Patent Publication Number: US-2022223827-A1

Title: Vapor-deposition mask, method for manufacturing vapor-deposition mask, and method for manufacturing display device

Description:
BACKGROUND 
     The present disclosure relates to a vapor deposition mask, a method for manufacturing a vapor deposition mask, and a method for manufacturing a display device. 
     Display elements included in an organic electroluminescent (EL) display are formed through vapor deposition using a vapor deposition mask. The vapor deposition mask includes mask holes extending through a front surface of the vapor deposition mask, a rear surface of the vapor deposition mask, and a section between the front surface and the rear surface. Each mask hole opens in the front surface and the rear surface. The front surface includes a large opening of each mask hole. The rear surface includes a small opening of each mask hole. When the vapor deposition mask is used to perform vapor deposition for a vapor deposition target, the front surface of the vapor deposition mask opposes a vapor deposition source, and the rear surface of the vapor deposition mask opposes the vapor deposition target (refer to, for example, Japanese Laid-Open Patent Publication No. 2015-55007). 
     Each mask hole of the vapor deposition mask has the shape of an inverted frustum in the cross-section along a plane that is orthogonal to the front surface of the vapor deposition mask. The small opening of each mask hole is shaped in conformance with a shape required for a display element. To form a polygonal (e.g., quadrilateral) display element, the small opening of each mask hole usually has a shape including corners in plan view opposing the front surface of the vapor deposition mask. 
     In a vapor deposition material that travels from the vapor deposition source to the vapor deposition mask, the traveling direction of the vapor deposition material and the front surface of the vapor deposition mask form a traveling angle of the vapor deposition material. The vapor deposition material traveling toward the vapor deposition mask includes vapor deposition materials with various traveling angles. Most of the vapor deposition materials entering the mask holes from the vicinity of the middle portion of the large opening reach the small opening from the large opening regardless of the traveling angles of the vapor deposition materials. 
     Some of the vapor deposition materials entering the mask holes from the vicinity of the edge of the large opening deposits on side surfaces defining the mask holes without reaching the small opening. In particular, many of the vapor deposition materials entering the mask holes from the vicinity of the corners in the edge of the large opening are prevented from reaching the small opening by the side surfaces that define the mask holes. This varies the thickness of a vapor deposition pattern formed by the passage of the vapor deposition material through the small opening. As a result, the luminance becomes uneven in the display element including the vapor deposition pattern. 
     Such a problem also occurs in a case where the mask hole has a shape other than the shape of an inverted frustum. In such a case, for example, the mask holes each include a large hole portion with the shape of an inverted frustum directed from the front surface toward the rear surface and a small hole portion with the shape of a frustum directed from the rear surface toward the front surface. In this case, the opening formed by connecting the large hole portion to the small hole portion functions as the above-described small opening. 
     SUMMARY 
     It is an objective of the present disclosure to provide a vapor deposition mask, a method for manufacturing a vapor deposition mask, and a method for manufacturing a display device capable of limiting variations in the thickness of a vapor deposition pattern. 
     A vapor deposition mask that solves the above-described problem is made of metal. The vapor deposition mask includes: a front surface configured to oppose a vapor deposition source; and mask holes each including a hole portion having a shape of an inverted frustum. The hole portion of each of the mask holes includes: a small opening including a polygonal edge as seen from a view opposing the front surface of the vapor deposition mask, the edge including corners and linear portions each located between adjacent ones of the corners; and a large opening located on the front surface, the large opening including an edge as seen from the view opposing the front surface of the vapor deposition mask, the edge being shaped such that the corners of the edge of the small opening project outward from the edge of the small opening. The large opening surrounds the small opening as seen from the view opposing the front surface. 
     A method for manufacturing a vapor deposition mask that solves the above-described problem includes: forming a resist mask on at least one of a front surface and a rear surface of a metal sheet; and forming mask holes in the metal sheet using the resist mask. The forming the mask holes forms the mask holes, each including a hole portion having a shape of an inverted frustum, in the metal sheet such that the hole portion of each of the mask holes includes: a small opening including a polygonal edge as seen from a view opposing a plane on which the metal sheet spreads, the edge including corners and linear portions each located between adjacent ones of the corners; and a large opening located on the front surface, the large opening including an edge as seen from the view opposing the front surface of the vapor deposition mask, the edge being shaped such that the corners of the edge of the small opening project outward from the edge of the small opening, and such that the large opening surrounds the small opening as seen from the view opposing the front surface. 
     A method for manufacturing a display device that solves the above-described problem includes: preparing a vapor deposition mask obtained through the method for manufacturing the vapor deposition mask; and forming a pattern through vapor deposition using the vapor deposition mask. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the structure of a vapor deposition mask according to an embodiment. 
         FIG. 2  is a plan view showing the shape of the mask hole seen from a view opposing the front surface of the vapor deposition mask. 
         FIG. 3  is a cross-sectional view taken along line III-III in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along line IV-IV in  FIG. 2 . 
         FIG. 5  is an enlarged plan view showing part of the small opening included in the mask hole. 
         FIG. 6  is a diagram illustrating a step of the method for manufacturing the vapor deposition mask according to the embodiment. 
         FIG. 7  is a diagram illustrating a step of the method for manufacturing the vapor deposition mask. 
         FIG. 8  is a plan view showing the shape of the mask hole included in the resist mask in plan view opposing the front surface of the metal sheet. 
         FIG. 9  is a diagram illustrating a step of the method for manufacturing the vapor deposition mask. 
         FIG. 10  is a plan view showing the structure of a mask device to which the vapor deposition masks are applied. 
         FIG. 11  is a plan view showing part of a first modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 12  is a plan view showing part of a second modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 13  is a plan view showing part of a third modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 14  is a plan view showing part of a fourth modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 15  is a plan view showing part of a fifth modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 16  is a plan view showing part of a sixth modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 17  is a plan view showing part of a seventh modification in the shape of the mask hole seen from the view opposing the front surface of the vapor deposition mask. 
