JP2011084760A - Film deposition method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To deposit a reflection-preventive film without any irregularity in the film thickness of a concave lens. <P>SOLUTION: The reflection-preventive film is deposited on a concave lens 1 by the sputtering method via a mask 2 for adjusting the film thickness installed parallel to a target 10. The mask 2 has a circular aperture 2a having an aperture diameter A smaller than the lens diameter D of the concave lens 1. The aperture 2a of the mask 2 is arranged on the inner side of the tangent plane at an edge of the concave lens 1. By executing the film deposition while rotating the concave lens 1, the film thickness to the concave lens 1 having a small radius R of curvature is unified. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a film forming method for forming an antireflection film or the like with a uniform film thickness on the surface of a concave lens having a small radius of curvature used in a projector, a camera, or the like.

  Conventionally, many antireflection films used for small and medium-diameter lenses such as projectors and cameras have been generally processed by a vacuum deposition method, an ion beam assist method, a sputtering method, or the like. As a method of making the film thickness unevenness uniform for a large number of processes using the sputtering method, there is a method of arranging a film thickness adjusting mask between the target and the substrate holder.

  In addition, as disclosed in Patent Document 1, for a large number of convex or concave optical lens base materials arranged concentrically on the substrate holder, the difference in film thickness is reduced in the optical lens base material between the concentric circles. What makes distribution uniform is known.

  In the case of a lens having a comparatively large lens diameter such as a mirror, it is general that one piece is processed by a vacuum deposition method, an ion beam assist method, a sputtering method or the like. For example, as disclosed in Patent Document 2, a mask provided immediately before a convex or concave spherical substrate is provided with an opening whose radial dimension changes, and the film thickness distribution is controlled by changing the film thickness in the radial direction. There is something to do.

Japanese Patent Laid-Open No. 10-317135 Japanese Patent Laid-Open No. 06-194575

  However, as the lens has a high NA, an improvement in antireflection performance has been demanded. In the case of a concave lens with a small radius of curvature, the conventional film formation method causes uneven film thickness at the periphery and increases the reflectivity. Generates ghost and flare.

  Although the method disclosed in Patent Document 1 can reduce the film thickness difference between the optical lens substrates between concentric circles and make the film thickness distribution uniform, the film thickness unevenness between the center and the periphery in each lens can be reduced. It cannot be resolved. In addition, what is disclosed in Patent Document 2 is effective when an aperture whose radial dimension changes is effective when applied to a concave lens having a small radius of curvature, but the center tip of the aperture is discontinuous. A lizard is generated at the center of the corresponding lens.

  An object of the present invention is to provide a film forming method capable of improving the film thickness distribution of an antireflection film or the like formed on a concave lens having a small curvature radius.

  In order to achieve the above object, the film forming method of the present invention forms a thin film having a uniform film thickness on the concave spherical surface of the concave lens by sputtering through a film thickness adjusting mask between the target and the target. In the method, the mask includes a circular opening smaller than the lens diameter of the concave lens, and the opening of the mask is disposed inside a tangential plane contacting the concave spherical surface at an edge of the concave lens. It is characterized by.

  By adjusting the opening diameter and position of the circular opening of the mask, it is possible to prevent the lens from being creased and to improve film thickness unevenness such as an antireflection film formed on a concave lens having a small radius of curvature. .

6 is a schematic diagram illustrating a film forming method according to Example 1. FIG. It is a graph which shows the actual measurement data by Example 1 and Example 2. FIG. It is a graph which shows the actual measurement data by Example 3 and Example 4. FIG. 10 is a schematic diagram illustrating a film forming method according to Example 5. FIG. 10 is a graph showing actual measurement data according to Example 5. 9 shows a film forming apparatus according to Example 6, wherein (a) is a plan view thereof and (b) is a cross-sectional view. FIG. 10 is a graph showing measured data according to Example 7 and reflectance characteristics according to Example 8. 5 is a graph showing actual measurement data according to Comparative Example 1 and Comparative Example 2. 10 is a graph showing reflectance characteristics according to Comparative Example 3.

