JP2015081968A - Pellicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pellicle which secures a large exposure area with a frame reduced in width, prevents occurrence of deformation or wrinkling of the frame during production and transportation of the pellicle and allows pasting a pellicle film to a photomask with a desirable dimensional accuracy.SOLUTION: A pellicle is rectangular, with side lengths of at least a pair of sides greater than 500 mm. A frame decreases in width owing to the concave shape of an inner wall of the frame in the region with the side center of 40-80% of the side lengths of the pair of sides as the center, and the width of the frame in the narrowed region is 3-6 mm.

Description

  The present invention relates to a pellicle used as a dust prevention when manufacturing a semiconductor device, an IC package, a printed board, a liquid crystal display, an organic EL display, or the like, and relates to a large pellicle whose side length exceeds 500 mm.

  In the manufacture of semiconductors such as LSI and VLSI, or liquid crystal displays, patterns are produced by irradiating a semiconductor wafer or glass plate for liquid crystal with ultraviolet light, and if dust adheres to the photomask used at this time, Since the dust blocks or reflects the ultraviolet light, there is a problem that the transferred pattern is deformed, short-circuited, and the quality is deteriorated.

  For this reason, these operations are normally performed in a clean room. However, since it is still difficult to keep the photomask always clean, exposure is performed after a pellicle is attached to the surface of the photomask to prevent dust. In this case, since the foreign matter does not adhere directly to the surface of the photomask but adheres to the pellicle, the foreign matter on the pellicle becomes irrelevant to the transfer if the focus is set on the photomask pattern during lithography.

  Such a pellicle is generally configured by adhering or adhering a transparent pellicle film made of nitrocellulose, cellulose acetate, fluororesin, or the like, which transmits light well, to the upper end surface of a frame made of aluminum, stainless steel, or the like. The lower end of the frame is provided with an adhesive layer made of polybutene resin, polyvinyl acetate resin, acrylic resin, silicone resin, etc. for mounting on a photomask, and a release layer (separator) for the purpose of protecting the adhesive layer. ing.

  Since the pellicle membrane is generally a thin resin, an appropriate amount of tension is applied to the frame in order to adhere and support the pellicle membrane to the frame without sagging. Accordingly, in a generally used rectangular pellicle, the frame after the pellicle film is attached is somewhat bent inward due to the tension of the pellicle film. This phenomenon occurs, for example, in large pellicles with large frame lengths used in printed circuit board and liquid crystal display manufacturing, as well as in small pellicles for semiconductor manufacturing that use low-rigidity frames due to material and dimensional constraints. Also appears prominently.

  On the other hand, there is a demand for a photomask to secure an exposure area as much as possible for cost reduction. However, if the deflection to the inside of the frame as described above is not made as small as possible, the usable exposure area is reduced. There is. For this reason, if a narrower frame is developed while suppressing the amount of bending as much as possible, and the inner exposure area can be increased, the effect of cost reduction is greater.

  By the way, the actual dimensions of the photomask and pellicle are determined for each exposure machine. For example, the outer shape of the pellicle is designed to have an inner dimension of about 5 to 30 mm from the outer dimension by subtracting the support area of the photomask and the area used for handling from the outer shape of the photomask. Such a design is based on the premise of not only the exposure machine but also the specifications of all equipment related to the exposure process, such as the inspection machine and pellicle mount device, and it is actually difficult to change this. is there.

  Therefore, conventionally, means for achieving a wider exposure area have been devised. For example, Patent Document 1 describes a frame for a large pellicle in which the frame width of the short side is narrower than the long side. In this frame, for example, a particularly large frame having a side length exceeding 1000 mm is described. In the case of a frame, there is a problem that it cannot be applied to a large frame because it is difficult to maintain rigidity. Further, this frame can be applied to the short side, but the long side has a problem that the exposure area cannot be enlarged in the short axis direction because the frame cannot be thinned because the frame is bent.

  As a means for solving such a bending of the frame, for example, in Patent Document 2, at least a pair of sides of the frame body, an arc-shaped portion having an outer convex central portion, an arc-shaped portion having an outer concave shape on both sides thereof, and A pellicle frame having a linear portion on the outside is described. According to the pellicle frame having such a shape, it is said that the inward bending of the frame due to the tension of the pellicle film can be prevented and the reduction of the exposure area can be prevented. This is limited to the case where the amount of bending of the pellicle frame can be controlled to a certain value or less by design.

