the low wetting angle area can be any of the shapes discussed above in relation to FIG. 4. The shape can have at least one vertex to provide an initiation point for the crystallization.

(47) Another means of controlling initiation of crystallization can be depositing a metallic grid on the substrate, at least over the areas of the substrate that come into contact with the molten globule. The deposited metal can act as an initiation point for the crystallization of the molten globule. The grid can be made of other materials, such as refractories or Ni. The deposited material can have other shapes such as dots or other patterns of deposited material that may not constitute a grid.

(48) In other implementations, after the crystalline islands form, the islands and the substrate can be over-coated. FIG. 11 shows a cross-section of substrate 1105 and crystalline island 1110 over-coated with layer 1115. This assembly forms a stack where the crystalline island 1110 is sandwiched between substrate 1105 and cover-coating layer 1115. The stack can then be planarized to remove some of the material comprising the stack and expose a cross-section of the crystalline island.

(49) The over-coating layer 1115 can be deposited using any suitable physical or chemical deposition method including but not limited to spin coating or electrostatically-applied powder coating. The layer can be a thin layer, such as layer 1115 in FIG. 11, or can be a thicker layer such as layer 1215 shown in cross-section in FIG. 12. When an over-coating layer is used, it can physically secure the crystalline island to the substrate by sandwiching the crystalline island between the substrate and the over-coating layer, as shown in FIGS. 11 and 12. This securing action of the over-coating layer can contribute to securing to the substrate crystalline islands having a large wetting angle, and therefore small contact area, with the substrate. Large wetting angles can, for example, be angles greater than about 90. Over-coating can be applied to a plurality of islands on a substrate.

(50) Another implementation of the present invention is reflected in method 1300 shown in FIG. 13. Box 1305 shows depositing particles of an island material on a first substrate. Particles can be deposited as loose powder or in a suspension as described above. The deposition can be patterned and/or at specified locations relative to the substrate. Next, as shown in box 1310, the particles of the island material can be sandwiched between the first substrate and a second substrate placed adjacent the first substrate. Next, as shown in box 1315, the substrate and the particles can be heated to melt and fuse the particles into a molten globule, without melting the first substrate or the second substrate.

(51) When the particles are deposited as a suspension in a carrier medium, the heating step can evaporate, burn off, or otherwise eliminate the carrier medium before melting and fusing the particles. Next, as shown in box 1320, the substrates and the molten globule can be cooled to crystallize the molten globule, thereby forming a crystalline island.

(52) The sandwiching, described in box 1310, and the heating described in box 1315 can be performed in the opposite order, i.e. the molten globule can form before it is sandwiched between the first and the second substrates. FIG. 14 shows molten globule 1415 sandwiched between first substrate 1405 and second substrate 1410.

(53) When the wetting angle between molten globule 1415 and both first substrate 1405 and second substrate 1410 is large, the crystallized island can adhere more weakly to the substrate. The weak adhesion can facilitate removing the crystallized island from the substrate to form a free-standing wafer.

(54) Features on the surfaces of one or both of the first and second substrates can be used to initiate crystallization of the molten globule. FIG. 15 shows an array of pyramids 1505 that can be formed on the region of the substrate that comes into contact with the molten globule. Pyramids 1505 can protrude from the substrate surface or form depressions into the substrate surface. Instead of a pyramid, other shapes can be used, such as a cone. These shapes can have a vertex, as does pyramid 1505, to act as an initiation site for the crystallization of the molten globule.

(55) Each pyramid 1505 initiates crystallization to form grain 1510, which eventually abuts upon neighboring grains at grain boundaries 1515. Using this method, a molten globule can be patterned into a poly-crystalline form. As discussed above, during crystallization at least some of the impurities in the molten globule can be pushed towards the grain boundary regions. This process can leave the central region of grain 1510 with relatively fewer impurities yielding a higher quality crystal for post-processing, such as device fabrication on grain 1510. This process can isolate the grain boundaries to regions where devices will not be fabricated in subsequent processing. Although FIG. 15 shows four grains 1510 of uniform size, grains 1510 can also be of different sizes.

