Patent Publication Number: US-2023161259-A1

Title: Pellicle for flat panel display photomask

Description:
RELATED APPLICATIONS 
     This application is a continuation application that claims priority to and the benefit of U.S. patent application Ser. No. 17/444,927, filed Aug. 12, 2021, and entitled PELLICLE FOR FLAT PANEL DISPLAY PHOTOMASK, which in turn is a continuation application that claims priority to and the benefit of U.S. patent application Ser. No. 16/568,365, filed Sep. 12, 2019, and entitled PELLICLE FOR FLAT PANEL DISPLAY PHOTOMASK, which claims priority to and the benefit of Provisional Application No. 62/730,119, filed Sep. 12, 2018 and entitled PELLICLE FOR FLAT PANEL DISPLAY PHOTOMASK, the contents of all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to photomask pellicles, and in particular to pellicles intended for use with large-size photomasks. 
     BACKGROUND 
     Flat-panel displays (FPDs) are electronic viewing technologies used to display content (e.g., still images, moving images, text, or other visual material) in a range of entertainment, consumer electronic, personal computer, and mobile devices, and many types of medical, transportation and industrial equipment. The current FPD types include, for example, LCD (Liquid Chrystal Display), AM LCD (Active Matrix Liquid Chrystal Display), OLED (Organic Light Emission Diode), LED (Light Emitting Diode), PDP (Plasma Display Panel) and AMOLED (Active Matrix OLED). 
     During manufacture of an FPD, an FPD lithography system irradiates light onto a photomask on which the original thin-film-transistor (TFT) circuit patterns are drawn, and the light exposes the patterns onto a glass plate substrate through a lens. On a large glass plate, the exposure process is repeated several times in order to form the patterns onto the entire plate. 
     Driven by end-user demands for better product quality and lower costs, FPD manufacturers are constantly searching for improved process equipment. Larger and thinner glass plates as well as tighter requirements lead to new challenges for equipment manufacturers. The glass plates are categorized by size and named by generations (GEN). For instance, Gen 8.5 glass plates have a size of 2200×2500 mm and can produce the panels needed for 55-inch LCD televisions. Photomasks must follow the size of FPD generations, because they are used as original plates to transfer patterns to TFT and color filter substrates. 
     As the size of photomasks used to manufacture large-size FPDs increases, a number of challenges arise in avoiding contamination of such photomasks by dust or other particles that might cause unwanted artifacts on the glass plate during the FPD lithography process. In this regard, conventional, smaller-sized photomasks may include a pellicle, which is a thin, transparent membrane or film that protects the photomask surface from contamination. However, the use of such pellicles with large-size photomasks requires an enhanced pellicle design that will function effectively over a large area. 
     SUMMARY OF THE INVENTION 
     A pellicle assembly for large-size photomasks according to an exemplary embodiment of the present invention comprises: a frame member configured to be affixed to a large-size photomask substrate; a substantially rigid and transparent pellicle membrane affixed to the frame member so as to protect at least a portion of the large-size photomask substrate from contamination during usage, storage and/or transport; and a coating on at least one of top and bottom surfaces of the pellicle membrane that binds the at least one of the top and bottom surfaces of the pellicle membrane to prevent separation of pellicle membrane material in the event of breakage. 
     In exemplary embodiments, the pellicle membrane is spaced from the photomask substrate by a distance of 3 mm to 20 mm. 
     In exemplary embodiments, the pellicle membrane is affixed to the frame member by adhesive. 
     In exemplary embodiments, the pellicle membrane is affixed to the frame member by a clamping mechanism. 
     In exemplary embodiments, the pellicle membrane has a transparency of at least 90% over a wavelength range of 190 nm to 500 nm. 
     In exemplary embodiments, the pellicle membrane has the following dimensions: outer dimension of 1146.0 mm×1366.0 mm and inner dimension of 1122.0 mm×1342.0 mm. 
     In exemplary embodiments, the pellicle membrane has the following dimensions: outer dimension of 1526.0 mm×1748.0 mm and inner dimension of 1493.0 mm×1711.0 mm. 
     In exemplary embodiments, the pellicle membrane has a thickness of 4 μm. 
     In exemplary embodiments, the pellicle membrane is made up of fused silica. 
     In exemplary embodiments, the coating meets wavelength requirements from 190 nm to 500 nm. 
     In exemplary embodiments, the pellicle assembly and photomask substrate are subjected to a compensation procedure within an exposure tool system to correct for any distortions induced by the pellicle. 
     In exemplary embodiments, the large-size photomask substrate is configured to manufacture a flat panel display. 
     In exemplary embodiments, the flat panel display is LCD (Liquid Chrystal Display), AM LCD (Active Matrix Liquid Chrystal Display), OLED (Organic Light Emission Diode), LED (Light Emitting Diode), PDP (Plasma Display Panel) or AMOLED (Active Matrix OLED). 
     These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example principles of the invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of this invention will be described in detail, with reference to the following figures, wherein: 
         FIG.  1    is a cross-sectional view of a large-size photomask assembly according to an exemplary embodiment of the present invention; 
         FIG.  2    is a top plan view of a large-size photomask assembly according to an exemplary embodiment of the present invention; 
         FIG.  3    is a partial cross-sectional view of a large-size photomask assembly according to an exemplary embodiment of the present invention; and 