  However, when trying to make the frame width as narrow as possible, theoretically speaking, any frame width should be able to balance the tension and obtain the linear shape of the frame. Even if the shape is obtained, the rigidity is surely reduced, so the displacement is too large to handle, and there is a problem that wrinkles occur on the pellicle film with a slight external force. However, there is a problem that it cannot be put into practical use.

Patent No. 4007752 Japanese Patent No. 4286194

  Therefore, in view of the circumstances as described above, the object of the present invention is to ensure a large exposure area with a narrow frame, and without causing deformation or wrinkling of the frame during manufacture and transportation of the pellicle, To provide a pellicle capable of attaching a pellicle film to a photomask with a desired dimensional accuracy.

  The present inventors have intensively investigated and studied the actual use situation of the photomask, and it is extremely rare that a wider exposure area is required in both the major axis direction and the minor axis direction of the photomask. Since it was found that one of the major axis direction and the minor axis direction is almost required in the actual exposure work, in order to obtain the substantial exposure area to the maximum, the exposure pattern The present invention has been conceived by deliberately sacrificing unused areas.

  That is, the pellicle of the present invention is a rectangular pellicle having at least a pair of side lengths of more than 500 mm, and at least in the region centered on the side center of 40 to 80% of the side lengths of the pair of opposing sides. Is narrowed by making the inner wall of the frame concave, and the frame width of the narrowed region is 3 mm or more and 6 mm or less.

  The pellicle of the present invention is characterized in that the reinforcing means is connected to the adjacent side from both ends of the narrow part of the frame or to the same part of the opposite side from both ends of the thin part.

  Further, the mask adhesive layer is not provided on the photomask side surface of the reinforcing means, and it is preferable that the reinforcing means is separated from the photomask surface after the pellicle is attached. It is preferable that the film side surface is on the same plane as the pellicle film side surface of the frame, and a pellicle film adhesive layer is provided on the surface to adhere the pellicle film.

  Further, the reinforcing means is preferably processed integrally with the frame, and the cross-sectional shape of the reinforcing means is a triangular shape having a vertex on the photomask side or a trapezoidal shape having a narrow surface width on the photomask side. It is preferable. The mask adhesive layer is preferably provided with the same width over the entire periphery of the pellicle and is arranged in a shape along the outer peripheral shape of the frame.

  According to the present invention, in the region of 40 to 80% of the side length, the frame width is narrowed by denting the inner wall of the frame, so that the exposure region actually used is enlarged while maintaining the rigidity of the entire pellicle frame. Therefore, there is no problem such as wrinkles on the pellicle film during handling, and the pellicle film can be attached to the photomask while maintaining the desired dimensional accuracy.

FIG. 1 is a top plan view showing an embodiment (1) of the present invention. FIG. 2 is a front view showing the embodiment (1) of the present invention. FIG. 3 is a right side view showing the embodiment (1) of the present invention. FIG. 4 is a lower plan view showing the embodiment (1) of the present invention. FIG. 5 is an upper plan view showing the embodiment (2) of the present invention. FIG. 6 is a front view showing the embodiment (2) of the present invention. FIG. 7 is a right side view showing the embodiment (2) of the present invention. FIG. 8 is a lower plan view showing the embodiment (2) of the present invention. FIG. 9 is a BB sectional view showing the embodiment (2) of the present invention. FIG. 10 is a top plan view showing the embodiment (3) of the present invention. FIG. 11 is a schematic view of the pellicle of the present invention attached to a photomask. FIG. 12 is a schematic view showing a conventional pellicle.

  Hereinafter, although embodiment of this invention is described, this invention is not limited to this.

  1 to 4 show a pellicle embodiment (1) of the present invention. The frame 11 constituting the pellicle 10 includes a long side 11a and a short side 11b, and the width of the central portion of the short side 11b is narrowed from the inner side toward the outer side in the region indicated by the arrow A. The area A is effective to be 40 to 80% of the inner length of the short side 11b, and the frame width in the area A is required to be 3 mm or more and 6 mm or less.