(56) FIG. 16 shows a cross-section of molten globule 1615 sandwiched between two substrates 1605 and 1610. As discussed above, one or both of the substrates can include depressions or protrusions to guide the crystallization process. Substrate 1605 can have protrusions 1620 in contact with molten globule 1615. Substrate 1610, in turn, can have depressions 1625 in contact with molten globule 1615. The protrusions 1620 and depressions 1625 can be of different shapes and any numbers of them can be used in/on one or both of first substrate 1605 and second substrate 1610. In addition or instead of protrusions and depressions, a grid or array of a metal or other material deposited on one or both of the first and the second substrate where those substrates come into contact with the molten globule can also be used to initiate and guide the crystallization process.

(57) The cooling as shown in box 1320 of FIG. 13 can also include super-cooling the molten globule. The super-cooling can include cooling the molten globule to a temperature below about 300 C. below its melting point before crystallization begins. A pressure pulse or mechanical impact can also be applied to the molten globule in a super-cooled state or otherwise. A seed crystal can also be added to the molten globule in a super-cooled state or otherwise.

(58) In some implementations, the coefficient of thermal expansion (CTE) of the substrate at a temperature within about 20 C. of the melting point of the island material can be matched to the CTE of the island material at the melting point of the island material. This matching of CTE can reduce stresses between the island material and the substrate as each one cools and contracts. Lower stresses can facilitate making of higher quality crystals with fewer defects, and can improve the adhesion of the crystalline island to the substrate.

(59) The island material can include, but is not limited to, semiconductors. Such semiconductor can include, but are not limited to, silicon. The substrate material can include, but is not limited to, silica, alumina, sapphire, niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, osmium, iridium, and combinations and alloys of these materials.

(60) The substrate can also be a ceramic or glasses with sufficiently high melting or softening temperatures. The substrate can also be a High-Temperature Co-fired Ceramic (HTCC). HTCC can be worked and mechanically patterned in its green phase. When the island material is silicon, alumina can be a relatively higher wetting angle material and silica a relatively lower wetting angle material.

(61) In some implementations, the molten globule of the island material can have a flattened or disk shape. For example, referring to FIG. 1, at step 110 heating the substrate and the particles of the island material can melt and fuse the particles to form a molten disk and/or a molten disk-shaped or flattened globule. At step 115, the cooling can then solidify and crystallize the molten disk, thereby securing the disk-shaped crystalline island of the island material to the substrate. The securing can comprise the crystalline island adhering directly to the substrate. In addition and/or instead, the securing can comprise the crystalline island adhering indirectly to the substrate by adhering to any intermediate and/or interfacial layer secured directly to the substrate. In some implementations, some portions of the crystalline island can adhere directly to the substrate material while other portions can adhere to an interfacial and/or intermediate layer covering at least a portion of the substrate. In some implementations, after the cooling step, at least a portion of the crystalline island can be planarized to expose a cross-section of the crystalline island.

(62) FIGS. 17a-d show various steps of an exemplary method for fabricating crystalline islands that are disk shaped and/or have a flattened shape. FIG. 17a shows island material 1710 being deposited on a substrate 1705. Substrate 1705 can be similar to the other substrates described herein, and island material 1710 can likewise be similar to other island materials described herein.

(63) While FIG. 17a shows island material 1710 deposited as a heap or mound, it is contemplated that island material 1710 can be deposited in any other suitable manner. For example, island material 1710 can be deposited in powder form. When island material 1710 is deposited in powder form, the powder can be deposited through a screen to form one mound or an array of mounds of powder at predetermined positions on substrate 1705. The size of the openings in the screen can determine the shape and size of the mounds of powder on substrate 1705.

(64) In other implementations, island material 1710 can be deposited as a layer of material on the substrate. In yet other implementations, the island material 1710 can be suspended in a carrier medium, and the suspension can be deposited on substrate 1705. Such layers and/or suspensions of island material 1710 can be patterned on substrate 1705 and/or deposited at predetermined positions on substrate 1705. In some implementations, island material 1710 can be printed on substrate 1705.

