Patent Publication Number: US-2021167338-A1

Title: Method and apparatus for producing flexible oled device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method and apparatus for producing a flexible OLED device. 
     BACKGROUND ART 
     A typical example of the flexible display includes a film which is made of a synthetic resin such as polyimide (hereinafter, referred to as “plastic film”), and elements supported by the plastic film, such as TFTs (Thin Film Transistors) and OLEDs (Organic Light Emitting Diodes). The plastic film functions as a flexible substrate. The flexible display is encapsulated with a gas barrier film (encapsulation film) because an organic semiconductor layer which is a constituent of the OLED is likely to deteriorate due to water vapor. 
     Production of the above-described flexible display is carried out using a glass base on which a plastic film is formed over the upper surface. The glass base functions as a support (carrier) for keeping the shape of the plastic film flat during the production process. Elements such as TFTs and OLEDs, a gas barrier film, and the other constituents are formed on the plastic film, whereby the structure of a flexible device is realized while it is supported by the glass base. Thereafter, the flexible device is delaminated from the glass base and gains flexibility. The entirety of a portion in which elements such as TFTs and OLEDs are arrayed can be referred to as “functional layer region”. 
     According to the prior art, a sheet-like structure including a plurality of flexible devices is delaminated from a glass base, and thereafter, optical parts and other constituents are mounted to this sheet-like structure. Thereafter, the sheet-like structure is divided into a plurality of flexible devices. 
     Patent Document No. 1 discloses the method of irradiating the interface between each flexible device and the glass base with laser light (lift-off light) in order to delaminate each flexible device from the glass base (supporting substrate). According to the method disclosed in Patent Document No. 1, after irradiation with the lift-off light, respective flexible devices are divided from one another, and each of the flexible devices is delaminated from the glass base. 
     CITATION LIST 
     Patent Literature 
     Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2014-48619 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to research conducted by the present inventors, a flexible OLED device produced by a conventional production method does not have sufficient moisture resistance. 
     The present disclosure provides a method and apparatus for producing a flexible OLED device which are capable of solving the above-described problems. 
     Solution to Problem 
     A flexible OLED device production method of the present disclosure includes, in an exemplary embodiment, providing a multilayer stack, the multilayer stack including a glass base, a functional layer region including a TFT layer and an OLED layer, and a synthetic resin film provided between the glass base and the functional layer region and bound to the glass base; and separating the multilayer stack into a first portion and a second portion in a dry gas atmosphere whose dew point is not more than −50° C., thereby exposing a surface of the synthetic resin film to the dry gas atmosphere, the first portion including the functional layer region and the synthetic resin film, the second portion including the glass base; and transporting the first portion of the multilayer stack from the dry gas atmosphere to a reduced-pressure atmosphere and forming a protection layer on the surface of the synthetic resin film in the reduced-pressure atmosphere. 
     In one embodiment, forming a protection layer on the surface of the synthetic resin film in the reduced-pressure atmosphere includes forming a layer of a dielectric and/or electric conductor on the surface of the synthetic resin film by physical vapor deposition. 
     In one embodiment, the protection layer includes a metal layer. 
     In one embodiment, the metal layer is made of aluminum or copper. 
     In one embodiment, the metal layer is deposited so as to have a thickness based on a surface roughness of the surface of the synthetic resin film. 
     In one embodiment, a thickness of the metal layer is not less than 5 nm and not more than 200 nm. 
     In one embodiment, a thickness of the metal layer is more than 200 nm and not more than 1 μm. 
     In one embodiment, separating the multilayer stack into the first portion and the second portion includes irradiating an interface between the synthetic resin film and the glass base with laser light. 
     In one embodiment, separating the multilayer stack into the first portion and the second portion includes sliding a blade at an interface between the synthetic resin film and the glass base. 
     In one embodiment, separating the multilayer stack into the first portion and the second portion includes supplying an ion into the dry gas atmosphere using an ionizer. 
     In one embodiment, the method further includes, after forming the protection layer on the surface of the synthetic resin film in the reduced-pressure atmosphere, mounting an electronic part or an optical part to the first portion of the multilayer stack in an environmental atmosphere. 
     In one embodiment, the method further includes, before exposing a surface of the synthetic resin film to the dry gas atmosphere, adhering a protection sheet to the functional layer region. 
     In one embodiment, the functional layer region includes a plurality of functional layer regions, the synthetic resin film includes a plurality of flexible substrate regions respectively supporting the plurality of functional layer regions and an intermediate region surrounding the plurality of flexible substrate regions, and the method further includes, after forming the protection layer on the surface of the synthetic resin film, dividing the intermediate region and respective ones of the plurality of flexible substrate regions of the synthetic resin film from one another. 
     In one embodiment, dividing the intermediate region and respective ones of the plurality of flexible substrate regions of the synthetic resin film from one another precedes exposing a surface of the synthetic resin film to the dry gas atmosphere. 
     A flexible OLED device production apparatus of the present disclosure includes, in an exemplary embodiment, a lift-off unit which includes a stage for supporting a multilayer stack, the multilayer stack including a glass base, a functional layer region including a TFT layer and an OLED layer, and a synthetic resin film provided between the glass base and the functional layer region and bound to the glass base, the lift-off unit being capable of forming a dry gas atmosphere whose dew point is not more than −50° C. and separating the multilayer stack into a first portion and a second portion in the dry gas atmosphere, thereby exposing a surface of the synthetic resin film to the dry gas atmosphere, the first portion including the functional layer region and the synthetic resin film, the second portion including the glass base; and a surface treatment unit capable of forming a reduced-pressure atmosphere, receiving the first portion of the multilayer stack from the lift-off unit without exposing the first portion of the multilayer stack to atmospheric air, and forming a protection layer on the surface of the synthetic resin film in the reduced-pressure atmosphere. 