  The length of the region A is preferably selected as appropriate according to the pattern range of the photomask to be used. However, when the length exceeds 80% of the side length, the rigidity is significantly reduced, and a reinforcing means described later is provided. Therefore, it is effective to set the length to 80% or less of the side length. Moreover, although the lower limit of the area A is not particularly provided, it is not preferable to cut below 40% from the viewpoint of cost. In addition, if the frame width in the area A is 3 mm or less, the area of the mask adhesive layer and the pressure area when the pellicle is attached cannot be secured. It is required to be 6 mm or less.

  The frame 11 can be manufactured using materials such as aluminum alloy, steel, stainless steel, various engineering plastics, and fiber reinforced plastics. However, the frame 11 can be integrally formed by mechanical cutting or thermal processing such as casting or injection molding. It is preferably manufactured and the surface thereof is subjected to surface treatment for preventing reflection, oxidation and dust generation.

  In the embodiment (1) of FIGS. 1 to 4, the region A having a narrow frame width is provided on the short side 11b, but this may be provided on the long side 11a side, and the long side and the short side. It may be provided in both. It is preferable that both ends of the region A are smoothly connected to the outer region by, for example, increasing the width in a tapered shape from the viewpoint of rigidity and dust generation prevention. Further, in the region A with a narrow frame width, it is particularly possible to reduce the deflection toward the inside of the frame by combining the configuration in which the frame protrudes outward as in the prior art described in Patent Document 2, for example. preferable.

  On the long side 11a of the frame 11, a sheet-like filter 13 made of a PTFE porous film or the like for preventing intrusion of air holes 12 and foreign matters is attached via an adhesive layer (not shown). On the frame 11, a non-penetrating jig hole 14 for handling is arranged on the outer side of the long side 11a, and a groove 15 for handling is arranged on the outer side of the short side.

  A mask adhesive layer 16 made of a rubber-based adhesive, a hot-melt adhesive, a silicone adhesive, or the like is provided on one surface of the frame 11, and a separator (not shown) for protecting the adhesive layer is provided on the surface as needed. Is provided. The mask adhesive layer 16 is provided along the outer peripheral shape of the frame 11 with the same width over the entire periphery, and is also arranged linearly in the region A where the frame width is narrowed.

  Here, “along the outer peripheral shape” includes not only that the mask adhesive layer is provided along the outer periphery but also that the mask adhesive layer is disposed at a certain distance from the outer periphery. Further, the idea of the present invention also includes a case where the corner portion of the frame 11 is locally different from the outer peripheral shape.

  A pellicle film adhesive layer 17 made of an acrylic adhesive, a fluorine adhesive, a silicone adhesive, or the like is provided on the opposite surface of the frame 11, and a thickness made of a cellulose resin, a fluorine resin, or the like is provided on the upper surface thereof. The pellicle film 18 having a thickness of 2 to 10 μm is adhered without slack with an appropriate tension.

  5 to 9 show another embodiment of the present invention (hereinafter referred to as embodiment (2)). In this embodiment (2), the pellicle 50 is composed of a frame 51 having a long side 51a and a short side 51b. The region indicated by the arrow C near the center of the short side 51b is the same as that in the above-described embodiment (1). Similarly, the pellicle frame width is narrowed from the inside to the outside of the frame 51. And in this embodiment (2), the reinforcement means 51c is further provided toward the long side 51a which adjoins from the both ends D of the area | region C. As shown in FIG.

  The frame 51 can be manufactured using a material such as aluminum alloy, steel, stainless steel, various engineering plastics, fiber reinforced plastic, etc., as in the embodiment (1). It is preferable that the surface is manufactured integrally by a thermal processing method such as the above, and the surface is subjected to surface treatment for preventing reflection, oxidation and dust generation.

  The reinforcing means 51 c can be made of the same material as the frame 51. The reinforcing means 51c manufactured separately can be attached by a fixing means such as an adhesive, a screw, and a rivet. However, since the dust generation prevention process is complicated, thermal cutting such as machining or casting, injection molding, etc. It is preferable that the long side 51a and the short side 51b are integrally processed by a forming unit. Further, it is more preferable that the connecting portion is provided with an R chamfering or the like as required, and is designed to have a shape that hardly generates dust.

