Biaxially oriented polyester film and method for producing the same

Abstract

To provide a biaxially-oriented polyester film that reduces the scattering of ultraviolet light during exposure and retains high handleability while preventing the occurrence of irregularities on a resist pattern wall surface, the biaxially-oriented polyester film suitable for a dry film resist support.SOLUTION: A biaxially-oriented polyester film has a laminated structure consisting of two or more layers. With the arithmetic average surface inclination of one film surface (A) being Δa(A), the geometric average surface inclination being Δq(A), and the refractive index in the film thickness direction being n(A), Δa(A)×n(A) is 3.4 or more and 3.8 or less and Δq(A)×n(A) is 3.9 or more and 4.5 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a biaxially oriented polyester film and a method for producing the same. [Background technology]

[0002] Dry film resist (hereinafter sometimes referred to as DFR) is used to form circuits on printed wiring boards, semiconductor packages, flexible boards, etc. DFR has a structure in which a photosensitive layer (photoresist layer) is laminated onto a polyester film support, and then sandwiched between protective films (cover films) made of polyethylene film, polypropylene film, polyester film, etc. Creating conductor circuits using this DFR generally involves the following process. 1) The process involves peeling off the protective film from the DFR and laminating it to the substrate and conductive base layer so that the surface of the exposed resist layer adheres tightly to the surface of the conductive base layer, such as copper foil, on the substrate. 2) Next, the photomask onto which the conductor circuit pattern is baked is placed on a support made of polyester film, and ultraviolet light is irradiated from above onto the resist layer, which is mainly made of photosensitive resin, to expose it. 3) After that, the photomask and polyester film are peeled off, and the unreacted portion of the resist layer is dissolved and removed using a solvent. 4) Next, a step of etching with acid or the like to dissolve and remove the exposed portion of the conductive substrate layer.

[0003] After step 4), the photoreactive portions of the resist layer and the corresponding portions of the conductive substrate layer remain intact. The remaining resist layer is then removed to form the conductor circuit on the substrate. Therefore, the polyester film support is required to be able to efficiently transmit UV light, allowing the conductor circuit pattern to be accurately reflected on the resist layer. In particular, in recent years, with the miniaturization and weight reduction of IT devices and other devices, printed wiring boards have become increasingly finer and denser. This has created a demand for polyester films for dry film resist supports that can form fine patterns with wiring widths and spacings of approximately 5 μm and achieve high resolution. Furthermore, it is important for polyester films used as supports to have adequate slip properties to ensure good handling when forming a resist layer on the support to produce a resist film. However, when particles are added as a lubricant to provide adequate slip properties, the particles aggregate, causing both transmission and reflection of scattered light during UV irradiation during the exposure process, resulting in a problem of reduced resist resolution.

[0004] Therefore, a polyester film for use as a dry film resist support has been proposed, characterized by a high film surface smoothness, a low haze value, and a transmittance at a wavelength of 365 nm within a certain range (Patent Document 1). Furthermore, a technology has been disclosed in which an antistatic agent is incorporated to improve the handleability and prevention of adhesion of dust and foreign matter during the production of polyester films for resists, as well as the handleability and prevention of adhesion of dust and foreign matter of resist films themselves using such films (Patent Documents 2, 3, and 4). Furthermore, a technology has been disclosed in which particles are added at any stage of production to improve dispersibility and prevent particle aggregation (Patent Document 5). Furthermore, a technology has been disclosed in which unevenness due to particle-based protrusions and depressions is set within a predetermined range, thereby suppressing defects in the resist pattern due to reflection and scattering near the film surface (Patent Document 6). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-87854 [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-117237 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-327158 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-254640 [Patent Document 5] Japanese Patent Application Laid-Open No. 2014-98136 [Patent Document 6] Japanese Patent Publication No. 2020-59242 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even these proposals have difficulty meeting the demand for higher resolution, and defects such as distortion, gaps, and poor resist pattern wall condition after development have not been fully resolved. Therefore, quality improvements toward higher resolution are still required. In particular, given the recent demand for fine patterns with wiring widths and spacings of approximately 5 μm, coarse particles of approximately 2 μm are believed to lead to resist defects. Examples of coarse particles include air bubbles (voids) formed during the stretching process, which are nucleated by particle aggregates applied to the base film. The presence of these particle-containing voids scatters the ultraviolet light irradiated during the exposure process, preventing the resist from curing according to the pattern. To prevent particle aggregation and reduce the particle-containing voids contained in the base film, the amount of particles added has been reduced and the particle size has been reduced. However, this has resulted in reduced yields in the film manufacturing process and the DFR manufacturing process. In particular, the deterioration of slipperiness has led to significant problems related to winding. Therefore, the demand for improved productivity (film winding ability) must also be met. In view of these circumstances, the present invention aims to provide a biaxially oriented polyester film suitable for use as a dry film resist support, which can reduce scattering of ultraviolet light during exposure by controlling the inclination of the film surface, maintain high handleability, and suppress the occurrence of unevenness on the resist pattern wall surface. [Means for solving the problem]

