Abstract:
A mask is disclosed. The mask includes at least one support base having at least one opening formed therein, where at least a portion of the boundary of the opening is tapered. The mask also includes at least one positioning layer disposed on the at least one support base, where at least one through opening corresponding to and aligned with the at least one opening is formed in the at least one positioning layer. In addition, at least a portion of the boundary of the through opening is tapered.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of priority to Chinese patent application No. 201310250635.2, entitled “MASK AND METHOD FOR FORMING THE SAME”, filed with the Chinese Patent Office on Jun. 21, 2013, the content of which is incorporated herein by reference in its entirety. 
       FIELD OF THE DISCLOSURE  
       [0002]    The present disclosure generally relates to display technology, and more particularly, to a mask and a method for forming the same. 
       BACKGROUND OF THE INVENTION  
       [0003]    Organic Light Emitting Diodes (OLED), as they have advantages of wide viewing angle, high contrast ratio and high response speed, etc, has drawn more and more attention. An OLED typically includes a first electrode, an organic light emitting layer and a second electrode. Currently, an organic light emitting layer of an OLED is generally formed by an etching process, in which an organic light emitting material is deposited on a substrate through evaporation for forming the organic light emitting layer. Such process needs a high resolution mask. To reduce production cost, a thin mask is a general choice. Further, to achieve a high resolution and high control accuracy, an opening with a mini-size may be needed to be formed in the mask. However, the conventional etching process has the following disadvantages: 1, because the mask is very thin, it is difficult to accurately control the size and location of the openings and to form the openings with a small size; 2, it is difficult and costly to manufacture a thin type mask, and the thin type mask has a relatively short lifetime. 
       BRIEF SUMMARY OF THE INVENTION  
       [0004]    One inventive aspect is a mask. The mask includes at least one support base having at least one opening formed therein, where at least a portion of the boundary of the opening is tapered. The mask also includes at least one positioning layer disposed on the at least one support base, where at least one through opening corresponding to and aligned with the at least one opening is formed in the at least one positioning layer, and where at least a portion of the boundary of the through opening is tapered. 
         [0005]    Another inventive aspect is a method of forming a mask. The method includes providing a support base having first and second opposing surfaces, and forming a first photoresist layer on the first surface of the support base. The method also includes forming at least one through opening in the first photoresist layer so as to form a positioning layer, and etching the second surface of the support base to form at least one opening corresponding to the through opening in the positioning layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]      FIG. 1  schematically illustrates a top view of a mask according to a first embodiment of the present invention; 
           [0007]      FIG. 2  schematically illustrates a cross-sectional view along the line L 1 -L 1  of  FIG. 1 ; 
           [0008]      FIG. 3  is a schematic flow chart of a method for forming the mask in  FIG. 1  according to one embodiment of the present invention; 
           [0009]      FIG. 4(   a ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0010]      FIG. 4(   b ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0011]      FIG. 4(   c ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0012]      FIG. 4(   d ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0013]      FIG. 4(   e ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0014]      FIG. 4(   f ) schematically illustrates an intermediate structure of a method for forming the mask in  FIG. 1 ; 
           [0015]      FIG. 5  schematically illustrates a partial cross-sectional view of a mask according to a second embodiment of the present invention; 
           [0016]      FIG. 6  schematically illustrates a top view of a mask according to a third embodiment of the present invention; 
           [0017]      FIG. 7  schematically illustrates a cross-sectional view along the line L 2 -L 2  of  FIG. 6 ; and 
           [0018]      FIG. 8  schematically illustrates a partial cross-sectional view of a mask according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    According to a first embodiment, a mask  100  and a method for forming the mask  100  are illustrated in accordance with  FIG. 1  to  FIG. 4(   f ). 
         [0020]    Referring to  FIG. 1  and  FIG. 2 ,  FIG. 1  schematically illustrates an overall structure of a mask  100 . Because  FIG. 1  is a top view, only a positioning layer  20  is shown in  FIG. 1 .  FIG. 2  schematically illustrates a cross-sectional view along the line L 1 -L 1  of  FIG. 1 , which is a partial cross-sectional view of the mask  100 . 