  The cross-sectional shape of the reinforcing means 51c is a triangular shape having a vertex on the photomask side as shown in FIG. 9A or a trapezoidal shape with a narrow surface width on the photomask side as shown in FIG. 9B. Preferably there is. Here, the two-dot chain line 90 is a virtual line indicating the photomask surface position when the pellicle 50 is attached, and the two-dot chain line 91 is a virtual line indicating a pattern on the photomask.

  In the triangular shape or trapezoidal shape of the reinforcing means 51c, one side surface is a vertical surface with respect to the photomask, and the opposite surface is an inclined surface. At this time, the vertical surface is a photomask pattern 91. It is preferable that it is arrange | positioned in the direction (inner side) arrange | positioned, and an inclined surface is arrange | positioned in the direction (outside) without the pattern 91. By arranging the reinforcing means 51c in this direction, the influence of irregular reflection during exposure on the exposure quality can be reduced. The width of the reinforcing means 51c and the connection position (angle) to the adjacent side may be determined from a viewpoint of improving the rigidity, but a range of approximately 3 to 15 mm is preferable.

  A mask adhesive layer 56 is provided on one surface of the frame 51, and a pellicle film adhesive layer 57 is provided on the opposite surface, but the mask adhesive layer 56 is not provided on the reinforcing means 51c. good. Further, the lower surface of the reinforcing means 51c is preferably on the same plane as or higher than the lower surfaces of the long side 51a and the short side 51b of the frame, and is preferably positioned away from the photomask surface after attaching the pellicle. This is to prevent the reinforcing means 51c from coming into contact with the photomask surface 90 due to an external force to generate dust, and it is preferable to secure a separation height of at least 1 mm.

  And it is preferable that the mask adhesion layer 56 is provided along the external shape of the flame | frame 51 similarly to the said embodiment (1). The mask adhesive layer 56 is not provided on the reinforcing means 51c, but is provided with the same width along the outer peripheral shape of the frame 51 over the entire periphery, and also in the region C where the frame width is narrowed. Has been placed. Here, “along the outer peripheral shape” means that a mask adhesive layer is provided along the outer periphery in the same manner as in the above embodiment (1), and is arranged at a certain distance from the outer periphery. It is also included. Further, the idea of the present invention also includes a locally different shape from the outer peripheral shape at the corner portion of the frame 51 or the like.

  The pellicle film side surface of the reinforcing means 51c is preferably on the same plane as the pellicle film side surface of the long side 51a and the short side 51b, and the long side 51a and the short side 51b are on the surface. It is preferable that a pellicle film adhesive layer 57 formed continuously with the pellicle film adhesive layer is provided, and that the pellicle film 58 is adhered to the surface thereof with an appropriate tension without slack.

  In this embodiment (2) as well, vent holes 52, filters 53, jig holes 54, grooves 55, and the like are provided on the frame 51, but these preferred embodiments are the same as in the above-described embodiment (1). Therefore, explanation is omitted.

  FIG. 10 shows a top plan view of still another embodiment (3). In a frame 101 composed of a long side 101a and a short side 101b, both ends F of a region E where the width on the long side 101a is narrowed. The reinforcing means 101c is connected across the opposite sides. And since the rigidity of the long side 101a is ensured by such a reinforcing means 101c, the film tension is maintained even in the region E where the frame width is narrow, and the frame attaching accuracy is also ensured. In this embodiment (3), the required embodiments other than the arrangement of the reinforcing means 101c are the same as those in the above-described embodiments (1) and (2), and the description thereof will be omitted.

  FIG. 11 is a plan view showing a state in which the pellicle 50 of the above embodiment (2) is attached to the photomask 110 on which the pattern 111 is drawn. In the above embodiments (1), (2) and (3), the presence or absence of the reinforcing means or the arrangement thereof is different, but these may be properly used depending on the shape of the pattern 111 of the photomask to be used.

  The embodiment (1) has the simplest configuration, the frame can be easily processed, and the number of components in the pellicle is small, so that there is an advantage that the quality can be easily maintained not only during manufacture but also during use. . Further, in the embodiments (2) and (3), the frame width can be designed to be narrower than in the case of the embodiment (1) due to the effect of the reinforcing means, so that a wider exposure area can be secured. effective. Further, the embodiment (3) has an advantage that the processing and manufacturing of the frame are relatively easy as compared with the embodiment (2). On the other hand, on the short side, the distance to the opposite side is long. However, since the reinforcing means is extremely long, there is a drawback that it is difficult to apply.

