JP5272791B2 - Manufacturing method of mold for nanoimprint - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a mold for nano-imprints, which forms a metal mold by reproducing a pattern on a resin layer when the production of a resin reproduction plate by methods such as an imprint while utilizing a high degree of freedom of the shape of a fine pattern using a silicon etching master is difficult. <P>SOLUTION: In the method for producing the mold for nano-imprints, the silicon etching master with a silicon fine pattern having a desired shape formed on the surface of a silicon substrate, a resin layer is formed on the surface of the silicon etching master, the surface of the fine pattern of the silicon etching master is exposed by partially removing the surface of the resin layer, a reproduction plate with a resin layer fine pattern in a desired shape formed on the resin layer on the silicon substrate is produced by etching the exposed silicon fine pattern, electroforming is carried out using the reproduction plate, so that the metal mold is produced. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for producing a nanoimprint mold.

  In recent years, expectations for nanoimprint technology are increasing as one of fine pattern transfer methods.

  S. Y. The nanoimprint method proposed by Chou et al. Mechanically forms a pattern by pressing an original mold (also called a mold, template, etc.) on which a pattern such as fine grooves and holes is formed, against the material to be transferred. This is a transfer method (see Non-Patent Document 1).

  The principle of the nanoimprint method is relatively simple, but in recent years, it has been demonstrated that it can transfer ultra-fine patterns of several tens to several nanometers, and is attracting attention as a candidate for next-generation lithography technology. is there. In addition, by utilizing the characteristics of a three-dimensional transfer method, practical application has started as a technology for transferring patterns with a high aspect ratio structure and complicated shapes such as curved and stepped shapes.

  In the pattern transfer by the nanoimprint method, as is clear from the principle, the pattern on the mold is transferred to the transfer material in a shape in which the unevenness is reversed, but the transfer fidelity is very high. This is one of the major characteristics of the nanoimprint method, which is different from the photolithography method in which the pattern shape to be formed changes due to the effects of light diffraction and the dissolution behavior of resist molecules.

  For this reason, in the nanoimprint method, a very high processing accuracy is required for the mold as the original plate. In order to achieve this accuracy, it is common to use a part of the manufacturing process of a semiconductor device or a photomask, and a silicon or quartz substrate formed with a fine pattern is often used as a mold. .

  In addition to these, molds using metals such as nickel are also used, but as this manufacturing method, replicas with concave and convex inverted by plating using fine resist, silicon, quartz, etc. patterns as masters are used. The electroforming process to manufacture is often employed. In this case, a part of a semiconductor device or photomask manufacturing process is generally used for forming a master fine pattern.

  As a nickel electroforming master, an advantage of using an etched silicon substrate is that the degree of freedom in pattern shape is high. A silicon substrate can be dry etched using a resist pattern formed by photolithography or electron beam lithography as a mask. By using this method, a pattern with a fine width of several tens of nanometers can be formed in any shape. can do.

  In addition, as a result of silicon dry etching technology being used in the manufacture of semiconductors and micromachines, methods for etching deeper than the opening width and methods for controlling the cross-sectional shape during etching have been established to some extent. As an example, a high aspect ratio structure in which the etching depth is 5 times or more the pattern width, a structure in which the side wall is tapered or a curved surface, and the like are realized. Such a structure is particularly important in fields such as micromachines, biotechnology, and optics, and a wide range of applications is expected.

  However, in the case of performing electroforming using a master having a fine pattern formed on the surface of a silicon substrate, there are many problems that hardly occur when a resist pattern is used as a master. One of them is that there are many cases where it is difficult to peel off the electroformed nickel mold from the silicon etching master. This is partly due to the fact that the shape of the silicon etching master is often special, but it is considered that this is essentially due to the difference in the ease of elastic deformation between the resist and silicon.

  In other words, since the resist master is likely to be elastically deformed, the external force applied during the peeling process causes a minute deformation in the resist master pattern, resulting in a gap between the nickel mold pattern and easy mold peeling. It is thought to become. On the other hand, since the silicon etching master is much less susceptible to elastic deformation than the resist, such a gap does not occur, and as a result, it is considered that it is difficult to remove the mold.