     In one embodiment, the apparatus further includes an ionizer for supplying an ion into the dry gas atmosphere. 
     In one embodiment, the lift-off unit includes a light source for irradiating an interface between the synthetic resin film and the glass base with laser light. 
     In one embodiment, the apparatus further includes a mechanism for sliding a blade at an interface between the synthetic resin film and the glass base. 
     Advantageous Effects of Invention 
     According to an embodiment of the present invention, the moisture resistance of a flexible OLED device is improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a plan view showing a configuration example of a multilayer stack used in a flexible OLED device production method of the present disclosure. 
         FIG. 1B  is a cross-sectional view of the multilayer stack taken along line B-B of  FIG. 1A . 
         FIG. 1C  is a cross-sectional view showing another example of the multilayer stack. 
         FIG. 1D  is a cross-sectional view showing still another example of the multilayer stack. 
         FIG. 2  is a diagram schematically showing a configuration example of a flexible OLED device production apparatus of the present disclosure. 
         FIG. 3A  is a diagram schematically showing a state immediately before a stage supports a multilayer stack. 
         FIG. 3B  is a diagram schematically showing a state where the stage supports the multilayer stack. 
         FIG. 3C  is a diagram schematically showing that the interface between a glass base and a plastic film of the multilayer stack with laser light (lift-off light) in the shape of a line. 
         FIG. 3D  is a diagram schematically showing the multilayer stack separated into the first portion and the second portion after irradiation with lift-off light. 
         FIG. 4  is a perspective view schematically showing irradiation with lift-off light. 
         FIG. 5A  is a diagram schematically showing evaporation of atoms or atom groups of a metal from a vapor deposition source. 
         FIG. 5B  is a diagram schematically showing a structure in which a protection layer is formed of a metal deposited on a surface of a plastic film. 
         FIG. 5C  is a diagram schematically showing a structure after an intermediate region and respective ones of a plurality of flexible substrate regions of the plastic film are divided from one another. 
         FIG. 6  is a diagram schematically showing an example where the “dividing” is carried out before the lift-off step. 
         FIG. 7  is another diagram schematically showing an example where the “dividing” is carried out before the lift-off step. 
         FIG. 8A  is a cross-sectional view illustrating a step of the flexible OLED device production method in an embodiment of the present disclosure. 
         FIG. 8B  is a cross-sectional view illustrating a step of the flexible OLED device production method in an embodiment of the present disclosure. 
         FIG. 8C  is a cross-sectional view illustrating a step of the flexible OLED device production method in an embodiment of the present disclosure. 
         FIG. 8D  is a cross-sectional view illustrating a step of the flexible OLED device production method in an embodiment of the present disclosure. 
         FIG. 9  is an equivalent circuit diagram of a single sub-pixel in a flexible OLED device. 
         FIG. 10  is a perspective view of the multilayer stack in the middle of the production process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Conventionally, a surface of a plastic film delaminated from a glass base by irradiation with lift-off light is thereafter exposed to the atmospheric air. According to experiments by the present inventors, it was found that if the surface of the plastic film immediately after being delaminated is brought into contact with the atmospheric air even in a short time period, the surface adsorbs and absorbs water vapor in the atmospheric air and, after that, the absorbed water vapor can affect the reliability of an OLED device. Particularly, the polyimide film has high hygroscopicity. When a polyimide film which has a thickness of about 15 μm is placed in an environmental atmosphere whose temperature is 23° C. and relative humidity is 50%, the polyimide film absorbs water vapor in the atmosphere for several minutes to several tens of minutes and bends, and the moisture permeates the polyimide film. The moisture permeating the polyimide film that functions as a flexible substrate can function as a source of moisture during the operation of the OLED device and therefore deteriorates the reliability of the OLED device and can shorten the product life. 
     In a method and apparatus for producing a flexible OLED device of the present disclosure, for the purpose of solving such problems, a plastic film is delaminated in a dry atmosphere and, thereafter, a surface treatment on the plastic film is carried out without exposing the plastic film to the atmospheric air. Thus, the reliability of the flexible OLED device can be improved, and the product life can be extended. 
     Hereinafter, an embodiment of the present disclosure is described. In the following description, unnecessarily detailed description will be omitted. For example, detailed description of well-known matter and repetitive description of substantially identical elements will be omitted. This is for the purpose of avoiding the following description from being unnecessarily redundant and assisting those skilled in the art to easily understand the description. The present inventors provide the attached drawings and the following description for the purpose of assisting those skilled in the art to fully understand the present disclosure. Providing these drawings and description does not intend to limit the subject matter recited in the claims. 
     EMBODIMENT 
     The outline of a method and apparatus for producing a flexible OLED device of the present disclosure is described with reference to the drawings. 
     See  FIG. 1A  and  FIG. 1B . In a flexible OLED device production method of the present embodiment, firstly, a multilayer stack  100  illustrated in  FIG. 1A  and  FIG. 1B  is provided.  FIG. 1A  is a plan view of the multilayer stack  100 .  FIG. 1B  is a cross-sectional view of the multilayer stack  100  taken along line B-B of  FIG. 1A . In  FIG. 1A  and  FIG. 1B , an XYZ coordinate system with X-axis, Y-axis and Z-axis, which are perpendicular to one another, is shown for reference. 