  FIG. 1 shows a film forming method according to an embodiment. The concave lens 1 faces the target 3 through a mask 2 having a circular opening 2a for film thickness adjustment, and is an antireflection film that is a thin film having a uniform thickness by sputtering using the target 3 as a cathode electrode. Is formed on the concave spherical surface of the concave lens 1. The lens diameter D of the concave lens 1 is preferably 1.3 times or more and 2 times or less of the radius of curvature R of the concave spherical surface to be formed, and the effect is small when it is less than 1.3R. Will get worse.

  The mask 2 is a circular opening mask installed below the lens, and the opening diameter A of the circular opening 2a is smaller than D. When A is close to D, the film formation speed increases, but the effect of improving the film thickness distribution is small. When A is small, the film thickness distribution is improved, but the film formation rate is slow. Practically, A is preferably about D / 2. The eccentric distance L of the opening center of the mask 2 with respect to the central axis O of the concave lens 1 is decentered within about D / 4, thereby greatly increasing the film thickness distribution with respect to the concave lens 1 rotating by a rotation mechanism (not shown). Can be improved. The separation distance H between the lens bottom surface, which is the edge of the concave lens 1 on which the antireflection film is formed, and the mask 2 is preferably about the same as the radius of curvature R. When the separation distance H must be separated from the holding of the mask 2 or the structure of the lens holder, the separation distance H is preferably 2R or less because the film thickness distribution deteriorates when the separation is separated.

  The target 10 is a cathode electrode for sputtering, and Ti, Nb, Ta, Al, Si, etc. can be used when the reactive DC sputtering method is used. As shown in FIG. 1, in the case of processing one piece, a circular target of about 4 to 6 inches can be used for a medium-aperture lens (about Φ20 to 60). For small-diameter lenses (Φ20 or less), it is advantageous to process a large number of targets with the same size target.

  Further, the pressure at the time of sputtering film formation is preferably about a fraction of Pa to several Pa, the film thickness around the lens is thin on the low pressure side, and the film around the lens due to the scattering effect on the high pressure side (poor vacuum). The thickness becomes thicker.

  Usually, the pressure at the time of sputtering film formation is about 1 Pa, and the mean free path of sputtered particles is about 1 cm. Most of the sputtered particles used for film formation are regarded as scattering particles, and the opening 2a of the mask 2 is effectively a circular opening having an opening diameter A smaller than the lens diameter D. When the aperture diameter A is larger than D, a lens with a large curvature radius R is not effective in improving the film thickness distribution, and with a lens with a small curvature radius R, the bottom portion becomes obscured, resulting in film thickness unevenness around the lens. As the opening diameter A is smaller, the film thickness unevenness is improved. However, if the opening diameter A is too small, the film formation speed is reduced. Practically, about 1/2 of the lens diameter D is preferable.

  Further, the opening 2 a of the mask 2 is disposed in a region inside the angle at which the tangent of the end of the concave lens 1 is expected, that is, inside the tangential plane that is in contact with the concave spherical surface at the edge of the concave lens 1. The shape of the opening 2a of the mask 2 is preferably circular. However, when the concave lens 1 rotates, it may be close to a circle.

  The distance H between the bottom surface of the lens supported by the lens holder and the mask is preferably not more than twice the radius of curvature R of the concave spherical surface of the concave lens 1 (H ≦ 2R). If the sputtered particles near the circular opening are scattered isotropically, the separation distance H is preferably closer to the lens bottom surface. However, when the pressure at the time of sputtering film formation is a fraction of 1 Pa, the mean free path of the sputtered particles becomes several times, and it is advantageous that the separation distance H is away from the lens bottom surface. If the lens diameter D is up to about 1.5 times the radius of curvature R, H ≦ 2R is preferable.

  As a lens shape, the lens diameter D is preferably larger than 1.3 times the radius of curvature R and smaller than twice that. When the lens aperture D is smaller than 1.3 times the radius of curvature R, the effect of the circular aperture is small. For example, when it is close to a flat surface, the film thickness unevenness is worsened. Further, the concave lens may be an aspherical lens represented by a reference spherical surface.