  Another problem is that when a silicon etching master is used, the silicon etching master is easily broken during electroforming or peeling. Unlike a silicon substrate having a smooth surface, which is used as a base material for a resist master, the silicon etching master has a concavo-convex pattern formed on the surface of the silicon substrate. For this reason, it is considered that when stress is generated due to a temperature change or the like during electroforming or peeling, a crack occurs due to stress concentration in the concavo-convex pattern portion, and the silicon substrate is cracked.

  Of course, cracking the substrate during electroforming is a major problem, but cracking of the substrate that occurs when the master is peeled off after electroforming is also a problem. Because the silicon etching master after electroforming and the nickel mold are in direct contact, and since silicon is harder than nickel, when a crack occurs in the silicon etching master, its fracture surface comes into contact with the nickel mold, This is because the nickel mold is easily damaged.

  Thus, it can be said that the resist master is preferable as the master for the nickel mold, but on the other hand, a structure with a high aspect ratio or a taper structure close to vertical is difficult to form with a resist pattern, and silicon etching You must use a master.

  As one of the solutions to this problem, a silicon etching master having an inverted inversion is first prepared, and this surface structure is applied to a resin substrate or silicon or silicon using a technique such as imprinting, hot pressing, UV replication, or injection molding. A process of replicating on the surface of the resin layer applied on the glass substrate and electroforming a nickel mold from this replica can be considered.

  If this method is used, the above-mentioned problem can be solved, but another problem may be caused in practical use. This is because, in the process of replicating the pattern from the silicon etching master to the resin, if stress is applied to the pattern on the silicon etching master, unlike silicon, silicon hardly undergoes elastic deformation and brittle fracture occurs. At the time of peeling, the fine pattern of the silicon etching master may be destroyed. Such destruction is particularly likely to occur in high aspect ratio structures. When such destruction occurs, broken silicon pieces remain on the surface of the duplicate plate, and cannot be used as an electroforming master as it is.

  Further, even if the above-described destruction does not occur, for example, in the case of a fine pattern in which the side wall shape forms a reverse taper shape, it may be quite difficult to peel off the duplicate plate. In such a pattern, even if a replica plate or an electroformed nickel mold can be formed, it is considered that there is no problem because subsequent imprinting is impossible. However, in reality, there may be cases where imprinting is possible even with such a reverse taper pattern depending on the deformation characteristics of the resin to be imprinted, so there is no need for such a reverse taper pattern. I can't say that.

  As described above, a method of once producing a replica plate made of resin by a method such as imprinting is an effective method, but a problem remains that it is difficult to apply depending on the shape of a fine pattern.

Appl. Phys. Lett. , Vol. 67, p. 3314 (1995)

  The present invention replicates a pattern on a resin layer when it is difficult to produce a resin replication plate by a method such as imprinting while taking advantage of the high degree of freedom of the fine pattern shape by using a silicon etching master. It is another object of the present invention to provide a method for producing a nanoimprint mold for forming a metal mold.

  The invention according to claim 1 of the present invention is to manufacture a silicon etching master in which a silicon fine pattern having a desired shape is formed on a silicon substrate surface, and to form a resin layer on the surface of the silicon etching master. The surface of the fine pattern of the silicon etching master is exposed by partially removing the substrate, and the exposed silicon fine pattern is etched to form a resin layer fine pattern having a desired shape on the resin layer on the silicon substrate. The method for producing a mold for nanoimprinting is characterized in that a duplicate plate is produced, electroforming is performed using the duplicate plate, and a metal mold is produced.

  In the invention according to claim 2 of the present invention, the silicon fine pattern is formed by forming an electron beam resist on a single crystal silicon substrate, selectively patterning the electron beam resist to form a fine resist pattern, The method for producing a mold for nanoimprinting according to claim 1, wherein the mask is formed by etching a single crystal silicon substrate.

  The invention according to claim 3 of the present invention is the method for producing a mold for nanoimprinting according to claim 1 or 2, wherein the silicon fine pattern has an inversely tapered cross section.

  The invention according to claim 4 of the present invention is characterized in that the etching of the single crystal silicon substrate is performed by reactive dry etching, so that the cross section of the silicon fine pattern has a reverse taper shape. Any one of the methods for manufacturing a mold for nanoimprinting is described.

  The invention according to claim 5 of the present invention is the method for producing a nanoimprint mold according to any one of claims 1 to 4, wherein the resin layer is a PMMA layer.