[0007] The present invention has the following features. [I] A biaxially oriented polyester film having a laminated structure of two or more layers, in which, when the arithmetic mean surface slope of one film surface (A) is Δa(A), the geometric mean surface slope is Δq(A), and the refractive index in the film thickness direction is n(A), Δa(A) × n(A) is 2.0 μm / mm or more and 4.0 μm / mm or less, and Δq(A) × n(A) is 3.0 μm / mm or more and 5.0 μm / mm or less. [II] The biaxially oriented polyester film according to [I], wherein Δa(A) of the film surface (A) is 1.5 μm / mm or more and 2.8 μm / mm or less, Δq(A) is 1.9 μm / mm or more and 3.2 μm / mm or less, and n(A) is 1.490 or more and 1.500 or less. [III] The biaxially oriented polyester film according to [I] or [II], wherein SRa(A) is the arithmetic mean surface roughness of the film surface (A) and SRz(A) is the 10-point average surface roughness, and SRa(A) is 1.5 nm or more and 2.5 nm or less, and SRz(A) is 15 nm or more and less than 35 nm. [IV] A biaxially oriented polyester film having a three-layer laminate structure, wherein when the layer without a surface is Layer B, Layer B has 40 or fewer coarse particles with a major axis of 2.0 μm or more when an area of ​​220 μm in the longitudinal direction and 290 μm in the transverse direction is observed in 200 fields of view using a laser microscope. [V] The surface (A) has voids with a major axis of 1.0 μm or more containing inert particles with a particle diameter of 0.05 μm or more, and the number of the voids is 2 / 10 mm 2 More than 6 pieces / 10mm 2The biaxially oriented polyester film according to any one of [I] to [IV], wherein the tensile strength is less than 1000 MPa. [VI] The biaxially oriented polyester film according to any one of [I] to [V], which has a film haze of 1.0% or less. [VII] The biaxially oriented polyester film according to any one of [I] to [VI], wherein the layer having the surface (A) contains particles, and when the particles contained in the layer are plotted with particle diameter on the horizontal axis and particle abundance ratio on the vertical axis, the particles have one or more maximum values ​​in the particle diameter range of 30 to 100 nm and in the particle diameter range of 100 to 200 μm. [VIII] A biaxially oriented polyester film according to any one of [I] to [VII], wherein the difference between SRa(A) and SRa(C) is 1 nm or less, when the arithmetic mean surface roughness of the film surface (C) opposite to the surface (A) is SRa(C). [IX] The biaxially oriented polyester film according to [VIII], wherein the static friction coefficient μs between the surface (A) and the surface (C) is 1.0 or more and 1.8 or less, and the dynamic friction coefficient μd between the surface (A) and the surface (C) is 0.6 or more and 1.0 or less. [X] The biaxially oriented polyester film according to [VIII] or [IX], wherein the layer having the surface (C) contains particles, and when the particle diameter of the particles contained in the layer is plotted on the horizontal axis and the particle abundance ratio on the vertical axis, the particle diameter has one or more maximum values ​​in the range of 30 to 100 nm. [XI] The biaxially oriented polyester film according to any one of [I] to [X], which is used as a dry film resist support. [XII] The biaxially oriented polyester film according to any one of [I] to [XI], which is used so as to be exposed to ultraviolet light from the surface (A) side when used as a dry film resist support. [XIII] A method for producing the biaxially oriented polyester film according to any one of [I] to [XII], which is obtained by melt-forming the film by co-extrusion. [Effects of the Invention]

[0008] According to the present invention, a biaxially oriented polyester film can be provided that is suitable for use as a dry film resist support, as it reduces reflection intensity during exposure, suppresses the occurrence of unevenness on the resist pattern wall surface, and maintains high handleability. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a conceptual diagram showing the geometric mean surface slope Δq(A) measured by a three-dimensional micro surface profile measuring instrument. DETAILED DESCRIPTION OF THE INVENTION

[0010] The biaxially oriented polyester film of the present invention has a laminate structure of two or more layers. From the viewpoint of improving the slipperiness of the film and also improving high permeability, the biaxially oriented polyester film of the present invention preferably has a laminate structure of three or more layers, and more preferably a laminate structure of three layers. When it has a three-layer laminate structure, it may be a two-kind three-layer structure of A layer / B layer / A layer, or a three-kind three-layer structure of A layer / B layer / C layer. A three-kind three-layer structure of A layer / B layer / C layer is preferred because it makes it easy to impart suitable properties to the film surface (A) of Layer A and the film surface (C) of Layer C, respectively.

[0011] In the biaxially oriented polyester film of the present invention, in order to reduce the reflection intensity during exposure, maintain high handleability, and suppress the occurrence of unevenness on the resist pattern wall surface, it is important that, when the arithmetic mean surface slope of one film surface (A) is Δa(A), the geometric mean surface slope is Δq(A), and the refractive index in the film thickness direction is n(A), Δa(A) × n(A) is 2.0 μm / mm or more and 4.0 μm / mm or less, and Δq(A) × n(A) is 3.0 μm / mm or more and 5.0 μm / mm or less.

[0012] The arithmetic mean surface slope Δa(A) represents the average absolute value of the slope of the line segment connecting the start and end points of the measurement curve in each measurement section (the slope angle in the Z-axis direction in a certain section parallel to the film surface) in a measurement curve obtained by measuring using the measurement method described below, with the direction parallel to the film surface as the X-axis, the direction parallel to the film surface and perpendicular to the X-axis as the Y-axis, and the direction perpendicular to the film surface as the Z-axis. The geometric mean surface slope Δq(A) represents the magnitude of the steepest slope in the planes equally spaced along the X-axis and Y-axis directions on the measurement curve obtained by measuring using the measurement method described below, with the direction parallel to the film surface as the X-axis, the direction parallel to the film surface and perpendicular to the X-axis as the Y-axis, and the direction perpendicular to the film surface as the Z-axis (Figure 1).

[0013] The inventors have conducted extensive research to suppress the occurrence of unevenness on the wall surface of the resist pattern that occurs in the resulting resist when a biaxially oriented polyester film is used as a dry film resist support, and have found that it is important to control the product of the inclination of the film surface of the biaxially oriented polyester film and the refractive index n(A) in the thickness direction of the biaxially oriented polyester film within a specific range.

[0014] Conventional techniques have reduced the scattering of UV light during exposure by limiting the protrusion shape and roughness of the film surface of biaxially oriented polyester films to specific ranges. However, while these techniques can reduce the scattering of UV light due to the unevenness present on the film surface, they have not been able to control the bending of UV light due to the inclination of the film surface. By limiting the product of the arithmetic mean surface slope Δa(A) and the refractive index n(A) of the polyester film in the thickness direction, and the product of the geometric mean surface slope Δq(A) and the refractive index n(A) of the polyester film in the thickness direction to specific ranges, it is possible to control the bending of UV light due to the inclination of the film surface. If Δa(A) × n(A) is less than 2.0 μm / mm or Δq(A) × n(A) is less than 3.0 μm / mm, it is possible to suppress the refraction of UV light passing through the film surface (A) during exposure, but this results in poor handleability during the production of polyester films for resists, handleability during the production of resist films by forming a resist layer on the film, and peelability during use as a photoresist base film. On the other hand, if Δa(A)×n(A) exceeds 4.0 μm / mm or Δq(A)×n(A) exceeds 5.0 μm / mm, the refraction of ultraviolet light passing through the film surface (A) during exposure will be intensified, resulting in unevenness on the resist pattern wall surface. Δa(A)×n(A) is preferably 3.4 μm / mm or more and 3.8 μm / mm or less, and Δq(A)×n(A) is preferably 3.9 μm / mm or more and 4.5 μm / mm or less.

[0015] In the biaxially oriented polyester film of the present invention, in order to reduce the reflection intensity during exposure, maintain high handleability, and suppress the occurrence of unevenness on the resist pattern wall surface, it is preferable that Δa(A) be 1.5 μm / mm or more and 2.8 μm / mm or less, and Δq(A) be 1.9 μm / mm or more and 3.2 μm / mm or less. If Δa(A) is less than 1.5 μm / mm or Δq(A) is less than 1.9 μm / mm, handleability during the production of a polyester film for resist and the handling during the production of a resist film by forming a resist layer on the film may be impaired. On the other hand, if Δa(A) is more than 2.8 μm / mm or Δq(A) is more than 3.2 μm / mm, scattering of ultraviolet light at the film surface (A) during exposure is intensified, making it difficult to form resist pattern wall surfaces suitable for forming fine circuits with wiring widths and spacings of approximately 5 μm. Δa(A) is more preferably 2.1 μm / mm or more and 2.7 μm / mm or less, and Δq(A) is more preferably 2.5 μm / mm or more and 3.1 μm / mm or less.