         [0021]    In some embodiments, the mask  100  may include a support base  10  and the positioning layer  20 . The support base  10  may include nickel iron alloy, i.e. Invar alloy, and may have a thickness ranging from 30 μm to 60 μm, preferably, 40 μm. A plurality of openings  12  are formed in the support base  10 . Apart from the openings  12 , the remaining part of the support base  10  is solid and defined as a first base  14 . The plurality of openings  12  may have substantially arc-shaped cross section. Referring to  FIG. 2 , the cross section is defined by cutting in a direction from the top to the bottom of the support base  10 . 
         [0022]    In some embodiments, the positioning layer  20  may be adhered onto the support base  10 . The positioning layer  20  is a high polymer material, which can withstand a temperature over 180° C. For example, the positioning layer  20  may be made of polyimide, and may have a thickness ranging from 4 μm to 8 μm. The positioning layer  20  may include a second base  24  and a plurality of through openings  22  extending through the positioning layer. The plurality of through openings  22  are formed one-to-one corresponding to the plurality of openings  12  in the support base. The plurality of through openings  22  may be tapered. For example, one or more through openings  22  may have a substantially trapezoidal cross section. Apart from the through openings  22 , the remaining part of the positioning layer  20  is solid and defined as a second base  24 . 
         [0023]    In some embodiments, the widths of the cross sections of the openings  12  and the through openings  22  may reduce gradually in a direction from the support base  10  to the positioning layer  20 . And the minimum width of each opening  12  is greater than the maximum width of a corresponding through opening  22 . 
         [0024]    In some embodiments, the geometry of the cross section of the opening  12  of the support base is not limited to arc-shaped, but may be tapered, for example, to have a substantially trapezoidal cross section or another shaped cross section. The geometry of the cross section of the through opening  22  of the positioning layer is not limited to trapezoidal, may also have a substantially arc-shaped or another shaped cross section. 
         [0025]    In the first embodiment, the support base  10  has a single-layer structure, which is not limited thereto. In other embodiments, the support base  10  may has a multi-layer structure as required. 
         [0026]    In the first embodiment, the mask  100  includes the support base  10  and the positioning layer  20 . The support base  10  including invar alloy is used to support the positioning layer  20 , which may enhance the tensile strength of the mask and prolong working lifetime of the mask  100 . The positioning layer  20  is disposed on the support base  10 . In conclusion, by adding only one layer having a predetermined thickness (i.e. the positioning layer  20  with a thickness ranging from 4 μm to 8 μm), the size, location and maching angle of the opening may be determined more accurately, which may promote the resolution of the mask  100 . Therefore, an organic material pattern having a mini-size may be obtained during evaporation, thus improving the evaporation accuracy of the mask  100 . 
         [0027]    Referring to  FIG. 3 , and  FIG. 4(   a ) to  FIG. 4(   f ), a method for forming a mask of the first embodiment is provided. For clarity, only one pair of an opening  12  and a corresponding through opening  22 , which are communicated to each other, are illustrated in the drawings. 
         [0028]    Step S 1 , a support base  10  which is made of invar alloy and has a thickness of 40 μm is provided, and a preprocessing is performed on the supports base  10 . The support base  10  may have a first surface A and a second surface B, both of which are disposed oppositely. 
         [0029]    Step S 2 , the first surface A of the support base  10  is coated with a first photoresist material. In some embodiments, referring to  FIG. 4(   a ), the first surface A of the support base  10  may be cleaned first. Then a polyimide material, which may have a thickness ranging from 4 μm to 8 μm, may be coated on the first surface A of the support base  10  using a roller brush R, so as to form the first photoresist layer  30  on the first surface A. To simplify the process, the first photoresist layer  30  is defined as a negative photoresist. 