         FIG.  4    is a flowchart of a process for compensating for mask or pellicle weight induced distortions according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    depicts a large-size photomask assembly, generally designated by reference number  1 , configured in accordance with an exemplary embodiment of the present invention. The photomask assembly  1  includes a large-size photomask (or reticle)  20  comprised of a substantially transparent substrate  22  to which one or more patterned layers of masking material  24  are affixed. The patterned layer of masking material  24  represents a scaled image of the pattern desired to be created on a glass panel of an FPD. The substrate may be comprised of fused silica and the masking material may be comprised of chromium. In exemplary embodiments, other types of materials may be used to form the photomask so that the present invention is not limited for use with photomasks having fused silica substrates and chromium masking material. Further, in exemplary embodiments, the pellicle of the instant invention can be used in conjunction with all types of photomasks including, but not limited to, binary masks and phase shift masks (PSM). 
     The large-size photomask  20  may be appropriately sized to accommodate photolithography processing of glass plate substrates used to form FPDs. In accordance with an exemplary embodiment, the large-size photomask  20  has dimensions of 1220 mm×1400 mm for Generation 8.5 size glass plates (e.g., glass plates having dimensions of 2200 mm×2500 mm). In another exemplary embodiment, the photomask  20  has dimensions of 3400 mm×3000 mm for Generation 10.5 size glass plates (e.g., glass plates having dimensions of 3370 mm×2940 mm). In exemplary embodiments, the large-size photomask  20  may be appropriately sized for photolithographic processing up to Generation 10.5 glass plate substrates and beyond as technology advances. For example, the large size photomask  20  may have dimensions in the range of 390 mm×610 mm (Generation 3) to 3400 mm×3000 mm (Generation 10.5). 
     As further shown in  FIG.  2   , photomask  20  also includes a pellicle frame or ring  26  which extends around the perimeter of the patterned masking material  24 . In an exemplary embodiment, frame  26  is made of anodized aluminum alloy, however, other materials may be used as well. Although shown as a continuous ring, in exemplary embodiments the frame  26  may have other shapes and may include various gaps or vents to ensure that pressure within the gap between pellicle and photomask comes to equilibrium at the end user site. Frame  26  is affixed to substrate  22  using adhesive  27 , such as, for example, hot-melt adhesive (HMA). In an exemplary embodiment, the HMA is styrene polymer. A liner  25  may be disposed between the adhesive  27  and the surface of the photomask substrate  22 . In an exemplary embodiment, the liner  25  is a polyester film. 
     In exemplary embodiments, the frame  26  may include one or more vents  21  configured to allow for equalization of pressure between the interior space formed below the pellicle membrane  28  and atmosphere. Each vent  23  may include a filter  23  that allows air and/or other gasses to pass through while filtering out particles. 
     The photomask assembly  1  further includes a pellicle membrane  28  disposed over the photomask  20 . In this regard, the pellicle membrane  28  may be affixed to the frame  26  by an adhesive  29 , such as, for example, a UV-curable adhesive. In an exemplary embodiment, the adhesive  29  used to affix the pellicle membrane  28  to the frame  26  has sufficient mechanical strength so as to withstand 30 psi air blow at a 1 inch distance. The pellicle membrane  28  generally conforms to the dimensions of the frame  26 . One or more of the edges or corners of the pellicle membrane  28  may be beveled or rounded for safety reasons. 
     The pellicle membrane  28  may be coated with one or more anti-reflective materials to give it suitable anti-reflective properties. The anti-reflective coating process can be done by spin-coating or vacuum deposition with low refractive index materials, examples of which include fluoropolymers, thin layers of oxides and oxynitrides such as TaO and TaON. In exemplary embodiments, the pellicle membrane  28  may include a coating that binds the surface to prevent the pellicle material from separating in the event of breakage. This coating preferably meets wavelength requirements from 190 nm to 500 nm. 
     In exemplary embodiments, the pellicle membrane  28  is made of cellulose ester or perfluoropolymer. In other exemplary embodiments, the pellicle membrane  28  may be a flat, polished, low birefringence slice of fused silica, as described in U.S. Pat. No. 6,524,754, the entire contents of which are incorporated herein by reference. The fused silica material used to form the pellicle membrane  28  may have the properties listed in Table 1. The transfer of the photomask image to the semiconductor wafer occurs through a process commonly referred to as photolithography. More specifically, a wafer exposure system is used to interpose the photomask between a semiconductor wafer which is coated with a layer of photosensitive material and an optical energy source. Energy from the wafer exposure system is inhibited from passing through the areas of the photomask in which the masking material is present. However, energy generated by the water exposure system passes through the portions of the substrate of the photomask not covered by the masking material and causes a reaction in the photosensitive material on the semiconductor wafer. Through subsequent processing, the image created on the photosensitive material is transferred to the semiconductor wafer. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Units of Measure 
                 SUMetrIc 
                 (Imperial) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Mechanical 
                   