  By the way, as described above, the range in which the pattern is actually drawn on the photomask varies depending on the product to be manufactured, but it is rare that the exposure area is required to the limit in both the major axis direction and the minor axis direction. As shown in FIG. 11, the area required for exposure is often only one of the major axis direction and the minor axis direction. Therefore, in most cases, since there is an area that does not affect the exposure, it is not necessary to reduce the frame width or it is possible to provide reinforcement means, so several models can be set. For example, most cases can be handled.

  Further, in the pellicle of the present invention, since the outer shape can be aligned with the same shape as a standard pellicle, it is possible to use a commonly used sticking device, and the process of an inspection machine, etc. Since only the inspection area needs to be changed, there is an advantage that it is very easy to introduce.

Examples of the present invention will be described more specifically below.
<Example 1>
A pellicle 10 according to the embodiment (1) as shown in FIGS. The frame 11 was manufactured by machining an aluminum alloy, and had an outer dimension of 750 × 904.5 mm, an inner dimension of 734 × 890.5 mm, and a height of 5.8 mm. The short side 11b of the frame 11 has a narrow area A, and the length of the area A is 580 mm. The width of the short side 11b is 7 mm, but the frame width in this area A is as thin as 4 mm. The frame 11 was provided with a vent hole 12, a jig hole 14, and a groove 15, and a black anodized treatment was applied to the entire surface.

  Next, the frame 11 was brought into a class 10 clean room, washed with a surfactant and pure water, and dried well. A silicone adhesive (manufactured by Shin-Etsu Chemical Co., Ltd.) was applied as a mask adhesive layer 16 to the lower surface of the frame 11 with a dispenser, and was cured by heating. The mask adhesive layer 16 is provided with a constant width of 4 mm along the outer shape of the frame 11.

  Further, a silicone pressure-sensitive adhesive (manufactured by Shin-Etsu Chemical Co., Ltd.) was applied as a pellicle film adhesive layer 17 to the opposite surface, and this was heated and completely cured. And the membrane filter 13 made from PTFE was attached via the acrylic adhesive sheet so that the ventilation hole 12 might be covered.

  Furthermore, after a fluororesin (trade name: Cytop, manufactured by Asahi Glass Co., Ltd.) is diluted with a solvent on a quartz substrate and uniformly applied by the die coating method, the solvent is removed by heating in an oven to dry the film. Got. Thereafter, a frame-shaped jig having the same dimensions as the substrate outer shape was adhered to the film on the substrate, and the film was peeled off by lifting the frame-shaped jig from the substrate to produce a pellicle film 18 having a thickness of about 4 μm. Then, the pellicle film 18 was adhered to the pellicle film adhesive layer 17 on the manufactured frame 11, and a surplus portion around the periphery was cut out with a cutter to complete the pellicle 10.

  When the pellicle film 18 of the completed pellicle 10 was closely observed, no wrinkles or slack were observed, and the pellicle film 18 was kept in a good state. After that, the pellicle 10 was affixed to a quartz substrate (not shown) having a size of 800 × 920 × 10 mm. However, during handling, the frame 11 is greatly bent or the pellicle film 18 is wrinkled. There was no such thing. Furthermore, when the dimension after sticking was confirmed, the dimension of the long side and the short side was as prescribed | regulated, and the squareness between the long side and the short side was maintained.

<Example 2>
A pellicle 50 according to the embodiment (2) as shown in FIGS. The size of this pellicle was 1146 × 1366 mm for outer dimensions, 1122 × 1342 mm for inner dimensions, and 5.8 mm in height. The short side 51b of the frame 51 has a narrow area C, and the length of the area C is 880 mm. Although the width of the short side 51b is 12 mm, the frame width in this region C is as thin as 5.5 mm.

  The reinforcing means 51c is integrally formed with the long side 51a and the short side 51c so as to be connected to the adjacent long side 51a from the D part at both ends of the region C. Here, the reinforcing means 51c has a width of 6 mm and a height of 5.8 mm, and the connection portion between the long side 51a and the short side 51b is smoothly connected with R having a radius of 2 mm, and the angle formed with the long side 51a is set to 30 degrees. did. Further, the frame 51 was provided with a vent hole 52 having a diameter of 1.5 mm, a jig hole 54, and a groove 55, and the entire surface was subjected to black alumite treatment.