(65) Referring to FIG. 17b, once island material 1710 is deposited on substrate 1705, island material 1710 and substrate 1705 can be heated to melt island material 1710 to form a first molten disk 1715. The heating can also form a second molten disk 1707 disposed between first molten disk 1715 and substrate 1705. Second molten disk 1707 can comprise oxygen and the island material.

(66) The molten disks can have any generally flattened shape, including but not limited to, a saucer, a pancake, a wafer, a platelet, a discus, a sheet, and/or an oblate shape. The molten disks can have a maximum thickness that is at least about ten times smaller than the smaller of their maximum length and maximum width. In some implementations, molten disks can have a maximum thickness that is at least about five times smaller than the smaller of their maximum length and maximum width. In other implementations, molten disks can have a maximum thickness that is at least about two times smaller than the smaller of their maximum length and maximum width. The first and second molten disks can be largely or entirely immiscible, thereby remaining largely or entirely phase-separated in the molten state. In addition, second molten disk 1707 can have a higher density in the molten state, thereby remaining, under the force of gravity, between substrate 1705 and first molten disk 1715.

(67) After the heating, substrate 1705, first molten disk 1715 and second molten disk 1707 can be cooled to solidify and/or crystallize first molten disk 1715 to form the crystalline island of the island material. In this process, second molten disk 1707 can also solidify to form an oxide disk. The crystalline island can also have a disk like (or generally flattened) shape similar to the shape of first molten disk 1715.

(68) The crystalline island can be single, poly, and/or nano crystalline. In some implementations, after forming the crystalline island, at least a portion of the crystalline island can be planarized to expose a cross-section of the crystalline island. In some implementations, the island can be mechanically (and/or chemo-mechanically) planarized without becoming detached from the substrate. This can be made possible because the crystalline island can adhere strongly to the oxide disk, which in turn can adhere strongly to the substrate.

(69) Several factors can contribute to the strong adhesion of the crystalline island to the substrate. One such factor can be the relatively small wetting angle and thereby relatively large contact area between the crystalline island and the oxide disk and also between the oxide disk and the substrate. If either one of the crystalline island and the oxide disk were to have a large wetting angle, and thereby a tendency to ball-up into a near-spherical shape, there would be much smaller contact area, and weaker adhesion, between the crystalline island, the oxide disk, and the substrate. Such balled-up, near-spherical crystalline islands can adhere only weekly to the substrate such that they would become detached from the substrate during planarization, such as mechanical and/or chemo-mechanical planarization.

(70) Another factor contributing to the strong adhesion can be porosity of the substrate, which can also increase the contact surface area between the substrate and the oxide disk and/or crystalline island in contact with the substrate. Yet another factor contributing to the strong adhesion of an alumina substrate to an oxide layer comprising silicon oxide is that often alumina substrates comprise some glass mixed in with the aluminum oxide. Since most glass comprises silicon oxide, the glass component of the alumina substrate can adhere strongly to the oxide disk which can also comprise silicon oxide.

(71) In some implementations, as shown in FIG. 17c, after the cooling the crystalline island and substrate 1705 can be over-coated with an over-coating layer 1720 to form a stack. This over-coating layer 1720 can be similar to other over-coating layers described herein. As shown in FIG. 17d, after the over-coating, the stack can be planarized to remove portions of the over-coating layer and the crystalline island, and as a result expose a cross-section 1725 of the crystalline island.

(72) In some implementations, second molten disk 1707 can be formed when island material 1710 is heated on substrate 1705 in the presence of oxygen. For example, oxygen can be present in gaseous form if island material 1710 and substrate 1705 are headed in an air atmosphere. In some implementations, second molten disk 1707 can comprise a molten oxide of the island material which phase separates from first molten disk 1715 comprising molten island material.

(73) The presence of the second molten disk, combined with temperatures in excess of island material's melting point during the heating step, can allow the molten globule of the island material to spread into a disk, instead of balling-up into a near-sphere under surface tension forces. Such temperatures can also reduce the viscosity of the molten island material, thereby promoting the ability of the molten island material to spread into a molten disk. In addition, the second molten disk can allow the first molten disk to cool and solidify into a disk and/or flattened shape. Without the second molten disk, as the temperature is reduced to approach the melting point of the island material, the decreasing temperature can cause an increase in the surface tension of the molten island material, thereby causing it to ball-up into a near-sphere.