         FIG. 18  is a cross-sectional view showing an eighth modification in the shape of the mask hole included in the vapor deposition mask. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A vapor deposition mask, a method for manufacturing a vapor deposition mask, and a method for manufacturing a display device according to an embodiment will now be described with reference to  FIGS. 1 to 10 . In the following description, the vapor deposition mask, the method for manufacturing the vapor deposition mask, a mask device, and examples will be described in this order. 
     Vapor Deposition Mask 
     The vapor deposition mask will now be described with reference to  FIGS. 1 to 5 . 
       FIG. 1  shows a pattern region and part of a surrounding region that are included in the vapor deposition mask. 
       FIG. 1  shows a vapor deposition mask  10  that is made of metal. The vapor deposition mask  10  includes a front surface  10 F that opposes a vapor deposition source, and includes a rear surface  10 R. The vapor deposition mask  10  includes mask holes  11  each including a hole portion having the shape of an inverted frustum. The vapor deposition mask  10  includes a pattern region R 1  that includes the mask holes  11  and a surrounding region R 2  that surrounds the pattern region R 1  and does not include the mask holes  11 . 
     In the present embodiment, the mask holes  11  are laid out in a staggered manner. The mask holes  11  may be laid out in an arrangement pattern other than a staggered pattern. The pattern other than a staggered pattern may be, for example, a square lattice. 
     The vapor deposition mask  10  has thickness T of, for example, between 1 μm and 20 μm inclusive. The thickness in the pattern region R 1  may be smaller than the thickness in the surrounding region R 2 . In this case, the thickness of the vapor deposition mask  10  is the thickness in the surrounding region R 2 . The vapor deposition mask  10  is made of an iron-nickel alloy. The iron-nickel alloy may be, for example, an alloy containing 36 mass % of nickel (i.e., Invar). In other words, the vapor deposition mask  10  is substantially made of an iron-nickel alloy. 
       FIG. 2  shows the shape of the mask hole  11  seen from a view opposing the front surface  10 F of the vapor deposition mask  10 . 
     As shown in  FIG. 2 , the mask hole  11  includes a small opening  11 S and a large opening  11 L. The small opening  11 S is one opening in the hole portion. The large opening  11 L surrounds the small opening  11 S as seen from the view opposing the front surface  10 F. In the present embodiment, the small opening  11 S is located on the rear surface  10 R of the vapor deposition mask  10 . As seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , the small opening  11 S includes a polygonal edge  11 SE. The edge  11 SE includes corners  11 SC and linear portions  11 SL each located between adjacent ones of the corners  11 SC. 
     In the present embodiment, the edge  11 SE of the small opening  11 S has a quadrilateral shape. The edge  11 SE of the small opening  11 S includes four linear portions  11 SL and four corners  11 SC. The four linear portions  11 SL include two pairs of linear portions  11 SL that are parallel to each other. The linear portions  11 SL included in each pair extend in a direction that is orthogonal to the linear portions  11 SL included in the corresponding pair. Each corner  11 SC is a line segment located between the linear portions  11 SL extending in directions that are orthogonal to each other, and has a predetermined curvature. In the small opening  11 S, the distance between the linear portions  11 SL parallel to each other is a small opening width WS. 
     The large opening  11 L is the other opening in the hole portion. The large opening  11 L includes an edge  11 LE shaped such that the corners in the polygonal shape of the edge  11 SE of the small opening  11 S project outward from the polygonal shape. That is, the large opening  11 L includes the edge  11 LE shaped such that the corners  11 SC of the edge  11 LE of the small opening  11 S project outward from the edge  11 SE of the small opening  11 S. The large opening  11 L is located on the front surface  10 F of the vapor deposition mask  10 . As seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , the large opening  11 L includes the edge  11 LE that is shaped to include corners  11 LC. The number of the corners  11 LC is the same as the number of the corners of the small opening  11 S. As seen from the view opposing the front surface  10 F, the maximum value of the distance between each corner  11 SC at the edge  11 SE of the small opening  11 S and the corresponding corner  11 LC at the edge  11 LE of the large opening  11 L is an inter-corner distance DC. 
     In the present embodiment, as seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , the edge  11 LE of the large opening  11 L includes four linear portions  11 LL and four corners  11 LC. The four linear portions  11 LL include two pairs of linear portions  11 LL that are parallel to each other. The linear portions  11 LL included in each pair extend in a direction that is orthogonal to the linear portions  11 LL included in the corresponding pair. As seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , the four linear portions  11 LL are included in an imaginary edge VE. The shape of the imaginary edge VE is substantially similar to the shape of the small opening  11 S. 
     The edge  11 LE of the large opening  11 L further includes four projections  11 LP that project from the imaginary edge VE in a direction in which the projections  11 LP are separated from the small opening  11 S. Each projection  11 LP is located between two of the linear portions  11 LL that are orthogonal to each other. As seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , the region defined by each projection  11 LP is substantially triangular. Each corner  11 LC belongs to a different one of the projections  11 LP. As seen from the view opposing the front surface  10 F of the vapor deposition mask  10 , each corner  11 LC includes a portion farthest from the small opening  11 S in the projection  11 LP to which that corner  11 LC belongs. In the projection  11 LP, the corner  11 LC has a third angle θ 3 . The third angle θ 3  is formed by the two of the linear portions between which the corner  11 LC is located. In detail, the third angle θ 3  is formed by a tangent of the projection  11 LP in each of the portions where the imaginary edge VE intersects the projections  11 LP. 
     The large opening  11 L and the small opening  11 S are connected by a side surface  11 SD that defines the mask hole  11 . The side surface  11 SD is inclined such that the area of the mask hole  11  in the cross-section parallel to the front surface  10 F of the vapor deposition mask  10  decreases from the large opening  11 L toward the small opening  11 S. That is, the mask hole  11  has the shape of an inverted frustum in the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10 . Thus, in the present embodiment, each mask hole  11  is formed by one hole portion having the shape of an inverted frustum. 
     The shape of the mask hole  11  will now be described in more detail with reference to  FIGS. 3 and 4 , each showing the cross-sectional structure of the vapor deposition mask  10 .  FIG. 3  shows the structure of the vapor deposition mask  10  in the cross-section that is taken along line III-III in  FIG. 2  and extends in the diagonal direction of the small opening  11 S.  FIG. 4  shows the structure of the vapor deposition mask  10  in the cross-section that is taken along line IV-IV in  FIG. 2  and extends in a direction that is orthogonal to one linear portion  11 SL of the edge  11 SE of the small opening  11 S. 