                   
                   
               
               
                 Density 
                 gm/cc (Ib/fe 3 ) 
                 2.2 
                 (137.4)   
               
               
                 Porosity 
                 % (%) 
                 0 
                 0   
               
               
                 Color 
                 — 
                 clear 
                 — 
               
               
                 Flexural Strength 
                 MPa (Ib/in 2  × 10 3 ) 
                 — 
                 — 
               
               
                 Elastic Modulus 
                 GPa (Ib/in 2  × 10 6 ) 
                 73 
                 (10.6)  
               
               
                 Shear Modulus 
                 GPa (Ib/in 2  × 10 6 ) 
                 31 
                 (4.5)  
               
               
                 Bulk Modulus 
                 GPa (Ib/in 2  × 10 6 ) 
                 41 
                 (6)   
               
               
                 Poisson&#39;s Ratio 
                 — 
                 0.17 
                 (0.17) 
               
               
                 Compressive Strength 
                 MPa (Ib/in 2  × 10 3 ) 
                 1108 
                 (160.7)   
               
               
                 Hardness 
                 Kg/mm 2   
                 600 
                 — 
               
               
                 Fracture Toughness K IC   
                 MPa · m 1/2   
                 — 
                 — 
               
               
                 Maximum Use Temperature 
                 ° C. (° F.) 
                 1100 
                 (2000)     
               
               
                 (no load) 
               
               
                 Thermal 
               
               
                 Thermal Conductivity 
                 W/m · ° K (BTU · in/ft 2  · hr · ° F.) 
                 1.38 
                 (9.6)  
               
               
                 Coefficient of Thermal 
                 10 −6 /C. (10 −6 /° F.) 
                 0.55 
                  (.31) 
               
               
                 Expansion 
               
               
                 Specific Heat 
                 J/Kg · ° (Btu/lb · ° F.) 
                 740 
                 (0.18) 
               
               
                 Electrical 
               
               
                 Dielectric Strength 
                 ac-kv/mm ((volts/mil) 
                 30 
                 (750)    
               
               
                 Dielectric Constant 
                 @ 1 MHz 
                 3.82 
                 (3.82) 
               
               
                 Dissipation Factor 
                 @ 1 MHz 
                 0.00002 
                   (0.00002) 
               
               
                 Loss Tangent 
                 @ 1 MHz 
                 — 
                 — 
               
               
                 Volume Resistivity 
                 ohm · cm 
                 &gt;10 10   
                 — 
               
               
                   
               
            
           
         
       
     