(74) Moreover, the second molten disk can allow the first molten disk to crystallize into a crystalline island while minimizing interference with crystal formation during the cooling step due to lattice and/or CTE mismatches between the island material and the substrate. Reducing these interferences and/or mismatches can also strengthen the mechanical adhesion of the crystalline island, via the oxide disk, to the substrate.

(75) In one particular example, crystalline islands of silicon can be fabricated on an alumina substrate. The alumina substrate can comprise Alumina Ceramic Substrate 10100.5 mm, one side polished (ALCeramic101005S1) sold by MTI Corporation. Particles of silicon can be deposited as heaps onto the alumina substrate. The deposition can be carried out using a screen with holes having a diameter of about 1 mm. Then the alumina substrate and the heaps of silicon island material can be heated in an air atmosphere, according to the temperature profile summarized in Table 1 below:

(76) TABLE-US-00001 TABLE 1 Temperature Profile Step Ramp rate ( C./min) Level temp ( C.) Dwell time (min) 1 3 1000 0.1 2 10 1600 60 3 10 1200 0.1 4 5 500 0.1

(77) As can be seen in Table 1, the maximum temperature is 1600 C., which is in excess of the melting point of silicon, which is 1414 C. In other implementations, the maximum temperature can be at least about 1500 C. In the case of silicon deposited on alumina, flattening of the first molten globule into the first molten disk, and consequent formation of a flattened/disk shaped crystalline islands has not been observed at temperatures below 1500 C. At the conclusion of step 4 in the temperature profile, the heater can be turned off, and the sample can be allowed to cool further to facilitate subsequent handling.

(78) In addition, while the dwell time at 1600 C. is 60 minutes, dwell times as short as at least 5 minutes at 1600 C. can cause flattening of the molten island material into the first molten disk, and consequent formation of a flattened/disk shaped crystalline island. Generally, in some implementations, the maximum temperature can be at least about 86 C. above the melting point of the island material. In other implementations, the maximum temperature can be at least about 186 C. above the melting point of the island material.

(79) FIG. 18 shows a top plan view optical micrograph of a flattened/disk shaped island 1810 of silicon formed on an alumina substrate 1805, fabricated using the temperature profile summarized in Table 1. When the island material comprises silicon and the substrate comprises alumina, in some implementations the second molten disk, and the oxide disk into which it solidifies, can comprise aluminum in addition to oxygen and silicon. In some implementations, this aluminum can originate from the alumina substrate. In general, in some implementations, the second molten disk can comprise one or more elements originating from the substrate.

(80) In the implementations described above, the second molten disk, and the oxide disk into which it solidifies, are formed by heating in the presence of oxygen the island material deposited on the substrate. It is also contemplated that in some implementations an oxide layer can be formed on the substrate before the island material is deposited on the substrate. This oxide layer can comprise the island material and oxygen. In other implementations, the oxide layer can also comprise additional materials including, but not limited to, elements originating from the substrate.

(81) In implementations where the oxide layer is initially formed on the substrate, the depositing step can comprise depositing the island material on the oxide layer. In such implementations, the second molten disk can comprise the oxide layer in a molten state. In some implementations, forming the oxide layer can comprise depositing on the substrate the oxide layer, which can comprise an oxide of the island material. In some implementations, the depositing can be according to a predetermined pattern, for example using a mask, printing, lithography, and the like.

(82) In other implementations, forming the oxide layer can comprise depositing an initial amount of the island material on the substrate and then oxidizing this initial amount of the island material to form the oxide layer. The deposition of the initial amount of the island material can be according to a predetermined pattern. The initial amount of the island material can be deposited as particles or as a layer of the island material. For example, in the case of silicon island material, the initial amount can be deposited as silicon particles/power and/or as a layer of amorphous, nano-crystalline, and/or poly-crystalline silicon.

