US20160045953A1

US20160045953A1 - Flexible metallic glass substrate with high resilience, manufacturing method thereof, and electronic device using same

Info

Publication number: US20160045953A1
Application number: US14/828,232
Authority: US
Inventors: Eun Soo Park; Wan Kim; Jinwoo Kim; Chae Woo RYU
Original assignee: Seoul National University R&DB Foundation; SK Innovation Co Ltd
Current assignee: SK Innovation Co Ltd; SNU R&DB Foundation
Priority date: 2014-08-18
Filing date: 2015-08-17
Publication date: 2016-02-18
Also published as: KR20160021579A

Abstract

Disclosed herein is a flexible substrate, made of metallic glass that is of high resilience suitable for use in electronic devices. The metallic glass is composed of a commercial alloy that can be produced in a continuous process on a mass scale, and may be selected from among Mg-, Ca-, Al-, Ti-, Zr-, Hf-, Fe-, Co-, Ni-, and Cu-based metallic glass. Preferably, its crystallization temperature, which determines the process allowable temperature, is 200° C. or higher. The flexible metallic glass substrate exhibits excellent fatigue properties as well as resilience of 1.5 MJ/m³or higher. Its coefficient of thermal expansion is within a small range of 1 to 20 ppm/° C., so that the flexible metallic glass substrate shows a better interfacial property with electronic devices.

Description

CROSS-REFERENCES TO RELATED APPLICATION

The present application claims priority of Korean Patent Application No. 10-2014-0107023, filed on Aug. 18, 2014, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a flexible substrate, a manufacturing method thereof, and an electronic device using the same. Particularly, the present invention relates to flexible metallic glass substrate that exhibits excellent fatigue properties as well as high resilience and process allowable temperature.
2. Description of the Related Art
In recent years, intensive efforts have been directed toward the use of flexible substrates in electronics and electronic devices. Advantageously, the application of flexible substrates makes it possible to perform processes continuously, as in a roll-to-roll process, instead of conventional discontinuous processes. Further, with the growth of demand for flexible electronics, attention has been focused on flexible substrates and electronic devices using the same.
As typical flexible substrates, polymer materials and iron-based metal materials, such as stainless steel (SUS), are currently used.
Polymers are light and exhibit excellent ductility. In addition, polymers have an elastic limit of at least 2%, which is relatively higher than that of other materials, and are not limited as to shape or thickness. Thanks to these advantages, polymers are suitable for use as flexible substrates. On the other hand, polymers are low in phase transition temperature, such as melting point (T_m) and glass transition temperature (T_g), limiting the temperatures available for processes of fabricating electronic devices to within 100 to 300° C. Further, since polymers are generally vulnerable to oxygen and water, an additional waterproof layer is required in order to ensure the durability of electronics.
In contrast, crystalline metal foil, such as SUS, is stable at high temperatures, extending the range of temperatures available for electronic device fabrication processes to up to 1,000° C. In addition, its relatively small coefficient of thermal expansion is advantageous for the formation of good interfaces with electronic devices. Crystalline metal foil is highly durable not only because it does not allow the permeation of water and oxygen thereto, but also because it has good impact resistance. However, a metal substrate with a thickness of 15 to 150 μm, used in flexible substrates, has a surface roughness of hundreds of nanometers or higher due to the manufacturing method thereof. Crystalline metal plates manufactured by rolling, for example, have rolling marks thereon, and cannot avoid defects causative of surface roughness, such as grain boundaries, due to the properties of the crystalline materials themselves. For a thick metal film formed on a glass substrate by deposition, surface roughness increases with thickness. Accordingly, it is difficult to achieve fine surface roughness on a metal film because surface roughness varies depending on the deposition method and conditions. Moreover, because of its low elastic limit of 0.5% or less, crystalline metal foil is apt to undergo plastic deformation even under small bending and thus to have surface defects such as wrinkles, which further deteriorate the surface roughness. The application of a polymer resin layer to crystalline metal substrates has been suggested in order to planarize and insulate the metal substrates. However, the presence of defects on the crystalline substrate requires the formation of a relatively thick polymer layer. In addition, the metal substrate retains the intrinsic problem whereby the metal substrate readily allows the occurrence of surface defects thereon because, whether coated with the polymer layer or not, it undergoes plastic deformation even when only slightly bent due to its low elastic limit of 0.5% or less. Alternatively, other efforts including polishing and alternative processing have been made with the goal of planarizing the surfaces of crystalline metal substrates, but in spite of all attempts, the physical limits of crystalline materials have not yet permitted results of a satisfactory level.