     As shown in  FIG. 3 , in the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends in the diagonal direction of the small opening  11 S, a first imaginary straight line L 1  connects the corner  11 SC of the edge  11 SE of the small opening  11 S to the corner  11 LC of the edge  11 LE of the large opening  11 L. The first imaginary straight line L 1  connects the corner  11 SC of the small opening  11 S to the corner  11 LC of the large opening  11 L that corresponds to that corner  11 SC of the small opening  11 S. That is, the first imaginary straight line L 1  connects the corner  11 SC of the small opening  11 S to the corner  11 LC of the large opening  11 L closest from the corner  11 SC as seen from the view opposing the front surface  10 F of the vapor deposition mask  10 . 
     In the example shown in  FIG. 3 , the side surface  11 SD defining the mask hole  11  has an arcuate shape recessed from the first imaginary straight line L 1  toward the rear surface  10 R. In the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends in the diagonal direction of the small opening  11 S, the side surface  11 SD defining the mask hole  11  may have an arcuate shape that protrudes from the first imaginary straight line L 1  toward the front surface  10 F. Alternatively, the side surface  11 SD may coincide with the first imaginary straight line L 1 . 
     The front surface  10 F of the vapor deposition mask  10  and the first imaginary straight line L 1  form a first angle θ 1 . Since the front surface  10 F of the vapor deposition mask  10  is substantially parallel to the rear surface  10 R of the vapor deposition mask  10 , the angle formed by the rear surface  10 R and the first imaginary straight line L 1  is equal to the first angle θ 1 . 
     The inter-corner distance DC is one to one and a half times greater than the distance between the front surface  10 F of the vapor deposition mask  10  and a plane including the edge  11 SE of the small opening  11 S. In the present embodiment, the distance between the front surface  10 F of the vapor deposition mask  10  and the plane including the edge  11 SE of the small opening  11 S is equal to thickness T of the vapor deposition mask  10 . As described above, thickness T of the vapor deposition mask  10  may be, for example, between 1 μm and 20 μm inclusive. Thus, the inter-corner distance DC is a value, for example, between 1 μm and 30 μm inclusive. 
     As shown in  FIG. 4 , in the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends in the direction that is orthogonal to one linear portion  11 SL of the edge  11 SE of the small opening  11 S, a second imaginary straight line L 2  connects the edge  11 SE of the small opening  11 S to the edge  11 LE of the large opening  11 L. 
     In the example shown in  FIG. 4 , the side surface  11 SD defining the mask hole  11  has an arcuate shape recessed from the second imaginary straight line L 2  toward the rear surface  10 R. In the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends in the direction that is orthogonal to one linear portion  11 SL of the edge  11 SE of the small opening  11 S, the side surface  11 SD defining the mask hole  11  may have an arcuate shape that protrudes from the second imaginary straight line L 2  toward the front surface  10 F. Alternatively, the side surface  11 SD may coincide with the second imaginary straight line L 2 . 
     The front surface  10 F of the vapor deposition mask  10  and the second imaginary straight line L 2  form a second angle θ 2 . Since the front surface  10 F of the vapor deposition mask  10  is substantially parallel to the rear surface  10 R of the vapor deposition mask  10 , the angle formed by the rear surface  10 R and the second imaginary straight line L 2  is equal to the second angle θ 2 . The second angle θ 2  is greater than the first angle θ 1 . 
       FIG. 5  is an enlarged view showing part of the small opening  11 S seen from the view opposing the front surface  10 F of the vapor deposition mask  10 . 
     As shown in  FIG. 5  and as described above, the corner  11 SC of the edge  11 SE of the small opening  11 S has a curvature. The corner  11 SC has a curvature such that a curvature center C is located in the small opening  11 S. The corner  11 SC has a curvature radius R of less than or equal to 4.5 μm. 
     Method for Manufacturing Vapor Deposition Mask 
     The method for manufacturing the vapor deposition mask  10  will now be described with reference to  FIGS. 6 to 9 . 
     The method for manufacturing the vapor deposition mask  10  includes forming a resist mask on at least one of the front surface and the rear surface of the metal sheet and forming mask holes in the metal sheet using the resist mask. The method for manufacturing the vapor deposition mask  10  will now be described in more detail with reference to the drawings.  FIGS. 6, 7, and 9  schematically show the mask holes formed in the metal sheet. 
     Referring to  FIG. 6 , a metal sheet  10 M is first prepared to manufacture the vapor deposition mask  10 . The metal sheet  10 M is made of, for example, an iron-nickel alloy. The iron-nickel alloy may be, for example, Invar. The thickness of the metal sheet  10 M is, for example, between 1 μm and 50 μm inclusive. When the thickness of the metal sheet  10 M is greater than thickness T of the vapor deposition mask  10 , the thickness of the metal sheet  10 M is reduced to the thickness required for the vapor deposition mask  10  by etching the metal sheet  10 M before forming a resist layer on the metal sheet  10 M. 
     Next, a resist layer RL is formed on a front surface  10 MF of the metal sheet  10 M. The resist layer RL may be formed using a positive resist or may be formed by using a negative resist. The resist layer RL may be formed on the front surface of the metal sheet  10 M by attaching a dry film resist on the front surface  10 MF of the metal sheet  10 M. Alternatively, the resist layer RL may be formed by applying, to the front surface  10 MF of the metal sheet  10 M, coating liquid that contains material used to form the resist layer RL. 
     As shown in  FIG. 7 , the resist layer RL is exposed and developed so as to form a resist mask RM from the resist layer RL. The resist mask RM includes mask holes RMh, each shaped in conformance with the shape of the mask hole formed in the metal sheet  10 M. 
       FIG. 8  is a plan view showing the resist mask RM as seen from a view opposing the front surface  10 MF of the metal sheet  10 M. The shape of the mask hole RMh of the resist mask RM shown in  FIG. 8  is an example of a shape that the mask hole RMh can have. 
     As shown in  FIG. 8 , the mask hole RMh includes an edge RMhE that defines the mask hole RMh. The edge RMhE of the mask hole RMh is shaped such that the corners of a polygonal imaginary edge RMhV project outward from the polygonal shape. In the example shown in  FIG. 8 , the edge RMhE of the mask hole RMh is shaped such that the corners of a quadrilateral imaginary edge RMhV project outward from the quadrilateral shape. The shape of the edge RMhE of the mask hole RMh is substantially equal to the shape of the edge  11 LE of the large opening  11 L formed using the resist mask RM. The shape of the imaginary edge RMhV is substantially equal to the shape of the edge  11 SE of the small opening  11 S. 
     The edge RMhE of the mask hole RMh includes four projections RMhP and four linear portions RMhL. In the edge RMhE of the mask hole RMh, one projection RMhP is located between two linear portions RMhL. The imaginary edge RMhV includes the four linear portions RMhL. Each projection RMhP includes one corner RMhC. As seen from the view opposing the front surface  10 MF of the metal sheet  10 M, the region defined by each projection RMhP is substantially triangular. The shape of each projection RMhP is substantially equal to the shape of the projection  11 LP of the edge  11 LE of the large opening  11 L. 