[0016] Furthermore, the biaxially oriented polyester film of the present invention preferably has a refractive index n(A) in the film thickness direction of 1.490 or more and 1.500 or less. The refractive index n(A) is an indicator of the substrate film removability after exposure of the dry resist film. If the refractive index n(A) is less than 1.490, the photoresist substrate film may be prone to cleavage when peeled off. Furthermore, scattering of ultraviolet light at the film surface (A) during exposure and refraction of light passing through the film layer are intensified, making it difficult to form resist pattern walls suitable for forming fine circuits with wiring widths and spacings of approximately 5 μm. On the other hand, if the refractive index n(A) exceeds 1.500, the film tends to stretch during peeling, resulting in peeling failure. The refractive index n(A) in the film thickness direction is more preferably 1.491 or more and 1.500 or less, and even more preferably 1.493 or more and 1.500 or less.

[0017] In the biaxially oriented polyester film of the present invention, SRa(A) is the arithmetic mean surface roughness of the film surface (A) and SRz(A) is the 10-point average surface roughness. Preferably, SRa(A) is 1.5 nm or more and 2.5 nm or less, and SRz(A) is 15 nm or more and less than 35 nm. When a resist layer is laminated on the film surface (A) and a cover film is laminated thereon and wound up, the surface (A) is the surface that comes into contact with the cover film. If SRa(A) is less than 1.5 nm or SRz(A) is less than 15 nm, handling during the resist coating process is impaired, coating becomes unstable, and coating spots occur. Furthermore, trapped air is difficult to remove during winding after coating, which can cause winding slippage. Furthermore, if SRa(A) is greater than 2.5 nm or SRz(A) is greater than 35 nm, unevenness on the surface or aggregation of the contained particles can cause the angle of incidence of light to become non-uniform during the exposure process, resulting in the reflection and scattering of light, which can cause gaps in the resist pattern. SRa(A) and SRz(A) can be set within the above ranges by incorporating specific amounts of specific organic or inorganic particles, as described below, into the polyester resin that constitutes the layer (A layer) having the surface (A).

[0018] In the biaxially oriented polyester film of the present invention, when the arithmetic mean surface roughness of the film surface (C) opposite to the surface (A) is defined as SRa(C), the difference between SRa(A) and SRa(C) is preferably 1 nm or less. It is preferable to set the arithmetic mean surface roughness SRa(A) and the 10-point average surface roughness SRz(A) of the film surface (A) within the above-mentioned ranges and set the difference between SRa(A) and SRa(C) to 1 nm or less, since this allows for both resist formation performance and ease of handling during processing. When the biaxially oriented polyester film of the present invention is used as a resist support, it is preferable to laminate a resist layer on the surface (C) side. The difference between SRa(A) and SRa(C) within the above-mentioned range can be achieved by incorporating specific amounts of specific organic or inorganic particles, as described below, into the polyester resin constituting the layer having the surface (A) (layer A) and the layer having the surface (C) (layer C).

[0019] When the biaxially oriented polyester film of the present invention has a laminate structure of three or more layers, and the layer without a surface is designated as Layer B, it is preferable that the number of coarse particles with a major axis of 2.0 μm or more present in Layer B when an area of ​​220 μm in the longitudinal direction and 290 μm in the transverse direction is observed in 200 fields of view using a laser microscope is 40 or less. If such coarse particles are present in Layer B, ultraviolet light irradiated during the exposure process may be scattered, resulting in voids in the resist pattern. In the present invention, "coarse particles" refers to solid particles that are not mixed in the polyester film or voids with such solid particles as their central nuclei. Coarse particles are considered to be composed of either a single particle or an aggregate of multiple particles. Furthermore, the major axis of the coarse particles referred to in the present invention refers to the longest measured length of the coarse particles detected by the measurement method described below. The method for achieving the number of coarse particles within the above range is not particularly limited, and examples include controlling the mesh size of the foreign matter collection filter used during melt film formation. For example, when a film is obtained by melt-forming using a co-extrusion method, it is effective to use an extruder with high precision that captures 95% or more of foreign matter of 2 μm or larger.

[0020] In the biaxially oriented polyester film of the present invention, the film surface (A) has voids with a major axis of 1.0 μm or more containing inert particles with a particle diameter of 0.05 μm or more, and the number of the voids is 2 / 10 mm. 2 More than 6 pieces / 10mm 2 The number of voids present on the film surface (A) is preferably less than 2 / 10 mm. 2 If the number of voids is less than 6 / 10 mm, the slipperiness will be significantly deteriorated, and the film surface will be scratched, which may cause exposure obstruction. 2 In the above cases, the ultraviolet light irradiated in the exposure step may be scattered, causing holes in the resist pattern.

[0021] In the biaxially oriented polyester film of the present invention, the layer having the surface (A) contains particles, and when the particles contained in the layer are plotted with particle diameter on the horizontal axis and particle abundance ratio on the vertical axis, it is preferable that the particles have one or more maximum values ​​in the particle diameter range of 30 to 100 nm and in the particle diameter range of 100 to 200 μm. If the layer having the surface (A) does not contain particles with such particle diameters, the handleability during production of a polyester film for a resist and the handleability of the resist film itself using the film will be deteriorated.

[0022] In the biaxially oriented polyester film of the present invention, the layer having the surface (C) contains particles, and when the particles contained in the layer are plotted with particle diameter on the horizontal axis and particle abundance ratio on the vertical axis, it is preferable that the particle diameter has one or more maximum values ​​in the range of 30 to 100 nm. If the layer having the surface (C) does not contain particles with such particle diameters, the winding properties of the polyester film for resist may deteriorate. Furthermore, uneven coating after coating the film with a resist and poor peeling of the substrate film after exposure of the dry resist film are likely to occur.

[0023] In the biaxially oriented polyester film of the present invention, the static friction coefficient μs between the surface (A) and the surface (C) is preferably 1.0 to 1.8, and the dynamic friction coefficient μd between the surface (A) and the surface (C) is preferably 0.6 to 1.0. By setting the dynamic friction coefficient μd between the surface (A) and the surface (C) in this range, both resist formation performance and handleability during processing can be achieved, which is preferable.