Document of Related Art

Patent Document

(Patent Document 1) Korean Patent Unexamined Application Publication No. 2009-0114195
(Patent Document 2) Korean Patent Unexamined Application Publication No. 2006-0134934
(Patent Document 3) Korean Patent Unexamined Application Publication No. 2004-0097228
(Patent Document 4) Korean Patent Unexamined Application Publication No. 2008-0024037
(Patent Document 5) Korean Patent Unexamined Application Publication No. 2009-0123164
(Patent Document 6) Korean Patent Unexamined Application Publication No. 2008-0065210
(Patent Document 7) Korean Patent No. 1271864
(Patent Document 8) Korean Patent Unexamined Application Publication No. 2013-0026007

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a flexible metallic glass substrate with high resilience, provided with the advantages of both conventional polymer materials and crystalline metal materials, and a method for manufacturing the same.
In order to accomplish the above object, an aspect of the present invention is to provide a flexible substrate for use in an electronic device, wherein the flexible substrate is made of metallic glass having high resilience.
In one embodiment of the present invention, the metallic glass preferably has a crystallization temperature (T_x) of 200° C. or higher at up to which a process of fabricating an electronic device using the flexible substrate can be conducted, and the metallic glass may be based on the IIA group element Mg or Ca, the IIIA group element Al, or the transition metal Ti, Zr, Hf, Fe, Co, Ni or Cu, which are all commercially available and suitable for use in a continuous process on a mass scale.
In another embodiment, the flexible substrate for use in electronic devices preferably has a thickness of at least 1 μm in order to support the electronic devices and up to 500 μm in order to avoid the ductile-brittle transition attributable to thickness.
For maximum elastic recovery, the flexible substrate has a yield strain of 1.5% or higher and ranges in strength from 0.3 to 5 GPa and in elastic modulus from 30 to 250 GPa. In a preferred embodiment, the flexible substrate has a resilience of 1.5 MJ/m³or higher.
In order for the flexible substrate to improve in interfacial properties with electronic devices, the flexible substrate ranges in coefficient of thermal expansion (CTE) from 1 to 20 ppm/° C., which is smaller than those of conventional substrates, and has a bending fatigue limit of 0.5% or higher.
In accordance with another aspect thereof, the present invention provides a method for manufacturing a flexible substrate having high resilience, comprising: preparing materials according to a composition of a metallic glass; and forming the materials into a metallic glass ribbon.
The formation of metallic glass ribbons may include a process of preparing metallic glass into a wide ribbon by controlling a melt spinneret nozzle, and a winding process, after which the resulting product is applicable to a roll-to-roll process and easy to carry and store.
The metallic glass ribbons are inosculated with each other or with a heterogeneous material in order to enlarge the area thereof. The area enlargement may be achieved by inosculating the metallic glass ribbon with a homogeneous ribbon or with a heterogeneous material through a thermo-plastic forming process.
In accordance with a further aspect thereof, the present invention provides method for manufacturing a flexible substrate for use in an electronic device, comprising: preparing materials according to a composition of a metallic glass; forming the materials into bulk metallic glass; and processing the bulk metallic glass into a thin plate.
Preferably, the bulk metallic glass can be processed into a thin plate suitable for use as a substrate by thermo-plastic forming process.
The method may further comprise planarizing a surface of the metallic glass substrate. In this regard, the substrate ranges in thickness of 500 μm or less in order to avoid the ductile-brittle transition attributable to thickness.
In accordance with still another aspect thereof, the present invention provides an electronic device fabricated with the flexible metallic glass substrate, and the electronic device may be a flexible device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph comparing the hardness and specific strength of a Zr-based metallic glass, manufactured according to one embodiment of the present invention, with those of other representative light alloys;

FIG. 2 is a schematic view comparing stress-strain curves of the flexible metallic glass manufactured according to an embodiment of the present invention with those of a conventional flexible SUS and polymer substrate;

FIG. 3 is an image showing the bending behavior of a wide metallic glass substrate manufactured according to an embodiment of the present invention;

FIG. 4 is a graph showing the strain-fatigue lifetime of the Zr-based metallic glass substrate manufactured according to an embodiment of the present invention, with inserts schematically representing ranges of fatigue strain of the substrate materials upon bending fatigue tests;

FIG. 5 is a graph showing the fatigue endurance limit of the Zr-based metallic glass substrate manufactured according to an embodiment of the present invention with those of SUS and Polymer substrate;

FIG. 6 is a view showing the correlation between the coefficient of thermal expansion and tensile modulus for the metallic glass substrate manufactured according to an embodiment of the present invention, a plastic, a typical crystalline metal material, Invar, and quartz;