     In the mask hole RMh, the distance between two of the linear portions RMhL that are substantially parallel to each other is a mask hole width WRMh. The distance between a corner of the imaginary edge RMhV and the corner RMhC of the projection RMhP, which projects from that corner, is a corner correction value RMhDC. In the projection RMhP, the corner RMhC has a fourth angle θ 4 . The shape of the mask hole formed in the metal sheet  10 M is changed by a change in at least one of the mask hole width WRMh, the corner correction value RMhDC, and the fourth angle θ 4  in the mask hole RMh. 
     Referring to  FIG. 9 , wet etching is performed using the resist mask RM so as to form mask holes  11 M in the metal sheet  10 M. This forms the mask holes  11 M, each including an opening in the front surface  10 MF and a rear surface  10 MR of the metal sheet  10 M. The above-described vapor deposition mask  10  is obtained by removing the resist mask RM from the metal sheet  10 M. In the metal sheet  10 M, the front surface  10 MF corresponds to the front surface  10 F of the vapor deposition mask  10 , the rear surface  10 MR corresponds to the rear surface  10 R of the vapor deposition mask  10 , and the mask holes  11 M correspond to the mask holes  11  of the vapor deposition mask  10 . 
     Mask Device 
     The mask device will now be described with reference to  FIG. 10 . 
     As shown in  FIG. 10 , a mask device  20  includes a frame  21  and vapor deposition masks  10 . In the example shown in  FIG. 10 , the mask device  20  includes three vapor deposition masks  10 . Instead, the mask device  20  may include two vapor deposition masks  10  or less, or may include four vapor deposition masks  10  or more. The frame  21  has a rectangular shape that supports the vapor deposition masks  10 . The frame  21  is coupled to a vapor deposition apparatus used to perform vapor deposition. The frame  21  includes a frame hole  21 H extending through the frame  21  in the substantially entire range where the vapor deposition masks  10  are located. 
     Each vapor deposition mask  10  has the shape of a belt extending in a single direction. Each vapor deposition mask  10  includes pattern regions R 1  and a surrounding region R 2  that surrounds the pattern region R 1 . In the example shown in  FIG. 10 , each vapor deposition mask  10  includes three pattern regions R 1 . Instead, each vapor deposition mask  10  may include two pattern regions R 1  or less, or may include four pattern regions R 1  or more. 
     In the surrounding region R 2  of each vapor deposition mask  10 , a pair of portions between which the pattern regions R 1  are located in the extending direction of each vapor deposition mask  10  is fixed to the frame  21 . Each vapor deposition mask  10  is fixed to the frame  21  through, for example, adhesion or welding. 
     The method for manufacturing the display device using the vapor deposition mask  10  includes preparing the vapor deposition mask  10  obtained through the above-described method for manufacturing the vapor deposition mask  10  and forming a pattern through vapor deposition using the vapor deposition mask  10 . 
     In the method for manufacturing the display device, the mask device  20  incorporating the vapor deposition mask  10  is first installed in a vacuum chamber of the vapor deposition apparatus. The mask device  20  is installed in the vacuum chamber such that the rear surface  10 R opposes a vapor deposition target (such as a glass substrate) and the front surface  10 F opposes a vapor deposition source. Then, the vapor deposition target is brought into the vacuum chamber of the vapor deposition apparatus so that the vapor deposition source causes the vapor deposition material to sublime. This causes the pattern shaped in conformance with the small opening  11 S to be formed in a region of the vapor deposition target opposing the small opening  11 S. In the present embodiment, the vapor deposition material is an organic light-emitting material used to form pixels of an organic electroluminescent display, which is an example of the display device. The vapor deposition material may be a conductive material used to form a pixel electrode included in a pixel circuit of the display device. 
     EXAMPLES 
     Examples will now be described with reference to Tables 1 to 3. 
     Example 1 
     An Invar metal plate having a thickness of 20 μm was prepared. The metal sheet was etched using 48% ferric chloride so as to reduce the thickness of the metal sheet to 3.5 μm. Next, a positive resist (THMR-iP5700, manufactured by TOKYO OHKA KOGYO CO., LTD.) (THMR is a registered trademark) was used to form a resist layer on the front surface of the metal sheet. The resist layer was exposed and then the exposed resist layer was developed so as to form a resist mask. In this manner, the resist mask was formed with mask holes having the same shape as the mask holes of  FIG. 8  as seen from the view opposing the front surface of the resist mask. 
     In Example 1, as described in the following Table 1, it was found in one mask hole that the mask hole width WRMh was 18.3 μm, the corner correction value RMhDC was 3.3 μm, and the specified value of the fourth angle θ 4  was 30.2°. Thus, mask holes are formed such that the target value of the small opening width WS of the vapor deposition mask was 20 μm and the target value of the pitch of each mask hole was 30 μm. 
     Wet etching was performed using the resist mask so as to form mask holes in the metal sheet. In this step, 48% ferric chloride was used as etchant. In the etching of the metal sheet, when the time for the hole portion formed in the metal sheet to reach the rear surface of the metal sheet was set to 1, the etching time of the metal sheet was set to 2. That is, the etching time was set to a length corresponding to thickness T of the metal sheet for which wet etching was performed. Lastly, the etched metal sheet was immersed for two minutes in 10% aqueous sodium hydroxide solution at 60° C. so as to remove the resist mask from the metal sheet. The vapor deposition mask of Example 1 was thus obtained. 
     Example 2 
     In Example 1, the same method as that of Example 1 was used to obtain the vapor deposition mask of Example 2 except that the thickness of the metal sheet was reduced from 20 μm to 4.0 μm, the mask hole width WRMh was 18.0 μm, the corner correction value RMhDC was 3.0 μm, and the fourth angle θ 4  was 30.1°. 
     Example 3 
     In Example 1, the same method as that of Example 1 was used to obtain the vapor deposition mask of Example 3 except that the thickness of the metal sheet was reduced from 20 μm to 4.5 μm, the mask hole width WRMh was 17.8 μm, the corner correction value RMhDC was 2.8 μm, and the fourth angle θ 4  was 30.2°. 
     Example 4 
     In Example 2, the same method as that of Example 2 was used to obtain the vapor deposition mask of Example 4 except that the corner correction value RMhDC was 2.0 μm and the fourth angle θ 4  was 30.3°. 
     Example 5 
     In Example 2, the same method as that of Example 2 was used to obtain the vapor deposition mask of Example 5 except that the corner correction value RMhDC was 2.5 μm and the fourth angle θ 4  was 29.9°. 
     Example 6 
     In Example 4, the same method as that of Example 4 was used to obtain the vapor deposition mask of Example 6 except that the corner correction value RMhDC was 1.5 μm and the fourth angle θ 4  was 29.8°. 
     Example 7 
     In Example 1, the same method as that of Example 1 was used to obtain the vapor deposition mask of Example 7 except that the corner correction value RMhDC was 4.3 μm and the fourth angle θ 4  was 30.0°. 
     Comparative Example 1 
     In Example 1, the same method as that of Example 1 was used to obtain the vapor deposition mask of Comparative Example 1 except that the corner correction value RMhDC was 0.8 μm and the fourth angle θ 4  was 29.9°. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Mask Hole 
                 Corner Correction 
                 Fourth 
               