[0024] The biaxially oriented polyester film of the present invention must be biaxially oriented. In the present invention, biaxial orientation refers to a state in which an unstretched (unoriented) film is stretched in two dimensions by a conventional method (showing a biaxially oriented pattern in wide-angle X-ray diffraction). Stretching can be performed by sequential biaxial stretching or simultaneous biaxial stretching. In sequential biaxial stretching, the stretching steps in the longitudinal direction (longitudinal) and the width direction (transverse) can be performed once longitudinally and once transversely, or twice, such as longitudinal-transverse-longitudinal-transverse.

[0025] In the biaxially oriented polyester film of the present invention, the term "polyester resin as the main component" refers to a polyester obtained by polymerization of at least 70 mol % of monomers or oligomers whose main components are dicarboxylic acids, diols, and their ester-forming derivatives. In the present invention, it is preferable to use an aromatic dicarboxylic acid as the dicarboxylic acid.

[0026] Examples of aromatic dicarboxylic acids include terephthalic acid and 2,6-naphthalenedicarboxylic acid, with terephthalic acid being particularly preferred. These acid components may be used alone or in combination of two or more, and may be partially copolymerized with other aromatic dicarboxylic acids such as isophthalic acid or fatty acids.

[0027] Examples of diol components include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, etc., and among these, ethylene glycol is preferred. These diol components may be used alone or in combination of two or more.

[0028] Preferred polyesters used in the biaxially oriented polyester film of the present invention include polyethylene terephthalate, polyethylene naphthalate and copolymers thereof, polybutylene terephthalate and copolymers thereof, polybutylene naphthalate and copolymers thereof, polyhexamethylene terephthalate and copolymers thereof, polyhexamethylene naphthalate and copolymers thereof, etc., with polyethylene terephthalate being particularly preferred.

[0029] The polyester used in the present invention can be produced by a conventionally known method. For example, a method of directly esterifying an acid component with a diol component, followed by polycondensation while heating the reaction product under reduced pressure to remove excess diol component, or a method of using a dialkyl ester as the acid component, esterifying it with a diol component, followed by polycondensation in the same manner as above, can be used. In this case, conventionally known alkali metals, alkaline earth metals, manganese, cobalt, zinc, antimony, germanium, titanium compounds, etc. can also be used as a reaction catalyst, if necessary.

[0030] The intrinsic viscosity of the polyester is preferably 0.5 dl / g (lower limit) to 0.8 dl / g (upper limit), more preferably 0.55 dl / g (lower limit) to 0.70 dl / g (upper limit).

[0031] The biaxially oriented polyester film of the present invention preferably has a total thickness of 10 μm or more and less than 50 μm, and particularly preferably 12 μm or more and less than 40 μm. If the total thickness is less than 10 μm, the strength may be insufficient, making it difficult to handle during processing. If the total thickness is 50 μm or more, it may be difficult to achieve a light transmittance and haze value within the ranges of the present invention, and economic efficiency may also be reduced.

[0032] In the biaxially oriented polyester film of the present invention, a dimensional change within the following range is preferred because it can suppress the occurrence of distortion and wrinkles due to thermal shrinkage during the DFR processing step. The dimensional change can be achieved by appropriately adjusting the film-forming conditions, such as relaxation and heat treatment, using known methods. The dimensional change at 150°C is preferably 3.0% or less in the longitudinal direction and 2.5% or less in the width direction, and more preferably 0.5% to 2.0% in the longitudinal direction and 0.8% to 1.8% in the width direction. Furthermore, the dimensional change at 100°C is preferably 1.0% or less in both the longitudinal and width directions, and more preferably 0.2% to 0.8%. If the dimensional change is below the lower limit of the above range, poor flatness occurs due to sagging when the resist layer is applied. If the dimensional change is above the upper limit, shrinkage occurs during application of the resist layer, resulting in galvanized iron-like shrinkage spots, resulting in poor flatness. In either case, unevenness in the coating thickness of the resist layer may occur.

[0033] Furthermore, the biaxially oriented polyester film of the present invention preferably has a film haze of 1.0% or less. If the film haze exceeds 1.0%, the polyester film, which serves as a support for the resist layer, will significantly scatter ultraviolet light when the resist layer is laminated on the polyester film and then exposed to ultraviolet light, which may result in distortion or voids in the resist patterning after development, deterioration of the resist pattern wall surface, or reduced transmittance of the polyester film.

[0034] The biaxially oriented polyester film of the present invention may contain particles. Both organic and inorganic particles can be used. Organic particles include, for example, polyimide resins, olefin or modified olefin resins, crosslinked polystyrene resins, and silicone resins. Inorganic particles include, for example, spherical silica, silicon oxide, calcium carbonate, agglomerated alumina, aluminum silicate, mica, clay, talc, and barium sulfate. When using these particles, surface modification with a surfactant or other agent to improve affinity with polyester is preferred to suppress increases in light transmittance and haze, thereby suppressing the generation of voids around the added particles. Furthermore, particles with a nearly spherical shape and a small difference in refractive index from polyester can suppress scattered light when ultraviolet light passes through the film layer. Colloidal silica and organic particles are particularly preferred, with silicone particles and crosslinked polystyrene particles being particularly preferred. Crosslinked polystyrene particles made of a styrene-divinylbenzene copolymer prepared by emulsion polymerization are particularly preferred because of their nearly spherical shape and uniform particle size distribution, which allows for the formation of uniform protrusions. Colloidal silica or crosslinked polystyrene particles made of a styrene-divinylbenzene copolymer are preferably incorporated into the polyester resin composition constituting the surface layers (A and C layers), with the content preferably being 0.01% by weight or more but less than 0.1% by weight of the total polyester resin composition constituting the surface layers (A and C layers). A content below 0.01% by weight is undesirable because it can lead to poor handling during the resist coating process, unstable coating, and uneven coating. Furthermore, it can be difficult to remove trapped air during winding after coating, which can lead to misalignment during winding. A content above 0.1% by weight is undesirable because it can lead to the crosslinked polystyrene particles tending to aggregate, which can lead to uneven light incidence during the exposure process, resulting in light reflection and scattering, which can lead to gaps in the resist pattern.