(83) In implementations where the oxide layer is formed on the substrate prior to depositing the island material on the oxide layer, no further oxide needs to be formed during the heating step. As such, the heating can be performed in a non-oxidizing atmosphere. For example, the atmosphere can be substantially oxygen-free. In some implementations, an inert atmosphere can be used during the heating. For example, the heating can be performed in an argon atmosphere.

(84) In some implementations, the island material can be deposited on the substrate in a predetermined pattern. This pattern can comprise an interconnected and/or contiguous pattern, including but not limited to the shape of a plurality of interconnected nodes, with each node connected to one or more other nodes. This can allow for the crystallization of the molten island material to start at one, or a few, nucleation sites in the pattern and then proceed throughout the pattern. This mode of crystallization can allow subsets of the crystalline islands to have similarly oriented crystal lattices. In some implementations, a single crystal can propagate through all or substantially all of the interconnected pattern of the island material. All the means and methods described herein for initiating and/or controlling crystallization can be used to initiate and/or control the crystallization of the molten island material deposited in the predetermined pattern.

(85) While the foregoing describes disks of molten material, it is contemplated that the molten material can be of any generally flattened shape, depending on the pattern according to which the island material and/or the oxide layer is deposited and/or formed on the substrate. For example, if the island material is deposited on the substrate according to a pattern of interconnected nodes, then the heating step can produce a generally flattened layer of molten island material also generally in the shape of interconnected nodes. The molten oxide layer can also be generally in the shape of interconnected nodes. Moreover, once the layer of molten island material crystallizes, the resulting crystallized island material can also comprise a generally flattened layer in the shape of interconnected nodes.

(86) In some implementations, the surface roughness of the substrate can be patterned in order to guide where on the substrate the molten oxide layer and/or the molten island material layer form. For example, the substrate can be polished and/or roughened according to a predetermined pattern. Regions of the substrate with different surface roughnesses can have different wettability by the molten oxide and/or molten island material. In addition, regions of the substrate with different surface roughnesses can adhere to the oxide layer with different mechanical strengths.

(87) Following the method depicted in FIGS. 17a-d, and/or the other similar methods described herein, a semiconductor device can be fabricated. Such a device comprises a substrate and an intermediary disk disposed on the substrate. The intermediary disk can comprise oxygen and an island material. The intermediary disk can comprise the oxide disk. The semiconductor device also comprises an island disk disposed on the intermediary disk. The island disk can comprise the island material, and can be crystalline. The island material can be formed separately from the substrate and then deposited on the substrate. The intermediary disk can be formed by melting and then solidifying the island material on the substrate. As discussed above, in implementations where the oxide intermediary disk is formed during the heating step, the heating and/or melting can be performed in the presence of oxygen and at a maximum temperature exceeding the melting point of the island material.

(88) In addition, while the above description refers to intermediary and island disks, it is contemplated that the intermediary oxide and/or the crystallized island material can be in any layer-like or otherwise flattened shape or configuration. The island disk can have a maximum thickness that is at least about ten times smaller than the smaller of its maximum length and maximum width. In some implementations, the island disk can have a maximum thickness that is at least about five times smaller than the smaller of their maximum length and maximum width. In other implementations, the island disk can have a maximum thickness that is at least about two times smaller than the smaller of their maximum length and maximum width.

(89) In some implementations, the substrate can comprise alumina and/or the island material can comprise silicon. Moreover, in some implementations the intermediary disk can also comprise aluminum. This aluminum can originate from the alumina substrate.

(90) The planarized cross-sections of the crystalline islands can be used to make electronic devices, such as transistors or other circuit components. As such, the methods and devices described herein can be used in backplanes for active matrix displays such as OLED displays, in electro-optical detector arrays such as X-ray detectors, and in fabricating certain integrated circuits such as those used in amplifiers and op-amps.

(91) When multiple crystalline islands are formed on a substrate, and/or when multiple electronic devices are fabricated on a given planarized cross-section, the islands and/or the devices respectively can be appropriately singulated to provide individual crystalline islands and/or electronic devices respectively. When separated crystalline islands (or arrays of crystalline islands) are used to make separate displays and/or detectors, those displays and/or detectors can be tiled together to form a larger tiled display and/or detector.

(92) The above-described implementations of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.