  According to the present invention, when it is difficult to produce a resin replication plate by a method such as imprinting while utilizing the high degree of freedom of a fine pattern shape by using a silicon etching master, a pattern is formed on a resin layer. The manufacturing method of the mold for nanoimprint which replicates and forms a metal mold can be provided.

(A)-(j) is a schematic sectional drawing explaining the process of the manufacturing method of the mold for nanoimprint which concerns on embodiment of this invention. (A)-(i) is a schematic sectional drawing explaining the process of the manufacturing method of the mold for nanoimprint which concerns on embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the embodiments, the same components are denoted by the same reference numerals, and redundant description among the embodiments is omitted.

  1 (a) to 1 (j) and FIGS. 2 (a) to 2 (i) are schematic cross-sectional views illustrating steps of a method for manufacturing a nanoimprint mold according to an embodiment of the present invention. The difference between FIGS. 1A to 1J and FIGS. 2A to 2I is that the shape of the silicon etching master shown in FIG. 1D is formed vertically toward the bottom surface of the single crystal silicon substrate 100. That is, the shape of the silicon etching master shown in FIG. 2D is formed in a reverse taper shape. Here, the reverse tapered shape means a shape in which the upper surface is wide and narrows toward the lower surface. Hereinafter, the process of the manufacturing method of the nanoimprint mold will be described.

  First, as shown in FIGS. 1A and 2A, electron beam resists 110 and 210 are applied to the surfaces of the single crystal silicon substrates 100 and 200. Next, as shown in FIGS. 1B and 2B, desired fine resist patterns 111 and 211 are formed by patterning the electron beam resists 110 and 210, respectively. As a patterning method of the electron beam resists 110 and 210, for example, a method by electron beam lithography is conceivable, but a known method may be appropriately used according to a desired design pattern. For example, known photolithography or nanoimprint technology may be used.

  Next, as shown in FIGS. 1C and 2C, the single crystal silicon substrates 100 and 200 are etched using the fine resist patterns 111 and 211 as a mask, and the single crystal silicon substrates 100 and 200 are etched. A desired microstructure is formed on the surface. As an etching method for the single crystal silicon substrates 100 and 200, reactive ion etching using a fluorine-based gas is most suitable. However, depending on the desired fine resist patterns 111 and 211, other known methods such as plasma are used. Etching or wet etching may be used.

  Next, as shown in FIGS. 1D and 2D, the single-crystal silicon substrate on which the fine resist patterns 111 and 211 used as the etching mask are removed and the desired silicon fine patterns 101 and 201 are formed. Cleaning to remove unnecessary foreign matters on the surfaces of 100 and 200 is performed to complete a silicon etching master. As this cleaning method, it is common to use ashing with oxygen plasma and wet cleaning using various organic and inorganic chemicals, but if there is no effect on the subsequent processes, either one can be used alone. Alternatively, other known methods such as cleaning using ozone water may be used.

  Next, as shown in FIGS. 1 (e) and 2 (e), resin layers 120 and 220 that will eventually become an electroforming mold are applied to the surface of the silicon etching master. As the resin layers 120 and 220, for example, PMMA (Poly (methyl methacrylate), polymethyl methacrylate resin) can be considered, but other known resins and resists may be used if there is no problem in the subsequent steps. .

  Next, as shown in FIGS. 1 (f) and 2 (f), the surfaces of the resin films 120 and 220 are scraped off until the upper surfaces of the silicon fine patterns 101 and 201 in the underlying silicon etching master are exposed. As this method, ashing using oxygen gas plasma is suitable, but other known methods may be used. For example, the resin layers 120 and 220 may be dissolved and removed using an organic or inorganic chemical, or may be removed by a physical method such as ion milling. Further, it may be scraped off by a method such as mechanical polishing.

  In this step, as long as the upper surfaces of the silicon fine patterns 101 and 201 are exposed, the resin may be removed to some extent, and the silicon fine patterns 101 and 201 may slightly protrude. Further, the silicon fine patterns 101 and 201 may be slightly shaved by mechanical polishing or the like.