     The multilayer stack  100  includes a glass base (motherboard or carrier)  10 , a plurality of functional layer regions  20  each including a TFT layer  20 A and an OLED layer  20 B, and a synthetic resin film (hereinafter, simply referred to as “plastic film”)  30  provided between the glass base  10  and the plurality of functional layer regions  20  and bound to the glass base  10 . The multilayer stack  100  further includes a gas barrier film  40  and a protection sheet  50  covering the entirety of the plurality of functional layer regions  20 . The multilayer stack  100  may include another unshown layer, such as a buffer layer. 
     The first surface  100   a  of the multilayer stack  100  is defined by the glass base  10 . The second surface  100   b  of the multilayer stack  100  is defined by the protection sheet  50 . The glass base  10  and the protection sheet  50  are materials temporarily used in the production process but are not constituents of a final flexible OLED device. 
     The plastic film  30  shown in the drawings includes a plurality of flexible substrate regions  30   d  respectively supporting the plurality of functional layer regions  20 , and an intermediate region  30   i  surrounding each of the flexible substrate regions  30   d . The flexible substrate regions  30   d  and the intermediate region  30   i  are merely different portions of a single continuous plastic film  30  and do not need to be physically distinguished. In other words, regions of the plastic film  30  lying immediately under respective ones of the functional layer regions  20  are the flexible substrate regions  30   d , and the other region of the plastic film  30  is the intermediate region  30   i.    
     Each of the plurality of functional layer regions  20  is a constituent of a final flexible OLED device. In other words, the multilayer stack  100  has such a structure that a plurality of flexible OLED devices which are not yet divided from one another are supported by a single glass base. Each of the functional layer regions  20  has such a shape that, for example, the thickness (size in Z-axis direction) is several tens of micrometers, the length (size in X-axis direction) is about 12 cm, and the width (size in Y-axis direction) is about 7 cm. These sizes can be set to arbitrary values according to the required largeness of the display screen. The shape in the XY plane of each of the functional layer regions  20  is rectangular in the example illustrated in the drawings but is not limited to this example. The shape in the XY plane of each of the functional layer regions  20  may include a square, a polygon, or a shape which includes a curve in the contour. 
     As shown in  FIG. 1A , the flexible substrate regions  30   d  are two-dimensionally arrayed in rows and columns. The intermediate region  30   i  consists of a plurality of stripes perpendicular to one another and forms a grid pattern. The width of the stripes is, for example, about 1-4 mm. The flexible substrate region  30   d  of the plastic film  30  functions as the “flexible substrate” in each flexible OLED device which is in the form of a final product. Meanwhile, the intermediate region  30   i  of the plastic film  30  is not a constituent of the final product. 
     In an embodiment of the present disclosure, the configuration of the multilayer stack  100  is not limited to the example illustrated in the drawings. The number of functional layer regions  20  supported by a single glass base  10  does not need to be plural but may be singular. If the number of functional layer regions  20  is singular, the intermediate region  30   i  of the plastic film  30  forms a simple frame pattern surrounding a single functional layer region  20 . 
     The size or proportion of each component illustrated in respective drawings is determined from the viewpoint of understandability. The actual size or proportion is not necessarily reflected in the drawings. 
     The multilayer stack  100  which can be used in the production method of the present disclosure is not limited to the example illustrated in  FIG. 1A  and  FIG. 1B .  FIG. 1C  and  FIG. 1D  are cross-sectional views showing other examples of the multilayer stack  100 . In the example illustrated  FIG. 1C , the protection sheet  50  covers the entirety of the plastic film  30  and extends outward beyond the plastic film  30 . In the example illustrated  FIG. 1D , the protection sheet  50  covers the entirety of the plastic film  30  and extends outward beyond the glass base  10 . As will be described later, after the glass base  10  is separated from the multilayer stack  100 , the multilayer stack  100  is a thin flexible sheet-like structure which has no rigidity. The protection sheet  50  serves to protect the functional layer regions  20  from impact and abrasion when the functional layer regions  20  collide with or come into contact with external apparatuses or instruments in the step of delaminating the glass base  10  and the steps after the delaminating. Since the protection sheet  50  is peeled off from the multilayer stack  100  in the end, a typical example of the protection sheet  50  has a laminate structure which includes an adhesive layer of a relatively small adhesive force (a layer of an applied mold-releasing agent) over its surface. 
     The more detailed description of the multilayer stack  100  will be described later. 
     See  FIG. 2 .  FIG. 2  is a diagram schematically showing a configuration example of a flexible OLED device production apparatus  200  of the present disclosure. The production apparatus  200  shown in the drawing includes a lift-off unit  210  and a surface treatment unit  220 . In the example illustrated in  FIG. 2 , the lift-off unit  210  and the surface treatment unit  220  are in communication with each other via a transportation space  230 . A part or the entirety of the transportation space  230  functions as a load lock chamber of the surface treatment unit  220 . 
     The lift-off unit  210  includes a stage  212  for supporting the multilayer stack  100 .  FIG. 3A  is a diagram schematically showing a state immediately before the stage  212  supports the multilayer stack  100 . The multilayer stack  100  is arranged such that the second surface  100   b  of the multilayer stack  100  faces the stage  212 . The multilayer stack  100  is supported by the stage  212 . 
       FIG. 3B  schematically shows a state where the stage  212  supports the multilayer stack  100 . The arrangement of the stage  212  and the multilayer stack  100  is not limited to the example illustrated in the drawing. For example, the multilayer stack  100  may be placed upside down such that the stage  212  is present under the multilayer stack  100 . 