FIG. 7 is a DSC analysis profile of the Zr-based metallic glass substrate manufactured according to an embodiment of the present invention;

FIG. 8 is photos showing bending procedure for Zr-based metallic glass substrate manufactured according to an embodiment of the present invention after annealing at 320° C. during 2 hours; and

FIG. 9 is graphs showing surface morphology and average roughness of metallic glass substrate manufactured according to an embodiment of the present invention before and after thermos-plastic forming process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The flexible metallic glass substrate in accordance with the present invention is described in detail with reference to the drawings.
The present invention addresses a flexible substrate having high resilience for use in electronic devices, wherein the flexible substrate is made of a commercial amorphous alloy that can be readily produced in a continuous process on a mass scale.
General metals or alloys form a crystalline structure, that is, a highly ordered structure occurring due to the intrinsic nature of its constituents to form symmetric patterns, whereas metallic glass (amorphous alloy) is a solid in which constituent atoms are in a disordered arrangement. That is, metallic glass is merely a structure in which the positions of constituent atoms in a liquid state are frozen as they are. Metallic glass is typically prepared by heating a master alloy to reach a liquid state and then quenching the molten alloy at a decrease rate of 10⁵to 10⁶K/sec into a solid. In spite of the same composition, metallic glass is different in physical properties from a crystalline alloy due to the structural difference therebetween. In consideration of the properties required for flexible substrates, the present inventor developed metallic glass substrate having high resilience by using commercial metal matrix metallic glass with high resilience as a material of the flexible substrate on the basis of the advantages and disadvantages of conventional polymer substrates and crystalline metal substrates.
First, an amorphous alloy is composed of multiple elements, that is, two or more elements, and typically more than three elements in order to improve glass-forming ability. In this regard, the amorphous alloy may be called “Zr-based metallic glass” on the basis of the main element having the highest percentage of the component elements.
Depending on the main constituent and the other constituent elements, the metallic glass varies in characteristic temperatures, such as the glass transition temperature (T_q), crystallization temperature (T_x), melting point (T_m), etc.
For metallic glass, the crystallization temperature is the maximum temperature at which the alloy still retains its amorphous properties while various processes can be performed. Because the crystallization temperature of metallic glass is higher than the maximum temperature available for flexible polymer substrates, metallic glass allows for the relatively easy fabrication of electronic devices without requiring improvements to existing fabrication processes or the development of new processes. Metallic glass that is available as a flexible substrate material is a commercial metal matrix amorphous alloy system having high resilience, which is suitable for use in a continuous process on a mass scale, and may be based on the IIA group element Mg or Ca, the IIIA group element Al, or the transition metal Ti, Zr, Hf, Fe, Co, Ni or Cu.
Metallic glass substrates having high flexibility and resilience according to the present invention are explained as follows.
First, free of defective regions existing in crystalline metal, such as grain boundaries, metallic glass exhibits a high strength (0.3 GPa˜5 GPa), which closely approximates the theoretical strength of materials for each alloy composition. As can be seen in FIG. 1, heavy Zr-based alloys, when amorphized, increase in strength and thus have high specific strength (strength/density), compared to various representative crystalline light alloys, such as Mg-, Al-, and Ti-based alloys. Materials that have high specific strength, such as these, can satisfy the load value required for a given product even when formed into a thinner film. Hence, a material with high specific strength is more suitable for use in flexible substrates.
In addition, the metallic glass substrate with flexibility in accordance with the present invention, as shown in FIG. 2, has a yield strain of 1.5% or higher, which is far superior to those of conventional crystalline SUS metal substrates (0.5% or less), so that the metallic glass substrate can be deformed at a level similar to polymer substrates. Also, the elastic modulus of the flexible metallic glass substrate in accordance with the present invention ranges from 30 to 250 GPa, and is lower than those of conventional crystalline SUS metallic substrates, which are approximately 250 GPa. Resilience is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading. On the basis of the mechanical properties, materials can be compared with regard to resilience in terms of the modulus of resilience (U=ρ_y ²/2E). It can be calculated by integrating the stress-strain curve from zero to the elastic limit. As is understood from the difference in areas under the stress-strain curves of FIG. 2, the metallic glass of the present invention has a far greater resilience than do conventional SUS or polymer materials, and can recover more quickly after elastic deformation.
FIG. 3 is a view illustrating the morphological change and surface shape of the metallic glass substrate of the present invention when it is artificially transformed. As described above, the metallic glass substrate is resilient enough to prevent the occurrence of surface defects thereon even at large deformation, and can maintain the original splendid appearance.
In order to examine the reliability of substrate in real service environment, the zirconium-based metallic glass substrate of the present invention was subjected to a bending fatigue test, as depicted in FIG. 4. All of the invented metallic glass substrates were found to have a fatigue limit of 0.5% or higher with regard to strain and to recover from the deformation even when they were repeatedly deformed under a strain greater than the elastic limit of SUS substrates, which means that they exhibit higher fatigue endurance limit than those of SUS and Polymer substrate as shown in FIG. 5. Accordingly, the zirconium-based metallic glass substrate of the present invention has excellent sustainability thanks to its high resilience and fatigue resistance. These material properties are summarized with conventional substrate materials, SUS 304 and 100HN Kapton, in Table 1, below.

