Patent Application
20150318403
A thin-film transistor (TFT) is a special kind of field-effect transistor made by depositing thin films of an active semiconductor layer over a supporting (but non-conducting) substrate. This differs from the conventional transistor, where the semiconductor material typically is the substrate, such as a silicon wafer. These TFT's are fundamental components in modem-age electronics, including, for example, sensors, imaging, and display devices. A common substrate is glass because the primary application of TFT's is in liquid-crystal displays.
Referring to
In some embodiments, the recrystallized aluminum substrate 110 comprises one of 1xxx., 3xxx, 5xxx or 8xxx aluminum alloy. In some embodiments, the recrystallized aluminum substrate 110 has an O Temper. In some embodiments, the recrystallized aluminum substrate 110 has a thickness in the range of 0.005-0.020 inches. In some embodiments, the recrystallized aluminum substrate 110 has a thickness in the range of 0.006-0.020 inches. In some embodiments, the recrystallized aluminum substrate 110 has a thickness in the range of 0.013-0.014 inches.
In some embodiments, the organic polymer 120 comprises one of an epoxy, acrylic, polyester or vinyl. In some embodiments, the organic polymer 120 has a molecular weight in the range of 800 to 2000 Daltons. In some embodiments, the organic polymer 120 has a molecular weight in the range of 1000-2000 Daltons. In some embodiments, the organic polymer 120 is able to be applied to a coil of aluminum via roll-coating. In some embodiments, the organic polymer 120 has a thickness in the range of 2.5-50 microns. In some embodiments, the organic polymer 120 has a thickness in the range of 5-12 microns. In some embodiments, the organic polymer 120 is adhered to the recrystallized aluminum substrate 110.
In some embodiments, instead of silicon dioxide, there is a layer of SiN on the organic polymer 120. In some embodiments, instead of silicon dioxide, there is a layer of Al2O3 on the organic polymer 120. The layer of silicon dioxide, SiN or Al2O3 130 is sufficiently thick so that electrodes 140 adhere to the layer of silicon dioxide, SiN, or Al2O3 130. In some embodiments, the layer of silicon dioxide, SiN or Al2O3 130 has a thickness in the range of 750-1500 angstroms. In some embodiments, the layer of silicon dioxide, SiN or Al2O3 130 has a thickness in the range of 1000-1250 angstroms.
Adhered means there is no lifting of gate dielectric layer or the gate electrode by visual inspection.
In some embodiments, the device comprises a thin film transistor 100.
Referring to
Referring to
Referring to
In some embodiments, depositing a layer of silicon dioxide, SiN or Al2O3 comprises radio frequency (“RF”) sputtering. In some embodiments, depositing a layer of silicon dioxide, SiN or Al2O3 comprises RF sputtering at room temperature. Room temperature is in the range of 60° F.-85° F. RF sputtering involves running radio waves through an inert gas to create positive ions. The target material, which will ultimately become the layer being deposited, is struck by these ions and broken up into a fine spray that covers the substrate.
Referring to
Rolling means use of machined rolls, oppositely opposed, wherein the metal substrate passes between the nip of the rolls. This reduces the thickness of the metal substrate, and under conditions where the rolls are sufficiently polished, the metal substrate will have a bright surface and a Ra value in the range of 25 to 200 nm.
Chemical brightening means use of acids at elevated temperatures, which selectively etch the metal surface, This etching removes the peaks on the metal surface, in turn yielding a surface with increased specularity.
A method comprises depositing a layer of silicon dioxide on a layer of an organic polymer on a recrystallized aluminum substrate; and adhering an electrode to the layer of silicon dioxide.
A substrate is a supporting material.
An electrode is a conductor through which electricity enters or leaves an object.
Roll coating is the process of applying a coating, to a flat substrate by passing it between rollers. Coating is applied by one auxiliary roller onto an application roil, which rolls across the conveyed flat substrate. There are two types of roll coating: direct and reverse roll coating. In direct roll coating, the applicator roll rotates in the same direction as the substrate moves. In reverse roll coating, the applicator roll rotates in the opposite direction of the substrate. Slot die coating comprises forcing a coating liquid out from a reservoir, through a slot by pressure and onto a substrate moving relative to the slot. Curtain coating comprises passing a horizontally fiat substrate on a conveyor underneath a steady stream of coating material falling onto the substrate. Spray coating comprises coating a substrate with a liquid spray. More information regarding these coating techniques can be found in Modern Coating. and Drying Technology, editors Edward Cohen & Edgar Cutoff, Wiley-VCH, Inc., isbn 1-56081-097-1, 1992, which is incorporated herein by reference.
The alloys mentioned herein are as defined by the Aluminum Association International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys as revised February 2009.