         [0030]    Step S 3 , the first photoresist layer  30  is patterned. Referring to  FIG. 4(   b ) and  FIG. 4(   c ), an exposure, development and baking processes may be performed on the first photoresist layer  30  successively, by using a reticle M 1 , a development device D and a baking device (not shown), respectively. As the first photoresist layer  30  is a negative photoresist, a through opening  22  is formed at a position corresponding to the reticle M 1 , so as to form a positioning layer  20 . In this embodiment, the through opening  22  is tapered and has a trapezoid shape, so the width of a cross section of the through opening  22  reduces gradually in a direction from the support base  10  to the positioning layer  20 . 
         [0031]    Step S 4 , a second photoresist material is coated on the second surface B of the support base  10 . In some embodiments, referring to  FIG. 4(   d ), the second surface B of the support base  10  may be cleaned first. Then a second photoresist layer  40  is formed on the second surface B. The second photoresist layer  40  may be an UV photosensitive material and defined as a positive photoresist. 
         [0032]    Step S 5 , the second photoresist layer  40  is patterned. Referring to  FIG. 4(   d ) and  FIG. 4(   e ), by using a reticle M 2  and a development device D, an exposure process and a development process may be performed on the second photoresist layer  40  successively, so as to form a grating structure on the second surface B. In  FIG. 4(   e ), four gratings  42  constitute the grating structure. In practice, parameters of the grating  42 , such as size, number and space between neighboring gratings, may be adjusted according to specific requirements. 
         [0033]    Step S 6 , the second surface B is etched to form an opening  12  extending trough the support base  10 . In some embodiments, referring to  FIG. 4(   e ), an etching rate is predetermined, and an etching line e may be designed according to the size, number and space of the gratings  42 . An etching device E is used to etch the second surface B along the etching line e, to form an opening  12  in the support base  10 . In the overall structure of the mask  100 , each of the openings  12  may respectively correspond to and be communicated to each of the through openings  22  in the positioning layer. And the openings  12  are formed under the through openings  22 . To avoid over etching and adverse influence on the adhesive force between the positioning layer  20  and the support base  10 , the minimum width of the cross section of the openings  12  is designed to be greater than the maximum width of the cross section of the through openings  22 . The opening  12  may have a substantially arc-shaped cross section, which has a width that reduces gradually in a direction from the support base  10  to the positioning layer  20 . Because  FIG. 4(   f ) is a partial cross-sectional view, only one opening  12  and its corresponding through opening  22  are shown in  FIG. 4(   f ). 
         [0034]    Step S 7 , the grating  42  on the support base  10  is removed to obtain the mask  100 . 
         [0035]    In order to enhance the adhesive force between the positioning layer  20  and the first surface of the support base  10 , a high temperature baking (e.g. above 100° C.) may be used to make the positioning layer  20  adhere more closely to the first surface A. 
         [0036]    In the method for forming a mask  100  according to the first embodiment, a polyimide material is coated on the first surface A of the base  10  to form the first photoresist layer  30 , and an UV photosensitive material is coated on the second surface B of the base  10  to form the second photoresist layer  40 . First, a positioning unit is obtained after an exposure process and a development process are performed on the first photoresist layer  30 . Then, a grating structure is obtained after an exposure process and a development process are performed on the second photoresist layer  40 . Further, the opening  12  is formed by etching the second surface B, which is communicated to the through opening  22 . The process described above for forming a mask is simple and is adapted for mass production. 
         [0037]    A second embodiment of a mask is illustrated in accordance with  FIG. 5 , the main structure of which may be similar to the mask  100  in the first embodiment. In the second embodiment, the positioning layer  20  is a double-layer structure, which includes a first-layer base  24  and an opening  22  corresponding to the first-layer base; and a second-layer base  24 ′ and an opening  22 ′ corresponding to the second-layer base. The opening  22  is communicated to the opening  22 ′. The opening  22 ′ is formed above the opening  22 . The width of a cross section of the opening  22 ′ is smaller than the width of a cross section of the opening  22 . In this way, a pattern of an organic light emitting material having a smaller size may be obtained through the opening  22 ′ through evaporation, thus improving the accuracy of the evaporation for forming a light emitting device. 