  Next, as shown in FIGS. 1 (g) and 2 (g), the silicon fine patterns 101 and 201 on the exposed silicon etching master surface are gradually etched from the upper surface to form resin layer fine patterns 121 and 221. Form. At this time, it may be stopped at a stage where a part of the silicon fine patterns 101 and 201 is left, or the entire silicon fine pattern 101 may be etched as shown in FIG. In any case, since the metal does not bite between the silicon fine patterns 101 and 201 of the silicon etching master during the electroforming process, it is possible to prevent the silicon etching master from being broken by the stress during electroforming.

  Similarly, as shown in FIG. 1 (h ′), even if etching is slightly advanced from the bottom surface of the resin layer 120, there is no significant problem in the manufacturing process. However, in this case, since metal slightly bites between the silicon fine patterns 101 and 201 of the silicon etching master during the electroforming process, there is a high possibility that the silicon etching master will break, so care must be taken.

The etching method is not particularly limited as long as the single crystal silicon substrates 100 and 200 are selectively etched with respect to the resin layers 120 and 220. The reactive ion etching using the above-mentioned fluorine-based plasma may be used, or other known methods may be used. For example, isotropic dry etching using fluorine-based plasma may also be used. If damage to the resin layers 120 and 220 due to plasma becomes a problem, plasma using XeF ₂ gas may be used. Isotropic dry etching that is not used may be used.

  In addition, as long as the resin layers 120 and 220 have sufficient resistance to a strong alkaline aqueous solution, etching may be performed with a potassium hydroxide aqueous solution, a tetramethylammonium hydroxide aqueous solution, or the like. However, in this case, it is necessary to sufficiently examine in advance the resistance of the resin layers 120 and 220 to the etching solution and the anisotropy of the etching shape.

  Next, although not shown, a very thin conductive film is formed on the surfaces of the resin layer fine patterns 121 and 221 in which fine openings are formed in the above-described process. This conductive film serves as a seed layer for an electroplating process described later. Metals such as nickel, copper, silver, gold, and aluminum are often used as the material for this conductive film, but the selection depends on the function of the final imprint mold and the film forming method. The best one can be selected. Moreover, as a film forming method, sputtering, vacuum deposition, electroless plating, etc. are generally used. However, a known method may be appropriately used as long as no problem occurs in the process.

  Next, as shown in FIGS. 1I and 2H, electrolytic plating is performed using the conductive film as a seed layer to form thick electroformed metal layers 130 and 230 having a fine structure on the surface. As a material of the electroformed metal layers 130 and 230, nickel is generally used, but nickel alloy plating containing other components may be used. In addition, other known metal electrolytic plating, for example, copper plating, may be used as long as necessary characteristics can be obtained as a nanoimprint mold.

  Finally, as shown in FIGS. 1 (j) and 2 (i), the electroformed metal layers 130 and 230 are peeled off to obtain final metal molds 131 and 231. The peeling method may be physically peeled off, or the silicon etching master and the resin layers 120 and 220 may be dissolved using a strong alkaline aqueous solution such as potassium hydroxide.

  Since the fine patterns formed on the metal molds 131 and 231 according to the embodiment of the present invention can replicate the silicon fine patterns 101 and 201 formed on the silicon etching master almost faithfully, The forming ability can be utilized as it is.

  Also, since the electroplating original plate is a replica plate in which fine patterns are formed on the resin layers 120 and 220 on the single crystal silicon substrates 100 and 200, there is a problem when the silicon etching master is used as it is. Problems such as poor peeling from the metal molds 131 and 231 and cracking of the silicon etching master can be solved.

  Furthermore, when manufacturing a duplicate plate, a mechanical etching method such as imprint or embossing is not used, but a scientific etching method is used. Problems can also be avoided.

  As described above, in the method of manufacturing a nanoimprint mold according to the embodiment of the present invention, once a silicon etching master is manufactured, the silicon fine patterns 101 and 201 of the silicon etching master are embedded with the resin layers 120 and 220. After removing the surfaces of the resin layers 120 and 220, the silicon fine patterns 101 and 201 exposed by dry etching or wet etching are removed, thereby producing a resin layer replica that faithfully reflects the shape of the silicon etching master. By producing metal molds 131 and 231 using nickel using this resin layer replica, damage to single crystal silicon substrates 100 and 200 during electroforming or peeling can be prevented.