Organic polymer layers were deposited on unannealed, H-temper aluminum substrates. These organic polymer layers provide insulating characteristics and planarizing (i.e., smoothness) properties required for TFT fabrication. The aluminum substrates with the organic polymer layers were annealed at elevated temperatures (i.e., 300-325° C.) to achieve the required thermal stability. Typically, organic polymer coatings show poor performance when exposed to temperatures above 260° C. for long periods of time. Heat treating/annealing studies at temperatures from 316-320° C. (600-610° F.) were conducted for three to four hours to investigate the thermal stability of the insulated (i.e., organic polymer coated) aluminum substrate. For those skilled in the art of the properties of organic coatings, it is not obvious to conduct this annealing step due to potential degradation of the organic layer.
Three different variables of aluminum substrates were tested: (Type-1): AA 8006 H25P temper substrate coated both sides with an organic polymer (epoxy polymer) and annealed for hrs at 316-320° C. (after annealing the substrate had a T-temper); (Type-2): AA 8006 H25P temper substrate coated with an organic polymer (epoxy polymer) both sides but not annealed; & (Type-3): AA 5657 H18 temper substrate coated on the front side only with ultraviolet (UV) curable organic polymer (epoxy acrylate polymer) but not annealed.
Several samples from each of the three variables were then RF-sputtered coated with 150 nanometers (nm) of Molybdenum (Mo) and then subjected to the photolithographic process to pattern/etch the Mo-layer in order to form Mo-electrodes. This step was performed successfully confirming that the integrity of the substrates is compatible with photolithographic processing.
After resist stripping, 100 nm thick silicon dioxide (SiO2) film was deposited via plasma enhanced chemical vapor deposition (PECVD) using silane and nitrous oxide as reactant gases. The deposition temperature was 270° C. and the total exposure at process temperature was 50 minutes. After the deposition step, the integrity of the substrates and deposited coatings was evaluated. After the PECVD step, only the Type-1 aluminum substrate did not have any cracks.
The dimensional stability of the Type-1 substrate after the above PECVD step was evaluated by performing a second lithographic processing step which results in a second pattern. Based on the registration error between the two patterns, it was determined that the Type-1 exhibits shrinkage of 3+1 μm across a 150 mm substrate. This result confirms that Type-1 is compatible with IGZO TFTs.
One reason for the increased dimensional stability is the change in the microstructure of the aluminum substrate after the annealing step. Recrystallization is defined as the formation of a new grain structure in a deformed material by the formation and migration of high angle grain boundaries driven by the stored energy of deformation. Strain/work hardened aluminum alloys were investigated in these studies. The annealing step provided the energy to form new grains that resulted in a thermally-stable substrate when exposed to the photolithographic steps. The change in recrystallized grain microstructure is depicted in
The adhesion of the gate dielectric layer over the gate electrode is important for operation of the TFT. In the investigations described above, the adhesion of the GaN2 (gallium nitride) gate dielectric layer was insufficient, resulting in a non-working TFT device. It was also found that substituting a different gate electrode (aluminum) in combination with same gallium nitride (GaN2) dielectric layer also resulted in adhesion issues. To improve the adhesion, an additional layer of SiO2 was deposited over the planarized annealed Type 1 aluminum substrate. This additional layer of SiO2 resulted in optimum adhesion between the gate dielectric layer and the gate electrode. The reason for this enhanced adhesion is that the additional layer (SiO2) serves to reduce mismatch in the coefficient of thermal expansion (CTE) between the gate electrode and the planarized aluminum substrate. The temperatures utilized during the deposition of the gate dielectric are such that the additional layer (SiO2) mitigates/reduces the tendency for the adhesion loss.
Testing Procedures:
An automated probe station enabled device characterization over the entire wafer.
An amorphous InGaZnO TFTs was fabricated on Type 1 aluminum substrates from Example 1 above. The aluminum substrate was coated with an organic layer which served to both planarize the aluminum surface and to provide an insulating coating for device fabrication upon it.
The challenge caused by the thermal expansion coefficient mismatch between the coated aluminum substrate and PECVD gate dielectrics deposited at 270° C., which resulted in stress and adhesion problems, was overcome. This problem was solved by RF-sputtering at room temperature a thin SiO2 layer on top of the organic coating of the aluminum substrate before the onset of TFT fabrication.
The TFT device structure, illustrated in
Mobility was extracted from the maximum transconductance at VDS=0.1 V. TFTs with a 26 μm channel length and 40 μm channel width displayed an average field effect mobility of 8.6 cm2 V-1s-1 (maximum of 13.3), threshold voltage of about 5 V, minimum off current less than 1 pA, and an on-off current ratio of more than 107 at Vds=+10 V (maximum of more than 108). Prior to the final thermal annealing, the TFTs exhibited no modulation and a high current due to the high conductivity of the IGZO Film.
While various embodiments of the present disclosure have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.
This patent application claims priority to U.S. Provisional Patent Application No. 61/986,640, filed Apr. 30, 2014, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61986640 | Apr 2014 | US |