     A form which allows the stage  212  to support the multilayer stack  100  is also arbitrary. The surface of the stage  212  may hold the multilayer stack  100  by suction using a vacuum chuck. Alternatively, a securing device (not shown) attached to the stage  212  may mechanically hold the multilayer stack  100 . In the example illustrated in  FIG. 3B , the second surface  100   b  of the multilayer stack  100  is in contact with the stage  212  and is adhered by suction to the stage  212 . In this example, the stage  212  functions as a “chuck stage”. 
     The lift-off unit  210  forms a dry gas atmosphere D whose dew point is not more than −50° C. and separates the multilayer stack  100  in the dry gas atmosphere D into two portions (the first portion  110  and the second portion  120  of  FIG. 3D ) as will be described later. 
       FIG. 3C  schematically illustrates irradiation of the interface between the glass base  10  and the plastic film  30  of the multilayer stack  100  with laser light (lift-off light) in the shape of a line extending in a direction vertical to the sheet of the drawing. A part of the plastic film  30  at the interface between the glass base  10  and the plastic film  30  absorbs the lift-off light and decomposes (disappears). By scanning the above-described interface with the lift-off light, the degree of binding of the plastic film  30  to the glass base  10  is reduced. The wavelength of the lift-off light is typically in the ultraviolet band. The wavelength of the lift-off light is selected such that the lift-off light is hardly absorbed by the glass base  10  but is absorbed by the plastic film  30  as much as possible. The light absorption by the glass base  10  is, for example, about 10% in the wavelength range of 343-355 nm but can increase to 30-60% at 308 nm. 
     When the lift-off unit  210  is a laser lift-off (LLO) unit for carrying out lift-off by irradiation with laser light (lift-off light), the lift-off unit  210  includes a line beam source  214 . The line beam source  214  includes a laser device and an optical system for shaping laser light emitted from the laser device into a line beam. 
       FIG. 4  is a perspective view schematically showing irradiation of the multilayer stack  100  with a line beam emitted from the line beam source  214 . For the sake of understandability, the stage  212 , the multilayer stack  100  and the line beam source  214  are shown as being spaced away from one another in the Z-axis direction of the drawing. During irradiation with the lift-off light, the multilayer stack  100  is in contact with the stage  212 . 
     Examples of the laser device include gas laser devices such as excimer laser, solid laser devices such as YAG laser, semiconductor laser devices, and other types of laser devices. A XeCl excimer laser device can generate laser light at the wavelength of 308 nm. When yttrium orthovanadate (YVO 4 ) doped with neodymium (Nd) or YVO 4  doped with ytterbium (Yb) is used as a lasing medium, the wavelength of laser light (fundamental wave) emitted from the lasing medium is about 1000 nm. Therefore, the fundamental wave can be converted by a wavelength converter to laser light at the wavelength of 340-360 nm (third harmonic wave) before it is used. 
     The irradiation with the lift-off light can be carried out with the power density (irradiance) of, for example, 250-300 mJ/cm 2 . The lift-off light in the shape of a line beam has a size which can extend across the glass base  10 , i.e., a line length which exceeds the length of one side of the glass base (long axis dimension, size in Y-axis direction of  FIG. 4 ). The line length can be, for example, not less than 750 mm. Meanwhile, the line width of the lift-off light (short axis dimension, size in X-axis direction of  FIG. 4 ) can be, for example, about 0.2 mm. These dimensions represent the size of the irradiation region at the interface between the plastic film  30  and the glass base  10 . The lift-off light can be emitted in the form of a pulsed or continuous wave. Irradiation with the pulsed wave can be carried out at the frequency of, for example, about 200 times per seconds. 
     The position of irradiation with the lift-off light moves relative to the glass base  10  for scanning with the lift-off light. In the lift-off unit  210 , the multilayer stack  100  may be movable while the light source from which the lift-off light is to be emitted and an optical unit are stationary and, alternatively, the light source may be movable while the multilayer stack  100  is stationary. In the example illustrated in  FIG. 4 , scanning with the lift-off light is carried out in the X-axis direction. 
       FIG. 3D  is a diagram schematically showing the multilayer stack  100  separated into the first portion  110  and the second portion  120  after irradiation with lift-off light. The first portion  110  includes functional layer regions  20  and a plastic film  30 . A final flexible OLED device is produced from the first portion  110 . Meanwhile, the second portion  120  includes a glass base  10 . Although a substantial constituent of the second portion  120  is the glass base  10 , a part of the plastic film  30  adhered to the glass base  10  may be included in the second portion. 
     Separating the multilayer stack  100  into the first portion  110  and the second portion  120  means delaminating the glass base  10  from the multilayer stack  100 . As a result of this separation (delamination), a surface (delamination surface)  30   s  of the plastic film  30  is exposed to the dry gas atmosphere D. As previously described, the plastic film  30  such as polyimide film has high hygroscopicity, but the vapor pressure of the dry gas atmosphere D whose dew point is not more than −50° C. is extremely low. Specifically, when the dew point is not more than −50° C., the mass of water vapor which can be present in a space of one cubic meter is not more than 0.0382 grams. That is, the absolute humidity of the dry gas atmosphere D is not more than 0.0382 g/m 3 . When the temperature of the dry gas atmosphere D is 25° C. and the pressure is 1 atm, the relative humidity is not more than 0.2% RH. In such a dry atmosphere gas, even the surface  30   s  of the plastic film  30  immediately after the delamination scarcely absorbs water vapor. 