TABLE 1

Zr₅₀Cu₄₀Al₁₀	Zr₆₀Cu₃₀Al₁₀	SUS	100HN

	Ribbon	Bulk	Ribbon	Bulk	304	Kapton^th

Yield strength (MPa)	1415	1860	1146	1720	290	69
Yield strain (%)	1.73	2.1	1.55	2.2	0.2	3
Young's modulus (GPa)	85	88	74	80	193	2.5
Fatigue-endurance limit (MPa)	712	752	486	—	240	40-60
Resilience (MJ/m³)	11.8	19.7	8.9	18.5	0.22	0.95

As shown in FIG. 6, the flexible metallic glass substrate of the present invention ranges in coefficient of thermal expansion (CTE) from 1 to 20 ppm/° C., which is small compared to those of conventional SUS or polymer substrates. Hence, the flexible metallic glass substrate of the present invention can stably endure electronic device fabrication processes and can increase production yield and sustainability under user environments, compared to conventional SUS or polymer substrates.
Moreover, the flexible metallic glass substrate of the present invention has a process allowable temperature of 200° C. or higher. As used herein, the term “process allowable temperature” refers to the maximum temperature allowable for a process of fabricating an electronic device using a flexible substrate. Having a process allowable temperature of 200° C. or higher, the flexible substrate of the present invention can be used in processes without requiring any additional improvement in the processes. Because the upper limit of the process allowable temperature of metallic glass, as shown in the differential thermal analysis curve of FIG. 7 for the Zr-based metallic glass substrate of the present invention, is determined by the crystallization temperature (T_x), at which the crystallization reaction initiates the release of energy upon heating, the upper limit varies depending on the material composition. Accordingly, when a high-temperature process is employed during the fabrication of an electronic device, a metallic glass material with a high crystallization temperature may be used as a substrate.
FIG. 8 shows bending procedure for Zr-based metallic glass substrate manufactured according to an embodiment of the present invention after annealing at 320° C. during 2 hours, which is higher than process allowable temperature of conventional polymer substrate and require to be used in electronic device manufacturing processes on the substrate without requiring any additional improvement in the processing procedure. As shown in FIG. 8 (a)-(d), the ribbon exhibit perfect elastic behavior even after high temperature annealing, which means that the metallic glass substrate of the present invention exhibit higher “process allowable temperature”.
As described above, the flexible metallic glass substrate having high resilience in accordance with the present invention is free of the disadvantages of conventional materials, but is provided with the advantages conferred by conventional materials. These material properties are summarized in Table 2, below.

TABLE 2

Plastic	SUS	Metallic Glass

Strength	<0.1 Gpa	~0.3 Gpa	0.3-5 Gpa
Thickness	Tens of μm-	tens of μm-	ones of μm-
	ones of mm	hundreds of μm	hundreds of μm
Yield
	2%<	<0.5%	1.5%<
strain
Elastic	2~4 GPa	~250 GPa	30~250 GPa
Modulus
CTE	~25 ppm/° C.	~20 ppm/° C.	1~20 ppm/° C.
Process
	100~300° C.	~1000° C.	200~800° C.
Temp.