[0035] In both the A and C layers, it is also desirable to use agglomerated alumina in combination with the particles. Here, agglomerated alumina refers to an agglomeration of several to several hundred particles with an average primary particle diameter of 5 nm or more and less than 30 nm. The average primary particle diameter of the agglomerated alumina is more preferably 8 nm or more and less than 15 nm. The agglomerated alumina can be produced by flame hydrolysis using anhydrous aluminum chloride as a raw material, or by hydrolysis of alumina alkoxide. Agglomerated alumina is known to have δ-, θ-, and γ-type crystal forms, with δ-alumina being particularly preferred. These agglomerated aluminas can be used by adding them during polyester polymerization. For example, agglomerated alumina with an average secondary particle diameter of 10 nm or more and less than 200 nm can be obtained by pulverizing and dispersing the agglomerated alumina in a sand grinder or the like, followed by microfiltration, as a slurry of ethylene glycol, which is a raw material used in polyester polymerization. When the agglomerated alumina obtained in this manner is added to a film, it is arranged in the plane direction by biaxial stretching, so it does not form substantial protrusions, has little effect on surface roughness, and has good transparency, thereby suppressing deterioration of light transmittance and haze value. The inclusion of agglomerated alumina significantly reinforces the surface of the film, improves abrasion resistance, and suppresses dent defects that occur when the film comes into contact with the roll during stretching. The agglomerated alumina is preferably incorporated into the polyester resin composition that constitutes the surface layer (A layer, C layer), and its content is preferably 0.1 wt% or more but less than 5.0 wt% of the total polyester resin composition. A content below 0.1 wt% is undesirable because it deteriorates handleability during the resist coating process, making the coating unstable and resulting in uneven coating, and may cause winding misalignment due to difficulty in escaping trapped air during winding after coating. If the content is 5.0 wt % or more, the aggregated alumina tends to aggregate even more easily, which is undesirable because it can cause holes in the resist pattern due to the effects of light reflection and scattering caused by uneven incident angles of light during the exposure step.

[0036] Next, the method for producing the biaxially oriented polyester film of the present invention will be described. In a method for incorporating inert particles into polyester during melt film formation by coextrusion, for example, the inert particles are dispersed in a predetermined ratio in the form of a slurry in ethylene glycol, a diol component, and then subjected to high-precision filtration capable of capturing at least 95% of coarse particles, for example, 2 μm or larger or 5 μm or larger. The ethylene glycol slurry is then added at any stage before the completion of polyester polymerization. When adding the particles, for example, adding the aqueous sol or alcohol sol obtained during particle synthesis without first drying it is preferable, as this improves particle dispersibility and suppresses the formation of coarse protrusions. Another effective method for achieving the effects of the present invention is to directly mix the aqueous slurry of particles with selected polyester pellets and feed them into a vented twin-screw kneading extruder to knead them into the polyester. An effective method for adjusting the particle content is to prepare a high-concentration master pellet using the above method, and then dilute it with PET, which essentially contains no particles, during film formation to adjust the particle content. In this case, the density of the particle-containing voids can be controlled by adjusting the intrinsic viscosity of the particle-free PET to be higher than that of the particle-containing pellets. Furthermore, if the intrinsic viscosity of the particle-containing pellets is higher or the same as that of the particle-free PET, the dispersibility of the particles decreases and the interparticle distance becomes shorter, which tends to increase the density of the particle-containing voids.

[0037] The particle-containing master pellets and pellets substantially free of particles prepared for each layer are mixed in a predetermined ratio, dried, and then fed into a known melt lamination extruder under a nitrogen stream or reduced pressure to prevent a decrease in intrinsic viscosity. The extruder used in producing the biaxially oriented polyester film of the present invention can be a single-screw or twin-screw extruder. A vented extruder equipped with a vacuum line can also be used to eliminate the pellet drying step. Furthermore, when an intermediate layer is to be formed, the extrusion volume is the highest, so a so-called tandem extruder can be used, in which each extruder has the function of melting the pellets and the function of maintaining the molten pellets at a constant temperature. A twin-screw vented extruder is preferably used to extrude the surface layer of the biaxially oriented polyester film of the present invention, as this maintains good particle dispersibility and suppresses particle aggregation.

[0038] The polymer melted and extruded in the extruder is filtered. Because even the smallest foreign particles can become large protrusions in the film, it is effective to use a high-precision filter capable of capturing at least 95% of foreign particles, for example, 2 μm or larger or 5 μm or larger. The polymer is then extruded into a sheet through a slit die and cooled and solidified on a casting roll to produce an unstretched film. Specifically, the polymer is layered using multiple extruders and multilayer manifolds or merging blocks (e.g., merging blocks with rectangular merging sections), and the sheet is extruded through a die and cooled on a casting roll to produce an unstretched film. In this case, installing a static mixer or gear pump in the polymer flow path is effective for stabilizing back pressure and suppressing thickness fluctuations.

[0039] The stretching method may be simultaneous biaxial stretching or sequential biaxial stretching. In the case of sequential stretching, the stretching temperature in the longitudinal direction is preferably 90°C or higher but lower than 130°C, more preferably 105°C or higher but lower than 110°C. A stretching temperature lower than 90°C is prone to breakage of the film, while a stretching temperature higher than 130°C is undesirable because the film surface is susceptible to thermal damage. Furthermore, from the viewpoint of preventing uneven stretching and scratches, it is effective to provide a preheating zone before stretching and heat the film in stages, with the preheating temperature in the longitudinal direction being preferably 65°C to 135°C, more preferably 70°C to 130°C.

[0040] The stretching ratio in the longitudinal direction is preferably 3 times or more and less than 4.5 times, and more preferably 3.5 times or more and less than 4.3 times.

[0041] The obtained uniaxially stretched film is first cooled. The cooling temperature is preferably 18°C ​​or higher and 40°C or lower, and more preferably 21°C or higher and 35°C or lower. Cooling stabilizes the width dimension, and even if the film surface is smooth according to the present invention, scratches caused by the film transport rolls and wrinkles can be suppressed.

[0042] The film is then stretched in the width direction in a known stenter oven to form a biaxially stretched film. The film is held by clips running on rails in the stenter oven and heated again to a temperature above the glass transition temperature of the resin in the oven, and is stretched in the width direction as the rails on which the clips run expand. At this time, the stretching ratio in the width direction is preferably 3.2 times or more and less than 5 times, more preferably 4.0 times or more and less than 4.6 times.

[0043] Here, it is important to heat the film in stages to control the surface roughness and surface gradient of the film. Uniaxially stretched films are heated to 90°C to 110°C before being stretched in the width direction, and the film, once sufficiently heated above its glass transition temperature, is then stretched in the width direction at 105°C to 110°C. This facilitates film stretching and suppresses minute stretching irregularities caused by particles forming on the film surface.

[0044] Next, the resulting biaxially stretched film can be heat-treated. The heat treatment can be carried out in the same stenter oven as the widthwise stretching, or in a different oven from the stenter oven used for widthwise stretching. The heat treatment temperature is preferably 170°C or higher and 250°C or lower. Heat treatment is preferred because it improves dimensional stability when exposed to high temperatures in subsequent processing steps or when used as a final product. It is also preferred to relax the film in the width direction by more than 0% but not more than 8% during heat treatment to further improve dimensional stability.