     To form the dry gas atmosphere D, the lift-off unit  210  is connected with a dry gas supplying unit  240  as shown in  FIG. 2 . A typical example of a gas that is a constituent of the dry gas is nitrogen. A part or the entirety of the dry gas atmosphere may be an inert gas such as argon or may contain a gaseous constituent which is a constituent of the atmospheric air, such as oxygen and carbon dioxide. The dry gas supplying unit  240  includes an unshown dehumidification rotor for dehumidification with the supply of a gas or atmospheric air which is not yet dehumidified from an external environment. The configuration of the dry gas supplying unit  240  is not particularly limited so long as the dry gas supplying unit  240  can generate a dry gas whose dew point is not more than −50° C. 
     The lift-off unit  210  does not need to be a LLO unit which performs lift-off by irradiation with laser light (lift-off light). The lift-off unit  210  may have a mechanical delamination (mechanical lift-off: MLO) mechanism which is capable of inserting a blade into the interface between the plastic film  30  and the glass base  10  and sliding the blade along the interface. When the delamination is carried out with a blade, the surface roughness of the surface  30   s  of the plastic film  30  is relatively low, and the surface  30   s  is smooth as compared with a case where the delamination is carried out by irradiation with lift-off light. When the delamination is carried out by irradiation with lift-off light, there are residues (ashes) of laser abrasion on the surface  30   s  of the plastic film  30  and, therefore, the surface roughness has a tendency to increase. As the surface roughness increases, the surface area increases, and the surface  30   s  becomes active. Thus, disadvantageously, the surface  30   s  is likely to adsorb water vapor in the atmospheric air. 
     Even when the delamination is carried out by irradiation with lift-off light, providing a delamination layer (sacrificial layer) between the plastic film  30  and the glass base  10  can improve the smoothness of the surface  30   s  of the plastic film  30  after the delamination. As the smoothness of the surface  30   s  of the plastic film  30  becomes higher, the thickness of a protection layer to be formed can advantageously be reduced. 
     When the delamination is carried out in the dry gas atmosphere D, static electricity is likely to occur because the atmosphere is dry. The lift-off unit  210  may include an ionizer for removal of electricity. The ionizer functions as a static electricity removing device. When the delamination surface gradually increases between the first portion  110  and the second portion  120  of the multilayer stack  100 , the largest static electricity is likely to occur at the position where the plastic film  30  is delaminated from the glass base (delamination beginning position). When the glass base  10  is large, moving the position of the ionizer according to movement of the delamination beginning position is effective. 
     The inside of the transportation space  230  can form a dry gas atmosphere D whose dew point is not more than −50° C. as can the inside of the lift-off unit  210 . The pressure inside the transportation space  230  is once reduced when the first portion  110  of the multilayer stack  100  is transported into the surface treatment unit  220 . Openable and closable partitions are provided between the transportation space  230  and the lift-off unit  210  and between the transportation space  230  and the surface treatment unit  220 . Each of the lift-off unit  210  and the surface treatment unit  220  can form a space temporarily separated from the transportation space  230 . 
     The surface treatment unit  220  of  FIG. 2  receives the first portion  110  of the multilayer stack  100  via the transportation space  230 . The surface treatment unit  220  generates a reduced-pressure atmosphere R and performs a surface treatment on the plastic film  30  of the first portion  110  in the reduced-pressure atmosphere R. One of specific examples of this surface treatment is vapor deposition of a metal. 
       FIG. 5A  is a diagram schematically showing evaporation of atoms or atom groups of a metal from a vapor deposition source  224 . The first portion  110  of the multilayer stack  100  is supported by a stage  222 . The first portion  110  of the multilayer stack  100 , which has been separated from the glass base  10 , lacks rigidity. Therefore, the stage  222  may have a mechanism of maintaining the first portion  110  of the multilayer stack  100  horizontal by applying tension to, for example, the edges of the protection sheet  50  in a horizontal direction. The stage  222  may tightly adhere to the protection sheet  50  via an electrostatic chuck or adhesive material (adhesive agent or the like) so as to hold the first portion  110  of the multilayer stack  100 . 
     The stage  222  may be a different stage from the stage  212  of the lift-off unit  210 . Alternatively, the stage  222  may be the stage  212  transported from the lift-off unit  210  into the surface treatment unit  220  via the transportation space  230 . When the same stage is used, an example of the stage  212  ( 222 ) is a stage which is capable of holding the first portion  110  using an electrostatic chuck or adhesive material, or the like. When different stages are used, an example of the stage  212  is a chuck stage, while an example of the stage  222  is a stage which is capable of holding the first portion  110  using an electrostatic chuck or adhesive material, or the like. 
     When the stage  222  and the stage  212  are different stages, the first portion  110  of the multilayer stack  100  which lacks rigidity after removal of the glass base  10  as shown in  FIG. 3D  needs to be moved from the stage  212  to the stage  222 . This moving needs to be carried out in the dry gas atmosphere D such that the first portion  110  is not exposed to the atmospheric air. To carry out an efficient process, it is desirable that the stage  212  of the lift-off unit  210  is transported into the surface treatment unit  220  via the transportation space  230  while the stage  212  keeps supporting the first portion  110  of the multilayer stack  100 , and is then used as the stage  222  for the surface treatment. 
     Atoms or atom groups of a metal evaporated from the vapor deposition source  224  so as to collide with the surface  30   s  of the plastic film  30  are deposited on the surface  30   s  of the plastic film  30 . 