  The frame 51 was provided with a mask adhesive layer 56 and a pellicle film adhesive layer 57, and the pellicle film 58 was bonded to finish the pellicle 50, and then the whole was observed. The stiffness of 51b was maintained, and no wrinkles or slack were observed in the pellicle film 58 even during handling. Further, when this pellicle 50 was attached to a 1220 × 1400 × 13 mm quartz substrate (not shown), the dimensions of each side and the perpendicularity between the long side and the short side were maintained with a predetermined accuracy.

<Comparative Example 1>
In Comparative Example 1, a pellicle with the same outer dimensions as in Example 1 was manufactured in the same process. However, this pellicle has an external shape as shown in FIG. 12 and has an outer dimension of 750 × 904.5 mm, an inner dimension of 734 × 896.5 mm, and a height of 5.8 mm, and the entire length of the short side is as narrow as 4 mm. .

  When the completed pellicle was confirmed, wrinkles occurred in the pellicle film near the center of the short side even when the short side of the frame was lightly touched with a finger, and handling was extremely careful. In addition, when attached to a quartz substrate (not shown) of 800 × 920 × 10 mm, a slight oblique wrinkle was observed in the pellicle film near the center of the short side, and the size was also shifted by about 1.5 mm from the normal position. It has a parallelogram shape and cannot be used.

<Comparative Example 2>
In Comparative Example 2, a pellicle having the same outer dimensions as in Example 2 was manufactured in the same process. However, this pellicle also has a conventional outer shape shown in FIG. 12, and has an outer dimension of 1146 × 1366 mm, an inner dimension of 1116 × 1354 mm, and a height of 5.8 mm, and the entire length of the short side is as narrow as 6 mm.

  When the completed pellicle was confirmed, the shape of the pellicle was maintained, but wrinkles occurred on the pellicle film nearby just by lightly touching the short side of the frame with a finger. Also, when trying to stand this pellicle upright from the horizontal state, the short side of the frame is too large and there is a risk of plastic deformation, so it cannot stand upright unless the center of the short side is supported from below. It was. Furthermore, the frame is deformed by a slight external force even when it is set up vertically, so that it is extremely difficult to use it normally, including pasting work.

10 Pellicle 11 Frame 11a Long side 11b Short side 12 Vent hole 13 Filter 14 Jig hole 15 Groove 16 Mask adhesive layer 17 Pellicle film adhesive layer 18 Pellicle film 50 Pellicle 51 Frame 51a Long side 51b Short side 51c Reinforcement means 52 Vent hole 53 Filter 54 Jig hole 55 Groove 56 Mask adhesive layer 57 Pellicle film adhesive layer 58 Pellicle film 90 Photomask surface 91 Pattern 101 Frame 101a Long side 101b Short side 101c Reinforcing means 110 Photomask substrate 111 Pattern part

Claims (7)

  At least a pair of side lengths of a rectangular pellicle exceeding 500 mm, and in a region centered on a side center of at least 40 to 80% of the side lengths of a pair of opposite sides, the width of the frame A pellicle characterized in that the pellicle is thinned by making it concave and the frame width of the thinned region is 3 mm or more and 6 mm or less.   2. The pellicle according to claim 1, wherein the reinforcing means is connected to the adjacent side from both ends of the part having a narrow width of the frame or to the same part of the opposite side.   3. The pellicle according to claim 2, wherein a mask adhesive layer is not provided on the photomask side surface of the reinforcing means, and the reinforcing means is separated from the photomask surface after the pellicle is attached. .   The pellicle film side surface of the reinforcing means is on the same plane as the pellicle film side surface of the frame, and a pellicle film adhesive layer is provided on the surface, to which the pellicle film is bonded. The pellicle according to claim 2 or 3.   The pellicle according to any one of claims 2 to 4, wherein the reinforcing means is processed integrally with a frame.   The pellicle according to any one of claims 2 to 5, wherein the cross-sectional shape of the reinforcing means is a triangular shape having a vertex on the photomask side or a trapezoidal shape with a narrowed surface width on the photomask side. The pellicle according to any one of claims 1 to 6, wherein the mask adhesive layer is provided with the same width over the entire periphery of the pellicle, and is arranged in a shape along the outer peripheral shape of the frame.

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