               
                   
                 Width 
                 Value 
                 Angle 
               
               
                   
                 WRMh (μm) 
                 RMhDC (μm) 
                 θ4 (°) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 18.3 
                 3.3 
                 30.2 
               
               
                 Example 2 
                 18.0 
                 3.0 
                 30.1 
               
               
                 Example 3 
                 17.8 
                 2.8 
                 30.2 
               
               
                 Example 4 
                 18.0 
                 2.0 
                 30.3 
               
               
                 Example 5 
                 18.0 
                 2.5 
                 29.9 
               
               
                 Example 6 
                 18.0 
                 1.5 
                 29.8 
               
               
                 Example 7 
                 18.3 
                 4.3 
                 30.0 
               
               
                 Comparative Example 1 
                 18.3 
                 0.8 
                 29.9 
               
               
                   
               
            
           
         
       
     
     Measurement Results 
     For the vapor deposition masks of Examples 1 to 7 and Comparative Example 1, various dimensions and the like were measured using a confocal laser microscope (OLS-4000, manufactured by OLYMPUS CORPORATION). The measurement results of the vapor deposition masks were as follows. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Small 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Opening 
                 First 
                 Second 
                   
                 Inter-Corner 
                   
                 Curvature 
                 Third 
               
               
                   
                 Width 
                 Angle 
                 Angle 
                 Thickness 
                 Distance 
                   
                 Radius 
                 Angle 
               
               
                   
                 WS (μm) 
                 θ1 (°) 
                 θ2 (°) 
                 T (μm) 
                 DC (μm) 
                 DC/T 
                 R (μm) 
                 θ3 (°) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 20.4 
                 30.2 
                 45.1 
                 3.5 
                 4.9 
                 1.4 
                 3.0 
                 51.6 
               
               
                 Example 2 
                 20.3 
                 31.1 
                 44.8 
                 4.0 
                 4.9 
                 1.2 
                 3.2 
                 51.5 
               
               
                 Example 3 
                 19.9 
                 30.5 
                 44.7 
                 4.5 
                 5.1 
                 1.1 
                 3.9 
                 51.6 
               
               
                 Example 4 
                 20.4 
                 31.0 
                 45.2 
                 4.0 
                 4.0 
                 1.0 
                 4.4 
                 51.8 
               
               
                 Example 5 
                 20.1 
                 30.1 
                 45.3 
                 4.0 
                 4.6 
                 1.2 
                 3.8 
                 51.1 
               
               
                 Example 6 
                 20.1 
                 45.0 
                 44.9 
                 4.0 
                 3.3 
                 0.8 
                 7.2 
                 51.0 
               
               
                 Example 7 
                 20.4 
                 29.9 
                 44.7 
                 3.5 
                 6.2 
                 1.8 
                 — 
                 51.3 
               
               
                 Comparative 
                 19.8 
                 45.2 
                 45.0 
                 3.5 
                 2.6 
                 0.7 
                 8.5 
                 51.1 
               