     In exemplary embodiments, the pellicle membrane  28  preferably has a transmittance of ≥90% over a wavelength range of 190 nm to 500 nm with a stand off distance of 3 mm to 20 mm, and filters out particles that are ≥10 μm in size. 
     In an exemplary embodiment, in order to accommodate Generation 8.5 glass plate substrates, the pellicle membrane  28  may have one or more of the following characteristics: outer dimension of 1146.0 mm×1366.0 mm (+0.0, −4.0); inner dimension of 1122.0 mm×1342.0 mm (+0.0, −4.0); pellicle thickness of 4 μm (±0.2 μm); pellicle transmittance of ≥95% (average between 360 nm and 440 nm); pellicle frame material is aluminum alloy (black anodized); stand off of 7.0 mm (±0.2 mm). For the purposes of the present disclosure, the term “inner dimension” of the pellicle membrane may be defined as an orthogonal measurement of the inner most parts of the frame and the “outer dimension” of the pellicle membrane may be defined as an orthogonal measurement of the outer most parts of the frame. 
     In an exemplary embodiment, in order to accommodate Generation 10.5 glass plate substrates, the pellicle membrane  28  may have one or more of the following characteristics: outer dimension of 1526.0 mm×1748.0 mm (+0.0, −4.0); inner dimension of 1493.0 mm×1711.0 mm (+0.0, −4.0); pellicle thickness of 4 μm; pellicle transmittance of ≥95% (average between 360 and 440 nm); pellicle frame material is aluminum alloy (black anodized); stand off of 8.0 mm (±0.2 mm). 
     In exemplary embodiments, the pellicle membrane  28  may be secured to the frame using a removable frame assembly so that the pellicle can be easily removed and cleaned. For example, as shown in the cross-sectional view of  FIG.  2   , frame  42  made from anodized aluminum is affixed to substrate  22  by means of an adhesive, applicable types of which being well known in the art. Those skilled in the art will understand frame  42  can be made from materials other than anodized aluminum. In the preferred embodiment frame  42  extends around the entire perimeter of the patterned masking material, however, frame  42  need not be contiguous and may include one or more gaps. Frame  42  includes a first receptive area  44  which forms a shelf parallel to the surface of substrate  22  for receiving the lower surface of the outer edges of pellicle  28 . Frame  42  also includes a second receptive area or detent  46  which receives lower protrusion  52  of flexible retainer  50  which may be constructed from a variety of materials including plastics and polytetrafluoroethylene (e.g., Teflon). An upper protrusion  54  of retainer  50  extends over the first receptive area  44  of frame  42  and over the upper surface of the outer edge of pellicle  28  thereby holding pellicle  28  securely in place. Accordingly, in this embodiment there may be no need for adhesive to affix the pellicle to the frame. For aid in the installation and removal of flexible retainer  50 , the corners of retainer  50  may include flexible tabs  56 . When an upward force is exerted on flexible tabs  56 , lower protrusion  52  is decoupled from second receptive area  46  of frame  42 . With lower protrusion  52  decoupled from frame  42 , retainer  50  can be removed thereby enabling pellicle  28  to be removed as well. 
     In this embodiment, no vent is necessary in frame  42  since pressure can be relieved through the gaps between frame  42 , pellicle  28 , and retainer  50 . Additionally, since no adhesive is used to secure the pellicle to the frame, the pellicle can be more readily removed, cleaned, and/or replaced. 
     In exemplary embodiments, compensation within or on the pellicle membrane material itself may be used to correct for mask or pellicle weight induced distortions. In this regard, finished blanks may be paired with flat panel design layers to optimize flat panel mask manufacturing. Large area mask blank manufacturing data may be used along with required display lithography pattern design data and an understanding of the mask and lithographic process to pair and tune finished blanks to design for mask making optimization and yield improvement. As shown in  FIG.  4   , in exemplary embodiments, this process may involve one or more of the following steps: 
     Step S1: Actual mask blank manufacturing data including but not limited to blank flatness, defects (size and placement), film properties are overlaid with the proposed mask design pattern; 
     Step S2: Pairing and optimization is performed. This pairing and optimization may include shifting and adjustment of flat panel lithography design data to best match with the measured blank characteristics with aim to improve finished mask yield and performance in the intended application. The pairing and optimization may be performed by simulating the overlay of manufactured blank or blanks properties with the intended design data. 
     Step S3: An optimum blank is selected from a batch based on the simulation for the specific use and/or the pattern data may be scaled, tuned, embellished, rotated or otherwise manipulated to be compatible with the proposed blank to be used in the flat panel mask making operation. 
     Once the blank and mask design pattern elements are optimally merged then the mask is committed to manufacturing using the selected blank and the optimization parameters. The subsequent mask manufacturing process may access and track the overlaid mask-blank conditions, and the inspection and other mask making steps for flat panel display masks may use these conditions to tune or optimize the manufacturing flow. 
     While in the foregoing specification a detailed description of a specific embodiment of the invention was set forth, it will be understood that many of the details herein given may be varied considerably by those skilled in the art without departing from the spirit and scope of the invention.