BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by reading the subsequent detailed description and examples with reference to the accompanying drawings, wherein:
FIG. 1A-1C show schematic cross sections of a mechanism for forming coffee rings;
FIG. 2A-2F show schematic cross sections of processes for forming an upper-gate TFT of a first embodiment of the invention;
FIG. 3 shows a schematic cross section of an upper-gate TFT of a second embodiment of the invention;
FIG. 4 shows a schematic cross section of a lower-gate TFT of a third embodiment of the invention; and
FIG. 5 shows a schematic cross section of a lower-gate TFT of a fourth embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best-contemplated mode of carrying out the invention. The description is provided for illustrating the general principles of the invention and is not meant to be limiting. The scope of the invention is best determined by reference to the appended claims.
The invention utilizes an inkjet printing micro-drop and the natural phenomenon of a solution drop drying into a coffee ring to make a micro-line structure of the coffee ring ridge. The thin central part of the coffee ring is removed by surface micro-etching to reduce the size of the micro-line structure of the coffee ring ridge to below the micrometer scale. Use the micro-line as an insulator or a semiconductor can yield a thin-film transistor (TFT).
The cross sections of fabrication steps of the first embodiment of the invention are shown in FIGS. 2A-2F. Referring to FIG. 2A, a substrate 20, such as glass or plastic substrate, is first provided. A semiconductor material solution is inkjet printed by a sprinkle-nozzle on the substrate 20 into a dot or a line shape, and then dried into a coffee ring film 21. The semiconductor material may be, but is not limited to, poly-(3-hexylthiophene) (P3HT) or poly-9(9dioctylfluorene-co-bithiophene) (F8T2). The semiconductor material is dissolved in a solvent into a solution for inkjet printing. The solvent includes watery liquid or an oily liquid such as xylene.
Referring to FIG. 2B, a central part 22 of the coffee ring film 21 is removed by a surface micro-etching method, and a coffee ring ridge 24 is left as a separating layer 24. If the semiconductor material solution is inkjet printed on the substrate 20 into the shape of a line, two parallel micrometer scale lines are formed on the substrate. The width of the coffee ring ridge line is below about 50 μm, and the height of the coffee ring ridge line is below about 10 μm. As shown in FIG. 2B, the coffee ring ridge 24 is treated with plasma 26 to make the coffee ring ridge 24 hydrophobic. The utilized plasma gas may be O2, N2, CF4, SF6 or combinations thereof. The surface micro-etching can be practiced by plasma, immersion, spraying, dispensing, printing or combinations thereof. The spraying, dispensing or printing is practiced by sprinkling a solvent on the substrate to etch the thin central part of the coffee ring.
Then referring to FIG. 2C, a solution of conductive material is inkjet printed on the coffee ring ridge into two separate areas due to the hydrophobic coffee ring ridge. The two separate areas are formed into films on the two sides of the ridge as a source layer 28 and a drain layer 30. The conductive material may be poly-3,4-ethylenedioxythiophene (PEDOT).
Referring to FIG. 2D, an active layer 32 is inkjet printed or coated on the coffee ring ridge 24, the source layer 28 and the drain layer 30. The active layer is formed from a solution of semiconductor material which can be inkjet printed, such as P3HT or F8T2 dissolved in a solvent. In the first embodiment of the invention, the active layer material solution can dissolve and micro-etch the separating layer 24 into a layer of graded concentration for enhancing TFT performance. As shown in FIG. 2D, the concentration of a part of the separating layer 24 near the active layer 32 is lower.
Referring to FIG. 2E, an upper gate dielectric layer 34 is inkjet printed or coated on the active layer 32. The upper gate dielectric layer can be formed of an insulating material, such as polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), or photoacrylate.
Referring to FIG. 2F, an upper gate layer 36 is finally inkjet printed or coated on the upper gate dielectric layer 34 and aligned with the separating layer 24 to form a TFT. The upper gate layer can be formed of a solution of conductive material such as PEDOT.
In the second embodiment of the invention the active layer 32 of the first embodiment is removed. As shown in FIG. 3, the coffee ring ridge (i.e., separating layer) 24 is formed on the substrate 20. The source layer 28 and the drain layer 30 are disposed on opposite sides of the ridge. The gate dielectric layer 34 is disposed on the source layer, the drain layer and the ridge. The gate layer 36 is then disposed on the gate dielectric layer. The material and fabrication method of the layers of the second embodiment of the invention are the same as the first embodiment. The coffee ring ridge is formed from an active material.
In the third embodiment of the invention a separating layer 38 thereof is formed of a solution of insulating material, such as PMMA, PVA or photoacryle dissolved in a solvent to form a solution for inkjet printing. The solvent may be a watery liquid or an oily liquid such as xylene. As shown in FIG. 4, the coffee ring ridge (i.e., separating layer) 38 is formed on the substrate 20. The source layer 28 and the drain layer 30 are disposed on opposite sides of the ridge. The active layer 32 is disposed on the source layer, the drain layer and the ridge. The gate dielectric layer 34 is disposed on the active layer and the gate layer 36 is disposed on the gate dielectric layer. The material and fabrication method of the layers of the third embodiment of the invention are the same as the first embodiment.
In the same way, a solution of material of the active layer 32 can dissolve and micro-etch the separating layer 38 into a layer of graded concentration for enhancing TFT performance. As shown in FIG. 4, the concentration of a part of the separating layer 38 near the active layer 32 is higher.
According to the first embodiment of the invention, the substrate can be replaced by a substrate having a conductive layer and a gate dielectric layer, and the upper gate layer and the upper gate dielectric layer can be removed to form a TFT with bottom gate structure. The structures of the second and third embodiments can be changed into a bottom gate TFT by following the described methods. For example, the fourth embodiment is a bottom gate TFT formed according to change the substrate of the first embodiment.
As shown in FIG. 5, a bottom gate layer 42 is disposed on a substrate 40. The bottom gate layer can be formed of PEDOT. A bottom gate dielectric layer 44 is disposed on the bottom gate layer and the substrate, which can be formed of an insulating material. In one embodiment, the bottom gate dielectric layer is formed of an inorganic insulating material such as SiO2 or Si3N4 etc.
There are several layers over the bottom gate dielectric layer forming a bottom gate TFT according to the material and fabrication method of the first embodiment. A separating layer 46 is disposed on the bottom gate dielectric layer. A source layer 48 and a drain layer 50 are disposed on opposite sides of the separating layer. An active layer 52 is then disposed on the source layer, the drain layer and the separating layer to obtain the bottom gate TFT. In the bottom gate TFT of the fourth embodiment of the invention, the separating layer is also a layer of graded concentration.
In one embodiment, the material of separating layer 46 may be the same as that of the third embodiment, wherein a solution of insulating material, such as PMMA, PVA or photoacrylate is dissolved in a solvent to form a solution for inkjet printing. The solvent may be a watery or oily liquid such as xylene. In the described bottom gate TFT, the separating layer is also a layer of graded concentration.
According to the invention, a micro-length of gate is formed by inkjet printing in combination with the coffee ring effect, and this has no need to be formed by photolithography. Use the micro-length of gate to achieve a reduced channel length of a TFT can get a high-current of the TFT.
Embodiments of the invention provide the following advantages. The micro-line of the coffee ring formed by inkjet printing can serve as a channel, source/drain, or gate of a TFT, wherein the micro-line can be formed of insulating, semiconductor, or conductive materials. TFTs with a circular or linear shape can also be provided. The channel of TFTs formed by inkjet printing can be dissolved by a solution of a subsequent process to improve the interfacial quality between source and drain, without requiring a lift-off process.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Patent Application
20080042200