               
                 Example 1 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, in the vapor deposition mask of Example 1, it was found through the transparent image captured in a direction opposing the rear surface that small openings, each having a width of approximately 20 μm and having a substantially square shape, were laid out at intervals of approximately 10 μm in a square lattice pattern. The distance between two linear portions parallel to each other was measured as the width of each small opening. Likewise, in the vapor deposition masks of Examples 2 to 6 and the vapor deposition mask of Comparative Example 1, it was found that small openings, each having a width of approximately 20 μm and having a substantially square shape, were laid out at intervals of approximately 10 μm in a square lattice pattern. In contrast, it was found that the vapor deposition mask of Example 7 had a quadrilateral shape in which the edge of each small opening was narrowed in the left-right direction and the up-down direction as a result of excessive etching of the corners while the opening of each small opening had a width of approximately 20 μm. 
     Further, it was found in Examples 1 to 7 that the edge of the large opening had a shape in which the corners of the quadrilateral shape projected outward from the square shape. In contrast, it was found in Comparative Example 1 that the edge of the large opening had a substantially square shape and the corners in the square shape did not project outward from the square shape. 
     It was found that the first angle θ 1  was 30.2° in Example 1, 31.1° in Example 2, and 30.5° in Example 3. Further, it was found that the first angle θ 1  was 31.0° in Example 4 and 30.1° in Example 5. Furthermore, it was found that the first angle θ 1  was 45.0° in Example 6, 29.9° in Example 7, and 45.2° in Comparative Example 1. 
     It was found that the second angle θ 2  was 45.1° in Example 1, 44.8° in Example 2, and 44.7° in Example 3. Further, it was found that the second angle θ 2  was 45.2° in Example 4 and 45.3° in Example 5. Furthermore, it was found that the second angle θ 2  was 44.9° in Example 6, 44.7° in Example 7, and 45.0° in Comparative Example 1. 
     Thus, it was found that the second angle θ 2  was larger than the first angle θ 1  in the vapor deposition masks of Examples 1 to 5 and 7. In contrast, it was found that the first angle θ 1  was larger than the second angle θ 2  in the vapor deposition masks of Example 6 and Comparative Example 1. 
     It was found that the inter-corner distance DC was 4.9 μm in Examples 1 and 2, 5.1 μm in Example 3, 4.0 μm in Example 4, and 4.6 μm in Example 5. Further, it was found that the inter-corner distance DC was 3.3 μm in Example 6, 6.2 μm in Example 7, and 2.6 μm in Comparative Example 1. 
     In other words, it was found that the ratio of the inter-corner distance DC to thickness T (DC/T) of the vapor deposition mask was 1.4 in the Example 1, 1.2 in Example 2, and 1.1 in Example 3. Further, it was found that the ratio of the inter-corner distance DC to thickness T of the vapor deposition mask was 1.0 in the Example 4 and 1.2 in Example 5. Furthermore, it was found that the ratio of the inter-corner distance DC to thickness T of the vapor deposition mask was 0.8 in the Example 6, 1.8 in Example 7, and 0.7 in Comparative Example 1. Thus, it was found that the ratio of the inter-corner distance DC to thickness T of the vapor deposition mask is included in a range between 1 and 1.5 inclusive in the vapor deposition masks of Examples 1 to 5 while the ratio of the inter-corner distance DC to thickness T of the vapor deposition mask is not included in the range between 1 and 1.5 inclusive in the vapor deposition masks of Examples 6, 7, and Comparative Example 1. 
     It was found that the curvature radius R of each corner included in the edge of the small opening was 3.0 μm in Example 1, 3.2 μm in Example 2, and 3.9 μm in Example 3. Further, it was found that the curvature radius R was 4.4 μm in Example 4, 3.8 μm in Example 5, and 7.2 μm in Example 6. Furthermore, it was found that the curvature radius R was 8.5 μm in Comparative Example 1. Thus, it was found that the curvature radius R was less than or equal to 4.5 μm in the vapor deposition masks of Examples 1 to 5 while the curvature radius R was greater than 4.5 μm in the vapor deposition mask of Example 6 and Comparative Example 1. Regarding Example 7, it was found that the edge of the small opening was narrowed in the left-right direction and the up-down direction and thus the corner had no curvature. 
     It was found that the third angle θ 3  was 51.6° in Example 1, 51.5° in Example 2, and 51.6° in Example 3. It was found that the third angle θ 3  was 51.8° in Example 4, 51.1° in Example 5, and 51.0° in Example 6. It was found that the third angle θ 3  was 51.3° in Example 7 and 51.1° in Comparative Example 1. In Comparative Example 1, the third angle θ 3  was set to an angle at the corner included in the large opening. 
     Evaluation Results 
     The vapor deposition pattern was formed on the vapor deposition target using each of the vapor deposition mask of Examples 1 to 7 and Comparative Example 1. A glass substrate was used for the vapor deposition target. An organic light-emitting material was used for the vapor deposition material used to form the vapor deposition pattern. 
     Each vapor deposition mask was used to form a quadrilateral vapor deposition pattern laid out in a grid pattern of fifteen rows and fifteen columns. In the middle of this vapor deposition pattern, variations in the thickness were calculated for the vapor deposition pattern of nine rows and nine columns. The thickness in the middle of each vapor deposition pattern was measured, and that thickness was regarded as a maximum value MM in the thickness of the vapor deposition pattern. Further, the thickness at the corner of each vapor deposition pattern was measured, and that thickness was regarded as a minimum value Mm in the thickness of the vapor deposition pattern. The thickness of each vapor deposition pattern was measured using a profilometer (Dektak 6M, manufactured by Veeco, Inc.). The following Equation (1) was used to calculate the variation in the thickness. The variation of less than or equal to 5% was marked with “∘”. The variation of less than or equal to 10% was marked with “Δ”. The variation of greater than 10% was marked with “×”. 
       100×{( MM−Mm )/( MM+Mm )}/2 (%)   Equation (1)
 
     Table 3 shows the variations in the thickness that were calculated using Equation (1). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Thickness 
                   
               
               
                   
                 Variation (%) 
                 Evaluation 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 1 
                 4.1 
                 ◯ 
               
               
                   
                 Example 2 
                 4.3 
                 ◯ 
               
               
                   