       FIG. 5B  is a diagram schematically showing a structure in which a protection layer  60  is formed of a metal deposited on the surface  30   s  of the plastic film  30 . The thickness of the protection layer  60  is adjusted according to the product of the deposition rate and the deposition duration. 
     As shown in  FIG. 2 , the surface treatment unit  220  is connected with a pressure reducing device  250 , such as vacuum pump, for forming the reduced-pressure atmosphere R. When the protection layer  60  is formed by vacuum vapor deposition, the pressure of the reduced-pressure atmosphere R is, for example, from 10 −4  Pa to 10 −1  Pa. 
     An example of the surface treatment unit  220  is a vacuum vapor deposition unit of a resistance heating or electron beam heating system. In the case of a resistance heating system, the protection layer  60  can be formed by depositing a metal, such as aluminum (Al), gold (Au), silver (Ag), copper (Cu), on the surface  30   s  of the plastic film  30 . The resistance heating system is a simple device and achieves a high deposition rate, but is limited to materials with which a sufficient vapor pressure can be achieved at a relatively low temperature such as previously described. Therefore, vapor deposition of a refractory metal whose melting point is higher than 1500° C. with the use of the resistance heating system is difficult. The electron beam heating system is capable of deposition of a refractory metal, such as titanium (Ti), molybdenum (Mo), tungsten (W), on the surface  30   s  of the plastic film  30 , thereby forming the protection layer  60 . Further, the electron beam heating system is excellent in controlling the deposition rate. 
     The thickness of the protection layer  60  formed on the surface  30   s  of the plastic film  30  can be determined according to the surface roughness of the plastic film  30 . If the surface roughness of the plastic film  30  is low so that the surface is smooth, the thickness of the protection layer  60  is, for example, not less than 5 nm and not more than 200 nm. On the other hand, if the surface roughness of the plastic film  30  is high so that the surface  30   s  is rough, the thickness of the protection layer  60  can be more than 200 nm and not more than 1 μm. The thickness of the protection layer  60  can be set in the above-described range from the viewpoint of suppressing absorption of moisture by the plastic film  30 , but can be set in a range of, for example, more than 1 μm and not more than 5 μm when the function of further improving the heat dissipation is added. To improve the heat dissipation, it is preferred that the protection layer  60  is made of a metal of high heat conductivity, e.g., Ag, Cu, Al, or the like. The heat conductivity of Cu is 403 W/m·K, which is higher than the heat conductivity of Al (236 W/m·K). When a vacuum vapor deposition unit of a resistance heating system is used, forming the protection layer  60  of Al and/or Cu, which have relatively low melting points and are inexpensive, is easy. 
     Another example of the surface treatment unit  220  is a sputtering device. The method of depositing a layer of Al and/or Cu on the surface  30   s  of the plastic film  30  may be a sputtering method. According to the sputtering method, uniform film formation can be easily realized with small deformation and variation as compared with the vacuum vapor deposition. According to the sputtering method, the kinetic energy of target atoms (molecules) is large as compared with the vacuum vapor deposition method and, therefore, a protection layer  60  which is unlikely to separate can be easily formed. 
     The protection layer  60  formed on the surface  30   s  of the plastic film  30  is not limited to a layer of a metal. The protection layer  60  can be a layer of various dielectrics and/or electric conductors which is formed on the surface  30   s  of the plastic film  30 . The protection layer  60  may be a multilayer film. The protection layer  60  may have, for example, a layer of an applied fluoric resin or silicone resin over its surface so long as it does not adversely affect the hygroscopicity. Such a resin layer has water repellency and, therefore, the moisture resistance of the plastic film  30  can improve. 
     The protection layer  60  may be a reformed surface layer of the plastic film  30 . The surface  30   s  of the plastic film  30  may be made hydrophobic by exposing the surface  30   s  of the plastic film  30  to, for example, a hydrocarbon gas plasma. The surface  30   s  of the plastic film  30  may be reformed by implanting argon ions in the surface  30   s  of the plastic film  30  such that the hygroscopicity decreases. The reformed surface  30   s  itself functions as a “protection layer” for the plastic film  30 . 
     When a “protection layer” is formed over the surface  30   s  of the plastic film  30  by the above-described surface treatment, the hygroscopicity of the plastic film  30  decreases. Therefore, even if exposed to the atmospheric air after the plastic film  30  is delaminated from the glass base  10 , the problem of decrease of the reliability of the OLED device which is attributed to the plastic film  30  can be solved. 
       FIG. 5C  is a diagram schematically showing a structure after an intermediate region  30   i  and respective ones of a plurality of flexible substrate regions  30   d  of the plastic film  30  are divided from one another. In the example illustrated in the drawing, the protection sheet  50  is also delaminated. Dividing (cutting) each of the flexible OLED devices  1000  from the first portion  110  of the multilayer stack  100  can be realized by irradiating borders to be divided with a laser beam for cutting. Such cutting can also be realized by a dicing saw instead of the laser beam irradiation. 
     Before the dividing, or after the dividing, various electronic or optical parts (not shown), such as driver integrated circuit, touchscreen, polarizer, heat dissipation sheet, electromagnetic shield, etc., can be mounted to each of the flexible OLED devices  1000  in, for example, the environmental atmosphere. 