  Hereinafter, the nanoimprint mold manufacturing method of the present invention will be described with reference to Examples 1 and 2.

  The first embodiment will be described with reference to FIGS. First, as shown in FIG. 1A, a 200 mmφ single crystal silicon substrate 100 made of 725 μm thick single crystal silicon is prepared, and an electron beam resist 110 is spun at a thickness of 200 nm on the surface of the single crystal silicon substrate 100. Coated.

  Next, as shown in FIG. 1B, the electron beam resist 110 was subjected to patterning processing such as electron beam drawing and development to form a linear fine resist pattern 111 having an opening width of 100 nm and a pitch of 200 nm. .

  Next, as shown in FIG. 1C, the single crystal silicon substrate 100 is made almost vertical and has a depth of 500 nm by reactive ion etching using a fluorocarbon-based mixed gas plasma using the fine resist pattern 111 as an etching mask. Etched until.

  Next, as shown in FIG. 1D, the remaining fine resist pattern 111 is removed by ashing treatment using oxygen plasma and wet treatment using various chemical solutions, and the single crystal silicon substrate 100 is washed. A silicon fine pattern 101 having an opening width of 100 nm, a pitch of 200 nm, and a depth of 500 nm was formed on the surface of the single crystal silicon substrate 100 to obtain a silicon etching master.

  Next, as shown in FIG.1 (e), PMMA is spin-coated by the thickness of about 1 micrometer-2 micrometers on the surface of the silicon | silicone fine pattern 101, and also the whole opening part of the silicon | silicone fine pattern 101 is baked. An embedded resin layer 120 was formed.

  Next, as shown in FIG. 1F, an ashing process using oxygen plasma was performed, and the surface of the resin layer 120 was removed until the upper surface of the silicon fine pattern 101 was exposed.

  Next, as shown in FIG. 1G, the silicon fine pattern 101 was etched downward from the exposed surface by reactive ion etching using a fluorocarbon mixed gas plasma. Next, as shown in FIG.1 (h), the resin layer fine pattern 121 was obtained by etching to the depth of 500 nm in which the convex part of the silicon fine pattern 101 lose | disappears.

  However, in the manufacturing process of the nanoimprint mold according to the first embodiment, etching may be performed in the middle of etching, for example, to a depth of 450 nm (FIG. 1G), and then the process may proceed to the next electroforming process. On the contrary, the state of further etching, for example, etching to a depth of 520 nm (FIG. 1 (h ′)) may proceed to the next electroforming process, but in this case, the single crystal in the subsequent process It is necessary to pay attention to cracking of the silicon substrate 100.

  Next, as shown in FIG. 1 (i), after a thin nickel layer is formed on the surface of the resin layer fine pattern 121 by electroless plating, nickel electroforming is performed using the resin layer fine pattern 121 as a mold, and nickel is used. An electroformed metal layer 130 was obtained.

  Next, as shown in FIG. 1 (j), the metal mold 131 was obtained by physically peeling the electroformed metal layer 130 from the single crystal silicon substrate 100 and the resin layer fine pattern 121.

  Since the fine pattern formed on the metal mold 131 according to Example 1 can replicate the silicon fine pattern 101 formed on the silicon etching master almost faithfully, a fine pattern with a high degree of freedom can be obtained.

  In addition, since the original used for electroforming is a replica plate in which a fine pattern is formed on the resin layer 120 on the single crystal silicon substrate 100, the metal mold 131 becomes a problem when the silicon etching master is used as it is. It was possible to solve problems such as poor peeling and cracking of the silicon etching master.

  As Example 2, an example of a method for manufacturing a nanoimprint mold having a tapered side wall will be described with reference to FIGS. First, as shown in FIG. 2A, a 200 mmφ single crystal silicon substrate 200 made of single crystal silicon having a thickness of 725 μm is prepared, and an electron beam resist 210 is spun at a thickness of 200 nm on the surface of the single crystal silicon substrate 200. Formed by coating.

  Next, as shown in FIG. 2B, the electron beam resist 210 was subjected to patterning processing such as electron beam drawing and development to form a linear fine resist pattern 211 having an opening width of 200 nm and a pitch of 400 nm. .