[0045] The biaxially stretched film is preferably cooled before leaving the oven. The cooling temperature is preferably 100° C. or higher and 130° C. or lower, more preferably 110° C. or higher and 125° C. or lower. Cooling stabilizes the width dimension, and prevents scratches and wrinkles on the film transport rolls even if the film surface is smooth. The film is then cut at the edges and wound up to obtain an intermediate product. During this transport process, the film thickness is measured and the data is used as feedback to adjust the film thickness by adjusting the die thickness, etc., and to detect foreign objects using a defect detector.

[0046] In the biaxially oriented polyester film of the present invention, it is preferable to suppress the generation of chips when cutting the edges. A round blade, a shear blade, or a straight blade can be used to cut the edges. When using a straight blade, it is preferable to avoid contact of the blade with the film at the same point all the time, as this reduces blade wear. Therefore, it is preferable to have a mechanism for oscillating the blade to its upper limit. It is also preferable to install a suction device at the film cutting point to suck up chips and chips generated when the film edges are scraped together after cutting.

[0047] The intermediate product is slit to an appropriate width and length in a slitting process and wound up to obtain a roll of the biaxially oriented polyester film of the present invention. When cutting the film in the slitting process, a cutting method similar to the edge cutting described above can be selected.

[0048] The intermediate product is slit to a desired width to obtain the biaxially oriented polyester film of the present invention. The biaxially oriented polyester film of the present invention thus obtained has good permeability and slip properties, and is therefore suitable for use as a dry film resist support.

[0049] In particular, the circuit wiring of electronic information devices is becoming increasingly finer. Therefore, in films for dry film resist supports used in circuit wiring production, it is necessary to improve wiring depictability by minimizing light scattering on the film surface during ultraviolet exposure. The biaxially oriented polyester film of the present invention is preferably used so that it is exposed to ultraviolet light from the film surface (A), since the film surface (A) can suppress the effects of light scattering and the like during ultraviolet exposure. [Example]

[0050] The present invention will be explained in more detail below with reference to examples, but the present invention should not be construed as being limited thereto.

[0051] (Measurement method) (1) Film surface gradient (Δa, Δq values) The surface (A) was measured using a three-dimensional micro surface profiler (ET-350K manufactured by Kosaka Manufacturing Co., Ltd.), and the arithmetic mean surface slope Δa value and the geometric mean surface slope Δq value were calculated from the obtained surface profile curve using the following calculation formulas (Formula 1 and Formula 2). The measurement conditions were as follows:

[0052]

number

[0053]

number

[0054] X-direction measurement length: 0.5 mm, X-direction feed rate: 0.1 mm / sec. Y-direction feed pitch: 5 μm, number of Y-direction lines: 40 Cutoff: 0.25mm. Stylus pressure: 0.02mN. Height (Z direction) magnification: 50,000 times (2) Refractive index A test piece was taken from an arbitrary position of the roll, and the refractive index in the thickness direction was measured. Instrument: Abbe refractometer Light source: sodium light source (3) Film surface roughness (SRa, SRz values) Surfaces (A) and (C) were measured using a three-dimensional micro surface profiler (ET-350K manufactured by Kosaka Seisakusho), and the arithmetic mean surface roughness (SRa) and ten-point mean surface roughness (SRz) were calculated from the obtained surface profile curves in accordance with JIS B0601. The measurement conditions were as follows: X-direction measurement length: 0.5 mm, X-direction feed rate: 0.1 mm / sec. Y-direction feed pitch: 5 μm, number of Y-direction lines: 40 Cutoff: 0.25mm. Stylus pressure: 0.02mN. Height (Z direction) magnification: 50,000 times (4) Number of large foreign objects A biaxially oriented polyester film was cut into a 5 cm x 5 cm piece, and an image of 220 μm in the longitudinal direction x 290 μm in the transverse direction was captured in layer C using a laser microscope (Keyence VK-X250) with a 50x objective lens. The captured image was binarized, and the number of coarse particles with a major axis of 2.0 μm or more present within a certain depth range from the film surface was counted using a particle analysis module (Keyence VKH1XG). The same procedure was repeated for 200 fields of view, and the total was calculated. Note that the major axis of the coarse particles in this invention refers to the longest measured length of the coarse particles detected by the laser microscope.

[0055] (5) Density of particle-containing voids The film surface (A) is observed for particles using a digital microscope (Keyence Corporation, VHX-7000 model) at a magnification of approximately 5000x. Voids that are confirmed to contain particles with a particle size of 0.05 μm or more are considered particle-containing voids, and the number of voids with a major axis of 1.0 μm or more is counted. 2 The major axis of the particle-containing void in the present invention refers to the longest measured length of the particle-containing void detected by the digital microscope.

[0056] (6) Film haze According to JIS K7105-1981, a sample measuring 4.0 cm in length and 3.5 cm in width is cut out from the center of the film width direction, and the haze is measured using a haze meter (HGM-2DP (for light source C) manufactured by Suga Test Instruments).

[0057] (7) Friction coefficient In accordance with JIS K7125-1987, two samples were prepared by cutting out from a biaxially oriented polyester film. After neutralizing the surface of each sample, the surface (A) of one sample was placed on the surface (C) of the other sample, the edges of the film were fixed, and a load of 200 g (normal force 1.96 N) was applied, and the film was moved 10 mm at 150 mm / min. The slip coefficients (static friction coefficient μs, kinetic friction coefficient μd) were calculated from the tension.