                 Example 3 
                 4.9 
                 ◯ 
               
               
                   
                 Example 4 
                 5.0 
                 ◯ 
               
               
                   
                 Example 5 
                 4.7 
                 ◯ 
               
               
                   
                 Example 6 
                 9.2 
                 Δ 
               
               
                   
                 Example 7 
                 4.0 
                 ◯ 
               
               
                   
                 Comparative Example 1 
                 10.3 
                 X 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 3, it was found that the results of evaluating the vapor deposition masks of Examples 1 to 5 and 7 were “∘”, the result of evaluating the vapor deposition mask of Example 6 was “Δ”, and the result of evaluating the vapor deposition mask of Comparative Example 1 was “×”. Accordingly, it was found that the vapor deposition masks of Examples 1 to 7 limited variations in the thickness of the vapor deposition pattern as compared with Comparative Example 1. 
     Further, it was found that variations in the thickness of the vapor deposition pattern tended to be larger when the ratio of the inter-corner distance DC to thickness T was less than 1 than when the ratio of the inter-corner distance DC to thickness T was greater than or equal to 1. When the ratio of the inter-corner distance DC to thickness T was greater than 1.5, it was found that variations in the thickness of the vapor deposition pattern were small while the edge of the vapor deposition pattern was narrowed relative to a desired polygonal shape. Thus, it is preferred that the ratio of the inter-corner distance DC to thickness T be between 1 and 1.5 inclusive in order to limit variations in the thickness of the vapor deposition pattern and increase the accuracy of the shape in the vapor deposition pattern. 
     Additionally, it was found that variations in the thickness of the vapor deposition pattern tended to be larger when the curvature radius R was greater than 4.5 μm than when the curvature radius R was less than or equal to 4.5 μm. Thus, it is preferred that the curvature radius R be less than or equal to 4.5 μm in order to limit variations in the thickness of the vapor deposition pattern. 
     As described above, the vapor deposition mask, the method for manufacturing the vapor deposition mask, and the method for manufacturing the display device according to the embodiment provide the following advantages. 
     (1) The vapor deposition material entering the mask hole  11  from the vicinity of each corner  11 LC of the large opening  11 L easily reaches the small opening  11 S. This limits variations in the thickness of the vapor deposition pattern. 
     (2) In the vapor deposition pattern, the thickness of each corner is prevented from being smaller than the thickness of the middle portion. This limits variations in the thickness of the vapor deposition pattern. 
     (3) The distance between the small opening  11 S and the vapor deposition target is reduced as compared with a configuration in which the small opening  11 S shaped in conformance with the vapor deposition pattern formed on the vapor deposition target is located between the front surface  10 F and the rear surface  10 R. This limits variations in the thickness in the vapor deposition pattern formed on the vapor deposition target. 
     (4) As compared with a configuration in which the thickness of the vapor deposition mask  10  is greater than 20 μm, the large opening  11 L capable of extending through the metal sheet  10 M is reduced in size. This allows the vapor deposition mask  10  to include the mask holes  11  with a higher density. 
     (5) The difference between the degree of expansion of the vapor deposition mask  10  and the degree of expansion of the glass substrate is prevented from being excessively increased by the heating of the vapor deposition mask  10  and the glass substrate. This prevents the accuracy of the vapor deposition pattern formed on the glass substrate from being decreased by the difference between the expansion rate of the vapor deposition mask  10  and the expansion rate of the glass substrate. 
     The above-described embodiment may be modified as follows. 
     Large Opening 
     The shape of each projection  11 LP included in the edge  11 LE of the large opening  11 L may be changed as follows. That is, as shown in the drawings referenced later, the region defined by the projection  11 LP can have various shapes seen from the view opposing the front surface  10 F of the vapor deposition mask  10 . 
     For example, the region defined by the projection  11 LP may have a substantially rectangular shape as shown in  FIG. 11  or may have an inverted-trapezoid shape as shown in  FIG. 12 . Instead, the region defined by the projection  11 LP may have a substantially trapezoid shape as shown in  FIG. 13  or may have a substantially rectangular shape with a linear corner  11 LC as shown in  FIG. 14 . As another option, as shown in  FIG. 15 , the region defined by the projection  11 LP may have a substantially square shape and the projection  11 LP may include three corners located outward from the imaginary edge VE. Alternatively, as shown in  FIG. 16 , the region defined by the projection  11 LP may have a substantially circular shape. 
     In any of these shapes of the region defined by the projection  11 LP, the edge  11 LE of the large opening  11 L has a shape in which the corners of the polygonal shape of the edge  11 SE of the small opening  11 S project outward from the polygonal shape. This provides an advantage similar to the above-described advantage (1). 
     Further, in any of these shapes of the region defined by the projection  11 LP, the inter-corner distance DC is one to one and a half greater than thickness T of the vapor deposition mask  10 . This allows the side surface  11 SD, which defines the mask hole  11 , to have a reduced inclination angle at a portion connecting each corner  11 SC of the edge  11 SE of the small opening  11 S to the corresponding corner  11 LC of the edge  11 LE of the large opening  11 L. 
     Number of Corners 
     The edge  11 SE of the small opening  11 S may have a polygonal shape other than a quadrilateral shape. The small opening  11 S may have a polygonal shape including, for example, five or more corners  11 SC. The edge  11 LE of the large opening  11 L may have, for example, five or more corners  11 LC. 
       FIG. 17  shows a modification in the shape of the edge  11 SE of the small opening  11 S. 
     As shown in  FIG. 17 , the edge  11 SE of the small opening  11 S has a substantially octagonal shape. The edge  11 SE of the small opening  11 S includes eight linear portions  11 SL and eight corners  11 SC. In the same manner as the above-described embodiment, the curvature radius R of each curvature radius R is less than or equal to  4 . 5 p.m. 