  Next, as shown in FIG. 2C, the single crystal silicon substrate 200 is etched to a depth of 500 nm by reactive ion etching using a fluorocarbon-based mixed gas plasma using the fine resist pattern 211 as an etching mask. The various conditions of reactive ion etching were adjusted so that the etching side wall at this time had an inversely tapered shape.

  Next, as shown in FIG. 2D, the remaining fine resist pattern 211 is removed by ashing treatment using oxygen plasma and wet treatment using various chemical solutions, and the single crystal silicon substrate 200 is washed. A silicon fine pattern 201 having an upper opening width of 200 nm, a lower opening width of 300 nm, a pitch of 400 nm, and a depth of 500 nm was formed on the surface of the single crystal silicon substrate 200 to obtain a silicon etching master.

  Next, as shown in FIG. 2 (e), PMMA is spin coated on the surface of the silicon fine pattern 201 at a thickness of about 1 μm to 2 μm, and further baked, so that the entire opening of the silicon fine pattern 201 is covered. An embedded resin layer 220 was formed.

  Next, as shown in FIG. 2 (f), polishing was performed for the purpose of removing the surface of the resin layer 220 to expose the upper surface of the silicon fine pattern 201.

Next, as shown in FIG. 2G, isotropic dry etching using XeF ₂ gas is performed to etch the silicon fine pattern 201 downward from the exposed surface to a depth of 400 nm. It was. As a result, a resin layer fine pattern 221 having a forward tapered side wall shape was obtained.

  Next, as shown in FIG. 2 (h), after a very thin nickel layer is formed on the surface of the resin layer fine pattern 221 by vacuum deposition or sputtering, nickel electroforming is performed using the resin layer fine pattern 221 as a mold. Then, an electroformed metal layer 230 using nickel was obtained.

  Next, as shown in FIG. 2I, a metal mold 231 was obtained by dissolving and removing the single crystal silicon substrate 201 and the resin layer fine pattern 221 using a potassium hydroxide aqueous solution.

  Since the fine pattern formed on the metal mold 231 according to the second embodiment was able to reproduce the silicon fine pattern 201 formed on the silicon etching master almost faithfully, a fine pattern with a high degree of freedom could be obtained.

  In addition, since a replica plate in which a fine pattern is formed on the resin layer 220 on the single crystal silicon substrate 200 is used as an electroforming original plate, a metal mold 231 poses a problem when the silicon etching master is used as it is. It was possible to solve problems such as poor peeling and cracking of the silicon etching master.

  As described above, according to the present invention, the application range of the silicon etching master can be expanded, and a metal mold having a fine structure with a particularly high aspect ratio or a metal mold having a special shape such as a reverse taper shape is manufactured. Can do.

  According to the present invention, application to a wide range of fields such as micromachines, biotechnology, and optical devices can be expected.

100, 200 ... single crystal silicon substrate 101, 201 ... silicon fine pattern 110, 210 ... electron beam resist 111, 211 ... fine resist pattern 120, 220 ... resin layer 121, 221 ... resin layer fine pattern 130, 230 ... electroformed metal Layer 131, 231 ... Metal mold

Claims (5)

A silicon etching master having a silicon fine pattern having a desired shape formed on the silicon substrate surface is manufactured,
Forming a resin layer on the surface of the silicon etching master;
By partially removing the surface of the resin layer, the surface of the fine pattern of the silicon etching master is exposed,
Etching the exposed silicon fine pattern to produce a replica plate having a resin layer fine pattern of a desired shape formed on the resin layer on the silicon substrate,
A method for producing a mold for nanoimprinting, wherein a metal mold is produced by performing electroforming using the duplicate plate. The silicon fine pattern forms an electron beam resist on a single crystal silicon substrate,
Selectively patterning the electron beam resist to form a fine resist pattern;
The method for producing a mold for nanoimprinting according to claim 1, wherein the single crystal silicon substrate is etched using the fine resist pattern as a mask.   The method for producing a mold for nanoimprinting according to claim 1, wherein the silicon fine pattern has a reverse taper in cross section.   The nanoimprint mold according to any one of claims 1 to 3, wherein the single crystal silicon substrate is etched by reactive dry etching so that the silicon fine pattern has an inversely tapered cross section. Method. The method for producing a mold for nanoimprinting according to any one of claims 1 to 4, wherein the resin layer is a PMMA layer.

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