[0058] (8) Particle size The particle size of particles contained on the film surface is measured as follows: The polymer is removed from the film using a low-temperature plasma ashing method, exposing the particles. The processing conditions are selected so that the polymer is ashed but the particles are minimally damaged. The processed sample is observed with a scanning electron microscope (SEM; Hitachi, Ltd., S-4000 model), and the particle images are imported into an image analyzer (Nireco Corporation, LUZEX_AP). The equivalent circle diameter is measured, and the volume average particle size of the particles is determined. The SEM magnification is appropriately selected from 5,000 to 20,000 times depending on the particle size. The volume average particle size of at least 5,000 particles is measured at various observation locations, and the average value is taken as the volume average particle size of the particles. The average value of these results is also taken as the volume average particle size of the particles. From these results, particle diameters expressed in classes at 10 nm intervals starting from 0 nm were plotted on the horizontal axis against the number of particles having these particle diameters (abundance ratio) on the vertical axis to create a graph of particle size distribution, and the particle diameter with the maximum value was determined. (9) Heat shrinkage rate Two lines, 10 mm wide and approximately 100 mm long, were drawn on the surface of the laminated film, and the distance between the two lines was measured at 23°C and designated L0. This laminated film sample was then left under a 3g load for 30 minutes in an Espec Corporation hot air oven "HIGH-TEMP-OVEN PHH-200" set to 100°C or 150°C (air flow gauge "7"), after which the distance between the two lines was measured again at 23°C and designated L1. The thermal shrinkage was calculated using the following formula. Measurements were performed on five samples in both the longitudinal and transverse directions, and the average value was used for evaluation. Heat shrinkage rate (%) = (L0-L1) / L0 x 100 When analyzing a film, if the longitudinal direction and width direction of the film are unknown, the direction in which the film has the maximum refractive index is considered to be the width direction, and the direction perpendicular to that direction is considered to be the longitudinal direction. The direction in which the film has the maximum refractive index may be determined by measuring the refractive indexes in all directions of the film using an Abbe refractometer, or may be determined by determining the direction of the slow axis using, for example, a retardation measurement device (birefringence measurement device).

[0059] (10) Visual Inspection of Resist Resolution The visual evaluation of the resist resolution of the biaxially oriented polyester film of the present invention was carried out as follows: (i) A negative resist "PMER N-HC600" manufactured by Tokyo Ohka Kogyo Co., Ltd. was applied to a one-side mirror-polished 6-inch Si wafer and spun using a large spinner to form a 7 μm-thick resist layer. This was then pre-heated for approximately 20 minutes at 70°C using a nitrogen-circulating ventilated oven. (ii) The A-layer side of the polyester film was placed on top of the resist layer, and a polyester film was laminated onto the resist layer using a rubber roller. A photomask patterned with chromium metal was placed on top of this, and exposure was performed using an I-line stepper through the photomask. (iii) After peeling the polyester film from the resist layer, the resist layer was placed in a container containing developer N-A5 and developed for approximately 1 minute. The film was then removed from the developer and washed with water for approximately 1 minute. (iv) The L / S (μm) (Line and Space) state of the resist pattern created after development was observed at 1000x magnification using a scanning electron microscope (SEM). The resist resolution was evaluated according to the following criteria. A rating of △ or higher is considered to be at a practical level. ◎: L / S = 8 / 8 μm is clearly visible. ○: L / S=8 / 8μm cannot be clearly confirmed, but L / S=10 / 10μm can be clearly confirmed. △: L / S=10 / 10μm cannot be clearly confirmed, but L / S=15 / 15μm can be clearly confirmed. Practical level. ×: L / S=15 / 15μm cannot be clearly confirmed (not applicable to production).

[0060] (11) Handling properties of resist film (evaluation of slipperiness) A resist film was produced by using the biaxially oriented polyester film of the present invention as a support and forming a resist layer made of a negative photosensitive resin on the A layer side by coating. The evaluation of slipperiness as handling properties during the production of the resist film was based on the following criteria. Note that an evaluation of △ or higher is considered to be at a practical level. ○: Appropriate slipperiness and good handling properties. △: Poor slipperiness and poor handling is at a practical level . ×: Handling is difficult due to lack of proper slipperiness (not applicable to production).

[0061] (raw materials) (Preparation of Polyester A) Esterification reaction was carried out between 86.5 parts by weight of terephthalic acid and 37.1 parts by weight of ethylene glycol at 255°C while distilling off water. After the esterification reaction was completed, 0.02 parts by weight of trimethyl phosphate, 0.06 parts by weight of magnesium acetate, 0.01 parts by weight of lithium acetate, and 0.0085 parts by weight of antimony trioxide were added. The mixture was then heated to 290°C under vacuum to carry out a polycondensation reaction, yielding polyester pellets with an intrinsic viscosity of 0.63 dL / g (Polyester A).

[0062] (Creating Polyester B) In producing polyester in the same manner as above, after transesterification, spherical silica with a volume average particle diameter of 0.06 μm, a volume shape factor f=0.51, and a Mohs hardness of 7 was added, and a polycondensation reaction was carried out to obtain silica-containing master pellets containing 1.0 wt % of particles relative to the polyester (polyester B). The spherical silica used was obtained by adding a mixed solution of ethanol and ethyl silicate while stirring the mixed solution, a mixed solution consisting of ethanol, pure water, and ammonia water as a basic catalyst, and stirring the resulting reaction solution to carry out the hydrolysis reaction of ethyl silicate and the polycondensation reaction of this hydrolysis product, followed by stirring after the reaction to obtain monodispersed silica particles.

[0063] (Creating Polyester C) In producing polyester in the same manner as above, after transesterification, spherical silica with a volume average particle diameter of 0.2 μm, a volume shape factor f=0.51, and a Mohs hardness of 7 was added, and a polycondensation reaction was carried out to obtain silica-containing master pellets containing 2% by weight of particles relative to the polyester (polyester C). The spherical silica used was obtained by adding a mixed solution of ethanol and ethyl silicate while stirring the mixed solution, a mixed solution consisting of ethanol, pure water, and ammonia water as a basic catalyst, and stirring the resulting reaction solution to carry out the hydrolysis reaction of ethyl silicate and the polycondensation reaction of this hydrolysis product, followed by stirring after the reaction to obtain monodispersed silica particles.

[0064] (Creating polyesters D, E, and F) Separately, a water slurry of divinylbenzene / styrene copolymer crosslinked particles with a volume average particle size of 0.15 μm and a volume shape factor f=0.51, obtained by a monomer adsorption method, was added to the above-mentioned substantially particle-free homopolyester pellets using a vented twin-screw kneader to obtain master pellets containing 1 wt% divinylbenzene / styrene copolymer crosslinked particles with a volume average particle size of 0.15 μm (Polyester D). Similarly, master pellets containing divinylbenzene / styrene copolymer crosslinked particles with volume average particle sizes of 0.3 μm and 0.45 μm were obtained in the same manner (Polyesters E and F), containing 0.2 wt% and 1 wt%, respectively, of the polyester.

[0065] (Creating Polyester G) Additionally, calcium carbonate with an average particle size of 1.0 μm was prepared and made into a 10% ethylene glycol slurry. This slurry was dispersed for one hour using a jet agitator and then highly accurately filtered through a filter with a 95% collection efficiency of 5 μm or larger particles. Dimethyl terephthalate was added with 1.9 mol of ethylene glycol, 0.05% magnesium acetate tetrahydrate, and 0.015% phosphoric acid to perform a thermal transesterification. The aforementioned calcium carbonate-containing slurry was then added after the transesterification, followed by 0.025% antimony trioxide. The mixture was heated and subjected to a polycondensation reaction under vacuum, yielding calcium carbonate-containing master pellets with an intrinsic viscosity of 0.63 dl / g and containing 1% by weight of calcium carbonate with an average particle size of 1.0 μm (Polyester G).