     The edge  11 LE of the large opening  11 L includes eight linear portions  11 LL and eight projections  11 LP. Each corner  11 LC belongs to a different one of the projections  11 LP. In the same manner as the modification shown in  FIG. 11 , the region defined by each projection  11 LP has a substantially rectangular shape and the corresponding corner  11 LC has a curvature. To these projections  11 LP, the projections  11 LP described above with reference to  FIGS. 2 and 12 to 16  are applicable. 
     When the edge  11 SE of the small opening  11 S has a polygonal shape other than a quadrilateral shape, the first imaginary straight line L 1  can be defined in the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends along a plane including one corner  11 SC of the small opening  11 S and the corner  11 LC of the large opening  11 L corresponding to that corner  11 SC. Further, when the edge  11 SE of the small opening  11 S has a polygonal shape other than a quadrilateral shape, the second imaginary straight line L 2  can be defined in the cross-section that is orthogonal to the front surface  10 F of the vapor deposition mask  10  and extends along a plane including one linear portion  11 SL of the small opening  11 S and the linear portion  11 LL of the large opening  11 L parallel to that linear portion  11 SL. 
     Mask Hole 
     The mask hole may include two hole portions. 
     More specifically, as shown in  FIG. 18 , each mask hole  31  in a vapor deposition mask  30  may include a large-hole portion  31   a  (one hole portion) and a small-hole portion  31   b  (the other hole portion). The large-hole portion  31   a  is connected to the small-hole portion  31   b  at a point in the thickness direction of the vapor deposition mask  30 . The large-hole portion  31   a  includes a large opening  31 L on a front surface  30 F and a small opening  31 S on a side opposite from the large opening  31 L. The small-hole portion  31   b  shares the small opening  31 S with the large-hole portion  31   a  and includes a rear surface opening  31 R. 
     The rear surface opening  31 R is located on a side opposite from the small opening  31 S and opens in a rear surface  30 R. The distance from the rear surface  30 R to the portion that connects the large-hole portion  31   a  to the small-hole portion  31   b  is a step height SH. It is preferred that the step height SH be small in order to limit the unevenness in luminance arising from a shadow effect on the vapor deposition pattern. The configuration in which the mask hole  31  includes two hole portions increases the thickness of the vapor deposition mask  30  while limiting a decrease in the resolution in the vapor deposition mask  30 . 
     In the same manner as the above-described embodiment, distance D between the front surface  30 F of the vapor deposition mask  30  and the plane including the edge of the small opening  31 S can be sized such that the inter-corner distance DC is one to one and a half times greater than the distance D. 
     Even in this case, the large opening  31 L of the vapor deposition mask  30  has a shape in which the corners of the polygonal shape of the edge of the small opening  31 S project outward from the polygonal shape. This provides an advantage similar to the above-described advantage (1). 
     Each mask hole  11  included in the vapor deposition mask  10  does not need to satisfy at least one of the following conditions. 
     (Condition A) The second angle θ 2  is greater than the first angle θ 1 . 
     (Condition B) The inter-corner distance DC is one to one and a half times greater than the distance between the front surface of the vapor deposition mask  10  and the plane including the edge  11 SE of the small opening  11 S. 
     (Condition C) The curvature radius R of the corner  11 SC at the edge  11 SE of the small opening  11 S is less than or equal to 4.5 μm. 
     Even if the mask hole  11  does not satisfy at least one of the conditions A to C, the edge  11 LE of the large opening  11 L has a shape in which the corners of the polygonal shape of the edge  11 SE of the small opening  11 S project outward from the polygonal shape. This provides an advantage similar to the above-described advantage (1). 
     Material for Vapor Deposition Mask 
     The vapor deposition mask  10  may be made of metal other than an iron-nickel alloy. The vapor deposition mask  10  may be made of, for example, an iron-nickel-cobalt alloy such as an alloy containing nickel of 32 mass % and cobalt of between 4 mass % and 5 mass % inclusive (namely, Super Invar). The vapor deposition mask  10  may be made of an iron-chrome-nickel alloy (namely, chrome-nickel stainless steel). The chrome-nickel stainless steel may be, for example, SUS304. An iron-chrome-nickel alloy has a larger thermal expansion coefficient than an iron-nickel alloy and an iron-nickel-cobalt alloy. Thus, an iron-chrome-nickel alloy may be used when the temperature of the vapor deposition mask  10  increases to a small extent during vapor deposition, and it is preferred that an iron-nickel alloy or an iron-nickel-cobalt alloy be used when the temperature of the vapor deposition mask  10  increases to a large extent. 
     Even in this case, the edge  11 LE of the large opening  11 L has a shape in which the corners of the polygonal shape of the edge  11 SE of the small opening  11 S project outward from the polygonal shape. This provides an advantage similar to the above-described advantage (1). 
     Thickness of Vapor Deposition Mask 
     The thickness of the vapor deposition mask may be greater than 20 μm. Even in this case, the edge  11 LE of the large opening  11 L has a shape in which the corners of the polygonal shape of the edge  11 SE of the small opening  11 S project outward from the polygonal shape. This provides an advantage similar to the above-described advantage (1). 
     Vapor Deposition Mask 
     The vapor deposition mask  10  may include mask portions corresponding to the pattern region R 1  and a sub-frame corresponding to the surrounding region R 2 . In this case, the sub-frame is separate from the mask portions and includes sub-frame holes. Each mask portion is coupled to the sub-frame so as to close a different one of the sub-frame holes. As a result, the number of the mask portions is the same as the number of the sub-frame holes in the vapor deposition mask  10 . The mask portions may be coupled to the sub-frame through adhesion or welding. 
     In this case, the sub-frame supports the mask portions. Thus, as compared with a configuration in which the mask portions are integrated with the surrounding portion, the thickness of each mask portion is reduced. To form each mask portion, the thickness of the mask portion is reduced by etching two opposite surfaces of the mask portion prior to forming mask holes in the mask portion. To form a thin mask portion, the mask portion may be stacked on a support layer that supports the mask portion in the etching for reducing the thickness of the mask portion and the etching for forming the mask holes in the mask portion in order to improve the handleability of the mask portion. The support layer simply needs to be removed from the mask portion after coupling the mask portion to the frame.