The method for manufacturing the metallic glass substrate of the present invention will be described below.
As long as it is typically used to manufacture metallic glass, any method may be applied without limitation. For example, a method may be selected from among melt-spinning, injection casting, thermal plastic forming, water quenching, high-pressure die casting, copper mold casting, cap-casting, suction-casting, squeeze-casting, arc-melting, zone melting, (single or twin) roll casting, and mechanical alloying.
Metallic glass is broadly divided into metallic glass ribbon and bulk metallic glass, depending on how it is formed. When formed into a ribbon, the metallic glass is thin. On the other hand, bulk metallic glass needs to be processed to form a thin plate. For use in electronic devices, the flexible metallic glass substrate preferably has a thickness of at least 1 μm in order to support the electronic devices and up to 500 μm in order to avoid the ductile-brittle transition attributable to thickness.
The formation of metallic glass ribbons may include a process of preparing metallic glass into a wide ribbon by controlling a melt spinneret nozzle, for example using planar flow casting method, and a coil winding process in order to be applicable to a roll-to-roll process and easy to carry and store. When narrow, the metallic glass ribbons are laterally inosculated with each other or with a heterogeneous material in order to enlarge the area thereof.
No particular limitations are imposed on the formation of bulk metallic glass into thin plates or the area enlargement of metallic glass ribbon. So long as it is used in the art, any method may be applied. A representative process is a thermo-plastic forming process using a supercooled liquid region, a characteristic temperature range of metallic glass.
The thermo-plastic forming process is performed by preheating a material in a short period in a supercooled liquid temperature region from glass transition temperature to crystallization temperature and pressurizing the material, while maintaining amorphous state of the material. With this process, bulk metallic glass can be formed into a thin plate.
In addition, metallic glass ribbons may be partially overlapped with each other or with a heterogeneous material and pressurized to give an enlarged area in a supercooled liquid temperature region.
The flexible metallic glass substrate prepared in accordance with the present invention has a smoother surface than do conventional flexible metallic substrates, but for an even smoother surface, a planarization process may be performed on the flexible metallic glass substrate. FIG. 9 shows surface morphology and average roughness profile of metallic glass substrate manufactured according to an embodiment of the present invention before and after thermos-plastic forming process. The measured surface roughness of the metallic glass substrate by atomic force microscope changes from about 203 nm to 3.27 nm before and after thermo-plastic forming process. As described above, the metallic glass substrate can be smoother and planarization after thermos-plastic forming process.
In accordance with another aspect thereof, the present invention addresses an electronic device fabricated using the flexible metallic glass substrate of the present invention.
The electronic device is not particularly limited. The flexible metallic glass substrate of the present invention may be applicable to all possible electronic devices, as exemplified by lighting devices, display devices, thin film transistors, microprocessors, and solar cells. Particularly, the flexible metallic glass substrate is suitable for use in a flexible electronic device that is itself bendable.
Made of commercial amorphous metal alloys, as described above, the flexible metallic glass substrate for electronic devices exhibits high resilience and is provided with advantages conferred by polymer materials and metal materials, but is free of the disadvantages of conventional materials.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

What is claimed is:

1. A flexible substrate for use in an electronic device, wherein the flexible substrate is made of metallic glass having high resilience.

2. The flexible substrate of claim 1, wherein the metallic glass is a material selected from among Mg-, Ca-, Al-, Ti-, Zr-, Hf-, Fe-, Co-, Ni-, and Cu-based metallic glass.

3. The flexible substrate of claim 1, ranging in strength from 0.3 to 5 GPa and in elastic modulus from 30 to 250 GPa, and having a yield strain of 1.5 or higher.

4. The flexible substrate of claim 1, having a resilience of 1.5 MJ/m^Jor higher.

5. The flexible substrate of claim 1, wherein the metallic glass has a crystallization temperature of 200° C. or higher.

6. The flexible substrate of claim 1, ranging in thickness from 1 to 500 μm.

7. The flexible substrate of claim 1, ranging in coefficient of thermal expansion (CTE) from 1 to 20 ppm/° C.

8. The flexible substrate of claim 1, having a bending fatigue limit of 0.5% or higher.

9. A method for manufacturing a flexible substrate having high resilience, comprising:

preparing materials according to a composition of a metallic glass with high resilience; and

forming the materials into a metallic glass ribbon.

10. The method of claim 9, wherein the forming step is carried out by controlling a melt spinneret nozzle to form a metallic glass ribbon having a wide width.

11. The method of claim 9, further comprising winding the metallic glass ribbon.

12. The method of claim 9, further comprising inosculating the metallic glass ribbon with another to achieve area enlargement.

13. The method of claim 12, wherein the area enlargement is achieved by inosculating the metallic glass ribbon with a homogeneous ribbon or with a heterogeneous material through a thermo-plastic forming process.

14. A method for manufacturing a flexible substrate for use in an electronic device, comprising:

preparing materials according to a composition of a metallic glass with high resilience;

forming the materials into bulk metallic glass; and

processing the bulk metallic glass into a thin plate.

15. The method of claim 14, wherein the forming step is carried out in a thermo-plastic forming process.

16. The method of claim 9, further comprising planarizing a surface of the substrate.

17. The method of claim 9, wherein the substrate ranges in thickness from 1 to 500 μm.

18. The method of claim 14, further comprising planarizing a surface of the substrate.

19. The method of claim 14, wherein the substrate ranges in thickness from 1 to 500 μm.

20. An electronic device, fabricated with the flexible substrate of claim 1.