         [0038]    In some embodiments, according to actual requirements, the positioning layer  20  may include more than two layers, which should not be limited to the double-layer structure shown in  FIG. 5 . All through openings respectively formed in the multiple layers of the positioning layer  20  are communicated to each other. And the width of a cross section of each through opening increases gradually in a direction away from the support base to approaching the support base. Such design may not only allow the mask having a thickness within a reasonable range, but also obtain a through opening having a relatively smaller size, thereby obtaining a pattern of light emitting material having a corresponding small size, further improving the accuracy of the evaporation of the mask. 
         [0039]    A third embodiment of a mask  100   a  is illustrated in accordance with  FIG. 6  and  FIG. 7 . 
         [0040]    Referring to  FIG. 6  and  FIG. 7 ,  FIG. 6  schematically illustrates an overall structure of the mask  100   a.    FIG. 7  schematically illustrates a cross-sectional view along the line L 2 -L 2  of  FIG. 6 , which is a partial cross-sectional view of the mask  100   a.    
         [0041]    The main structure of the mask  100   a  is substantially the same as the mask shown in the first and second embodiments, which is not described in detail herein. The difference is that the positioning layer  20  of the first and second embodiments is a continuous structure, while a positioning layer  20   a  of the third embodiment has a plurality of separated structures. Further, a step is formed between the top surface of the positioning layer  20   a  and the top surface of the support base  10   a,  which has a height difference H in a longitudinal direction. The step having the height difference H can prevent an organic layer from being crushed by the mask during evaporation. 
         [0042]    A method for forming the mask  100   a  may be similar to the method for forming the mask  100  shown in  FIG. 3 . In the method of forming mask  100   a,  in the step S 3 , the photoresist layer on the support base  10   a  is patterned to form multiple separated positioning layers  20   a.  In some embodiments, a space between neighbouring positioning layers  20   a  is predetermined. Then, a plurality of separated reticles (not shown) are additionally provided, which may have their locations corresponding to the spaces of the multiple separated positioning layers  20   a.  By using the plurality of additional reticles, the photoresist layer on the support base  10   a  is divided into multiple separated photoresist layers through exposure and development processes. Then, a through opening  22   a  is formed in each separated photoresist layer, such that a plurality of separated positioning layers  20   a  are obtained. 
         [0043]    According to the third embodiment, a modified structure of the mask is provided. The positioning layers  20   a  having a plurality of separated structures is formed on the support base  10   a,  such that a step is formed between the top surface of the positioning layer  20   a  and the top surface of the support base  10   a,  which has a height difference H in a longitudinal direction. The step having the height difference H can prevent an organic layer from being crushed by the mask during evaporation. 
         [0044]    A fourth embodiment of a mask is illustrated in accordance with  FIG. 8 , the main structure of which may be similar to the mask  100   a  of the third embodiment. In the third embodiment, the positioning layer  20   a  is a double-layer structure, which includes a first-layer base  24   a  and an opening  22   a  corresponding to the first-layer base  24   a,  and a second-layer base  24   a ′ and an opening  22   a ′ corresponding to the second-layer base  24   a ′. The opening  22   a  is communicated to the opening  22   a ′. The opening  22   a ′ is formed above the opening  22   a.  The width of a cross section of the opening  22   a ′ is smaller than the width of a cross section of the opening  22   a.  In this way, a pattern of an organic light emitting material having a relatively smaller size may be obtained through the opening  22   a ′ during evaporation, thus improving the accuracy of the evaporation for forming a light emitting device. 
         [0045]    Likewise, in some embodiments, the positioning layer  20   a  may include more than two layers, which should not be limited to the double-layer structure shown in  FIG. 8 . All through openings respectively formed in the multiple layers of the positioning layer  20   a  are communicated to each other upper and lower. And the width of a cross section of each through opening increases gradually in a direction away from the support base to approaching the support base. Therefore, a through opening having a relatively smaller size can be obtained, thereby improving the accuracy of the evaporation of the mask. 
         [0046]    Although certain features have been disclosed above with reference to specific embodiments, it should be understood that the disclosure is presented by way of example only, and not limitation. Those skilled in the art can modify and vary the embodiments without departing from the spirit and scope of the present disclosure.