[0066] (Creation of Polyester H) Furthermore, δ-alumina was used as agglomerated alumina to prepare a 10% ethylene glycol slurry, which was then crushed and dispersed using a sand grinder, and filtered using a 3 μm filter with a collection efficiency of 95%. This was added to the transesterification product prepared in the same manner as above, followed by the addition of antimony trioxide, and a polycondensation reaction to obtain master pellets containing 1.5% by weight of agglomerated alumina and having an intrinsic viscosity of 0.62 dL / g (Polyester H).

[0067] Example 1 The raw materials for each layer, formulated according to the formulations shown in Table 1, were mixed in a blender. The mixed raw materials for Layers A and C were then fed into vented twin-screw extruders for Layers A and C, respectively. The raw materials for Layer B were dried under reduced pressure at 120-140°C for at least one hour and then fed into a single-screw extruder for Layer B. The materials were then melt-extruded at 275°C, filtered through a high-precision filter capturing at least 95% of particles 5 μm or larger for Layer A, and a high-precision filter capturing at least 95% of particles 2 μm or larger for Layer C. The resulting mixtures were then laminated in a rectangular three-layer merging block to form a three-layer laminate consisting of Layers A, B, and C. The resulting mixture was then passed through a slit die maintained at 285°C onto a chilled roll using an electrostatic casting method, and wrapped around a casting drum at a surface temperature of 23°C, where it was cooled and solidified to produce an unstretched laminate film.

[0068] This unstretched film was preheated with a heating roll at 68 to 132°C and then stretched 4 times in the longitudinal direction at 104 to 110°C using a stretching roll with a surface roughness Ra of 0.2 μm. The uniaxially stretched film was then cooled at 81 to 89°C below the stretching temperature. The film was then stretched 4.3 times in the width direction using a stenter under hot air at 104 to 111°C, followed by heat treatment at 232°C for 3 seconds under constant tension. This intermediate product was then relaxed by 0.1% in the longitudinal direction and 3.3% in the width direction to obtain a 16 μm thick biaxially oriented polyester film intermediate product. This intermediate product was slit using a slitter to obtain a 16 μm thick biaxially oriented polyester film roll. The evaluation results of the obtained film are shown in Table 2. As described above, the biaxially oriented polyester film of the present invention exhibited excellent smoothness and resist resolution.

[0069] Examples 2 to 5 A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the compositions of Layer A, Layer B, and Layer C were changed as shown in Table 1. The evaluation results of the obtained film are shown in Table 2. As such, the biaxially oriented polyester film of the present invention, like that of Example 1, was excellent in slip properties and resist resolution.

[0070] (Comparative Examples 1 to 3) A biaxially oriented polyester film was obtained in the same manner as in Example 1, except that the compositions of Layer A, Layer B, and Layer C were changed as shown in Table 1. The evaluation results of the obtained film are shown in Table 2.

[0071] [Table 1]

[0072] [Table 2] [Industrial Applicability]

[0073] The biaxially oriented polyester film of the present invention has good permeability and slipperiness, and therefore can be suitably used as a support for a dry film resist.

Claims

1. A biaxially oriented polyester film having a laminated structure of two or more layers, The arithmetic mean surface slope of the film surface (A) is Δa(A), the geometric mean surface slope is Δq(A), When the refractive index in the film thickness direction is n(A), Δa(A)×n(A) is 3.3 μm / mm or more and 3.8 μm / mm or less, Δq(A)×n(A) is 3.9 μm / mm or more and 4.5 μm / mm or less A biaxially oriented polyester film characterized by:

2. A biaxially oriented polyester film as described in claim 1, wherein n(A) is 1.490 or more and 1.500 or less.

3. 3. The biaxially oriented polyester film according to claim 1, wherein SRa(A) is the arithmetic mean surface roughness of the film surface (A) and SRz(A) is the 10-point average surface roughness, and SRa(A) is 1.5 nm or more and 2.5 nm or less, and SRz(A) is 15 nm or more and less than 35 nm.

4. 4. The biaxially oriented polyester film according to claim 1, wherein the biaxially oriented polyester film has a three-layer laminate structure, and the layer without a surface is designated as layer B. When an area of ​​220 μm in the longitudinal direction and 290 μm in the width direction is observed in layer B over 200 fields of view using a laser microscope, the number of coarse particles with a major axis of 2.0 μm or more present in layer B is 40 or less.

5. The film surface (A) has voids with a major axis of 1.0 μm or more containing inert particles with a particle diameter of 0.05 μm or more, and the number of the voids is 2 / 10 mm ２ 6 or more pieces / 10mm ２ The biaxially oriented polyester film according to any one of claims 1 to 4, wherein the tensile strength is less than 1

6. 6. The biaxially oriented polyester film according to claim 1, wherein the film haze is 1.0% or less.

7. 7. The biaxially oriented polyester film according to claim 1, wherein the layer having the film surface (A) contains particles, and when the particles contained in the layer are plotted with particle diameter on the horizontal axis and particle abundance ratio on the vertical axis, the particles have one or more maximum values ​​in the particle diameter range of 30 to 100 nm and one or more maximum values ​​in the particle diameter range of 100 to 200 μm.

8. The biaxially oriented polyester film according to any one of claims 1 to 7, wherein the difference between SRa(A) and SRa(C) is 1.0 nm or less, where SRa(C) is the arithmetic mean surface roughness of the film surface (C) opposite to the film surface (A).

9. The biaxially oriented polyester film according to claim 8, wherein the static friction coefficient μs between the film surface (A) and the film surface (C) is 1.0 or more and 1.8 or less, and the dynamic friction coefficient μd between the film surface (A) and the film surface (C) is 0.6 or more and 1.0 or less.

10. The biaxially oriented polyester film according to claim 8 or 9, wherein the layer having the film surface (C) contains particles, and when the particles contained in the layer are plotted with particle diameter on the horizontal axis and particle abundance ratio on the vertical axis, the particle diameter has one or more maximum values ​​in the range of 30 to 100 nm.

11. The biaxially oriented polyester film according to any one of claims 1 to 10, which is used for a dry film resist support.

12. 12. The biaxially oriented polyester film according to claim 1, wherein when used as a dry film resist support, the film surface (A) is exposed to ultraviolet light.

13. The method for producing the biaxially oriented polyester film according to any one of claims 1 to 12, wherein the film is obtained by melt-forming using a co-extrusion method.

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