       FIG. 6  and  FIG. 7  are diagrams schematically showing examples where the “dividing” is carried out before the lift-off step. In the example illustrated in the drawing, as shown in  FIG. 6 , before adhering the protection sheet  50 , the intermediate region  30   i  and respective ones of the plurality of flexible substrate regions  30   d  of the plastic film  30  are divided from one another by a laser beam. Thereafter, the protection sheet  50  is adhered to the multilayer stack  100 . Respective ones of the divided functional layer regions  20  are sandwiched between a single protection sheet  50  and the glass base  10 . Note that the method of the dividing is not limited to this example. 
     The multilayer stack  100  which has the above-described configuration is supported by the stage  212  as shown in  FIG. 7 , and the same process as that previously described can be carried out. After the glass base  10  is delaminated from the multilayer stack  100 , each of the functional layer regions  20  is still supported by the protection sheet  50 , and the protection sheet  50  functions as a carrier during the production process. 
     When a protection layer  60  is formed on the surface  30   s  of the plastic film  30  by the surface treatment unit  220 , this protection layer  60  can also perform the function of suppressing separation of each functional layer region  20  from the protection sheet  50 . 
     Multilayer Stack 
     Hereinafter, the configuration of the multilayer stack  100  is described in more detail. 
     First, see  FIG. 8A .  FIG. 8A  is a cross-sectional view showing the glass base  10  with the plastic film  30  provided on the surface of the glass base  10 . The glass base  10  is a supporting substrate for processes. The thickness of the glass base  10  can be, for example, about 0.3-0.7 mm. 
     In the present embodiment, the plastic film  30  is a polyimide film having a thickness of, for example, not less than 5 μm and not more than 100 μm. The polyimide film can be formed from a polyamide acid, which is a precursor of polyimide, or a polyimide solution. The polyimide film may be formed by forming a polyamide acid film on the surface of the glass base  10  and then thermally imidizing the polyamide acid film. Alternatively, the polyimide film may be formed by forming, on the surface of the glass base  10 , a film from a polyimide solution which is prepared by melting a polyimide or dissolving a polyimide in an organic solvent. The polyimide solution can be obtained by dissolving a known polyimide in an arbitrary organic solvent. The polyimide solution is applied to the surface  30   s  of the glass base  10  and then dried, whereby a polyimide film can be formed. 
     In the case of a bottom emission type flexible display, it is preferred that the polyimide film realizes high transmittance over the entire range of visible light. The transparency of the polyimide film can be represented by, for example, the total light transmittance in accordance with JIS K7105-1981. The total light transmittance can be set to not less than 80% or not less than 85%. On the other hand, in the case of a top emission type flexible display, it is not affected by the transmittance. 
     The plastic film  30  may be a film which is made of a synthetic resin other than polyimide. Note that, however, in the embodiment of the present disclosure, the process of forming a thin film transistor includes a heat treatment at, for example, not less than 350° C., and therefore, the plastic film  30  is made of a material which will not be deteriorated by this heat treatment. 
     The plastic film  30  may be a multilayer structure including a plurality of synthetic resin layers. In one form of the present embodiment, in delaminating a flexible display structure from the glass base  10 , laser lift-off is carried out such that the plastic film  30  is irradiated with ultraviolet laser light transmitted through the glass base  10 . A part of the plastic film  30  at the interface with the glass base  10  needs to absorb the ultraviolet laser light and decompose (disappear). Alternatively, for example, a sacrificial layer which is to absorb laser light of a certain wavelength band and produce a gas may be provided between the glass base  10  and the plastic film  30 . In this case, the plastic film  30  can be easily delaminated from the glass base  10  by irradiating the sacrificial layer with the laser light. 
     &lt;Polishing&gt; 
     When there is an object (target) which is to be polished away, such as particles or protuberances, on the surface  30   x  of the plastic film  30 , the target may be polished away using a polisher such that the surface becomes flat. Detection of a foreign object, such as particles, can be realized by, for example, processing of an image obtained by an image sensor. After the polishing process, a planarization process may be performed on the surface  30   x  of the plastic film  30 . The planarization process includes the step of forming a film which improves the flatness (planarization film) on the surface  30   x  of the plastic film  30 . The planarization film does not need to be made of a resin. 
     &lt;Lower Gas Barrier Film&gt; 
     Then, a gas barrier film may be formed on the plastic film  30 . The gas barrier film can have various structures. Examples of the gas barrier film include a silicon oxide film and/or a silicon nitride film. Other examples of the gas barrier film can include a multilayer film including an organic material layer and an inorganic material layer. This gas barrier film may be referred to as “lower gas barrier film” so as to be distinguishable from a gas barrier film covering the functional layer regions  20 , which will be described later. The gas barrier film covering the functional layer regions  20  can be referred to as “upper gas barrier film”. 
     &lt;Functional Layer Region&gt; 
     Hereinafter, the process of forming the functional layer regions  20 , including the TFT layer  20 A and the OLED layer  20 B, and the upper gas barrier film  40  is described. 
     First, as shown in  FIG. 8B , a plurality of functional layer regions  20  are formed on a glass base  10 . There is a plastic film  30  between the glass base  10  and the functional layer regions  20 . The plastic film  30  is bound to the glass base  10 . 
     More specifically, the functional layer regions  20  include a TFT layer  20 A (lower layer) and an OLED layer  20 B (upper layer). The TFT layer  20 A and the OLED layer  20 B are sequentially formed by a known method. The TFT layer  20 A includes a circuit of a TFT array which realizes an active matrix. The OLED layer  20 B includes an array of OLED elements, each of which can be driven independently. The thickness of the TFT layer  20 A is, for example, 4 μm. The thickness of the OLED layer  20 B is, for example, 1 μm. 
       FIG. 9  is a basic equivalent circuit diagram of a sub-pixel in an organic EL (Electro Luminescence) display. A single pixel of the display can consist of sub-pixels of different colors such as, for example, R (red), G (green), and B (blue). The example illustrated in  FIG. 9  includes a selection TFT element Tr 1 , a driving TFT element Tr 2 , a storage capacitor CH, and an OLED element EL. The selection TFT element Tr 1  is connected with a data line DL and a selection line SL. The data line DL is a line for transmitting data signals which define an image to be displayed. The data line DL is electrically coupled with the gate of the driving TFT element Tr 2  via the selection TFT element Tr 1 . The selection line SL is a line for transmitting signals for controlling the ON/OFF state of the selection TFT element Tr 1 . The driving TFT element Tr 2  controls the state of the electrical connection between a power line PL and the OLED element EL. When the driving TFT element Tr 2  is ON, an electric current flows from the power line PL to a ground line GL via the OLED element EL. This electric current allows the OLED element EL to emit light. Even when the selection TFT element Tr 1  is OFF, the storage capacitor CH maintains the ON state of the driving TFT element Tr 2 . 
     The TFT layer  20 A includes a selection TFT element Tr 1 , a driving TFT element Tr 2 , a data line DL, and a selection line SL. The OLED layer  20 B includes an OLED element EL. Before formation of the OLED layer  20 B, the upper surface of the TFT layer  20 A is planarized by an interlayer insulating film that covers the TFT array and various wires. A structure which supports the OLED layer  20 B and which realizes active matrix driving of the OLED layer  20 B is referred to as “backplane”. 
     The circuit elements and part of the lines shown in  FIG. 9  can be included in any of the TFT layer  20 A and the OLED layer  20 B. The lines shown in  FIG. 9  are connected with an unshown driver circuit. 
     In the embodiment of the present disclosure, the TFT layer  20 A and the OLED layer  20 B can have various specific configurations. These configurations do not limit the present disclosure. The TFT element included in the TFT layer  20 A may have a bottom gate type configuration or may have a top gate type configuration. Emission by the OLED element included in the OLED layer  20 B may be of a bottom emission type or may be of a top emission type. The specific configuration of the OLED element is also arbitrary. 
     The material of a semiconductor layer which is a constituent of the TFT element includes, for example, crystalline silicon, amorphous silicon, and oxide semiconductor. In the embodiment of the present disclosure, part of the process of forming the TFT layer  20 A includes a heat treatment step at 350° C. or higher for the purpose of improving the performance of the TFT element. 
     &lt;Upper Gas Barrier Film&gt; 
     After formation of the above-described functional layer, the entirety of the functional layer regions  20  is covered with a gas barrier film (upper gas barrier film)  40  as shown in  FIG. 8C . A typical example of the upper gas barrier film  40  is a multilayer film including an inorganic material layer and an organic material layer. Elements such as an adhesive film, another functional layer which is a constituent of a touchscreen, polarizers, etc., may be provided between the upper gas barrier film  40  and the functional layer regions  20  or in a layer overlying the upper gas barrier film  40 . Formation of the upper gas barrier film  40  can be realized by a Thin Film Encapsulation (TFE) technique. From the viewpoint of encapsulation reliability, the WVTR (Water Vapor Transmission Rate) of a thin film encapsulation structure is typically required to be not more than 1×10 −4  g/m 2 /day. According to the embodiment of the present disclosure, this criterion is met. The thickness of the upper gas barrier film  40  is, for example, not more than 2.0 μm. 
       FIG. 10  is a perspective view schematically showing the upper surface side of the multilayer stack  100  at a point in time when the upper gas barrier film  40  is formed. A single multilayer stack  100  includes a plurality of flexible OLED devices  1000  supported by the glass base  10 . In the example illustrated in  FIG. 10 , a single multilayer stack  100  includes a larger number of functional layer regions  20  than in the example illustrated in  FIG. 1A . As previously described, the number of functional layer regions  20  supported by a single glass base  10  is arbitrary. 
     &lt;Protection Sheet&gt; 
     Next, refer to  FIG. 8D . As shown in  FIG. 8D , a protection sheet  50  is adhered to the upper surface of the multilayer stack  100 . The protection sheet  50  can be made of a material such as, for example, polyethylene terephthalate (PET), polyvinyl chloride (PVC), or the like. As previously described, a typical example of the protection sheet  50  has a laminate structure which includes a layer of an applied mold-releasing agent at the surface. The thickness of the protection sheet  50  can be, for example, not less than 50 μm and not more than 150 μm. 
     After the thus-formed multilayer stack  100  is provided, the production method of the present disclosure can be carried out using the above-described production apparatus  200 . As a result, a flexible OLED device of high encapsulation property can be obtained. 
     INDUSTRIAL APPLICABILITY 
     An embodiment of the present invention provides a flexible OLED device of high encapsulation property. Such a flexible OLED device is broadly applicable to smartphones, tablet computers, on-board displays, and small-, medium- and large-sized television sets. 
     REFERENCE SIGNS LIST 
       10  . . . glass base,  20  . . . functional layer region,  20 A . . . TFT layer,  20 B . . . OLED layer,  30  . . . plastic film,  40  . . . gas barrier film,  50  . . . protection sheet,  100  . . . multilayer stack,  200  . . . production apparatus,  210  . . . lift-off unit,  212  . . . stage,  220  . . . surface processing unit,  1000  . . . flexible OLED device