  According to the aperture diameter A of the mask 2, the eccentric distance L from the lens center axis O at the center of the aperture and the pressure at the time of sputtering film formation are adjusted to obtain the target shape, target material, target-to-lens distance, and the like. Corresponds to membrane conditions.

  The mask having a circular opening may be a mask having a plurality of circular openings for simultaneously processing a plurality of concave lenses. For example, in the magnetron sputtering method, a circular target sufficiently larger than the lens diameter is used, and a plurality of masks having a plurality of circular openings are provided below the lens corresponding to the plurality of lenses installed above the erosion track. Processing is possible. At this time, if the sputtering speeds of the erosion track portions are substantially the same, a fixed lens may be provided above the fixed mask. If importance is attached to film thickness unevenness in each lens, the lens can be rotated by the fixed mask method. If importance is attached to film thickness unevenness between lenses, the lens and mask are kept in the same position while maintaining the positional relationship. You can do it. Alternatively, the lens may be rotated while revolving as a unit while maintaining the positional relationship between the lens and the mask. In addition, a plurality of targets can be installed, moved to another target when switching the film formation, the next film formation is performed, and this can be repeated to form a multilayer film. The target may be moved.

By the reactive DC magnetron method, an Nb ₂ O ₅ film was formed on a rotating concave lens (material is S-BSL7) having a radius of curvature R of 20 mm and a lens diameter D of 40 mm by the reactive DC magnetron method. The target was Nb of Φ5 inch, and the pressure during film formation was 1 Pa. At this time, the case where the opening diameter A of the mask was Φ20 mm, no eccentricity, and the separation distances H = R, H = 2R, and H = 2.5R between the lens and the mask was examined. The distance between the mask and the target is constant and 90 mm. The result of measuring the film thickness distribution on the lens is shown in FIG. The vertical axis represents the film thickness ratio with respect to the lens center, and the horizontal axis represents the distance from the lens central axis. As can be seen from the graph of FIG. 2A, the difference in film thickness distribution is within about 10% in the region where the distance is up to 15.3 mm (opening angle 50 ° from the lens central axis). This is the same even with a concave lens having the same curvature and a lens aperture of 30 mm or less.

  In the region where the distance from the lens center is 20 mm, the film thickness is smaller in the case of 2R than in the case where the separation distance H between the lens and the mask is R, and the distance is 18.8 mm (open from the lens center axis). The film thickness ratio with respect to the center is 70%. In the case of H = 2.5R, the film thickness distribution further deteriorates as it approaches the periphery.

An Nb ₂ O ₅ film was formed by the same film forming method as in Example 1. The target was Φ5 inch Nb, and the pressure during film formation was 0.5 Pa. At this time, the case where the opening diameter A of the mask was Φ20 mm, no eccentricity, and the separation distances H = R, H = 2R, and H = 2.5R between the lens and the mask was examined. The result of measuring the film thickness distribution on the lens is shown in FIG. The vertical axis represents the film thickness ratio with respect to the lens center, and the horizontal axis represents the distance from the lens central axis. As can be seen from the graph of FIG. 2B, when H = 2R compared to H = R, the peripheral film thickness is thicker, and the increase from the center when the distance from the lens center is 15 mm is 20%. Degree. From comparison with Example 1, it can be seen that improvement by pressure adjustment or improvement by adjustment of the separation distance H is possible. When H = 2.5R, the difference in film thickness distribution is 40% or more, which is not preferable.

An Nb ₂ O ₅ film was formed by the same film forming method as in Example 1. The target was Nb of Φ5 inches, the pressure during film formation was 1 Pa, and the separation distance H = R between the lens and the mask. The opening diameter A of the mask was Φ30 mm and Φ40 mm, which was compared with the case of Φ20 mm. The result of measuring the film thickness distribution on the lens is shown in FIG. As can be seen from the graph of FIG. 3A, the peripheral film thickness tends to decrease as the aperture diameter A increases, and the effect of improving the film thickness distribution is small at aperture diameters greater than the lens aperture. That is, the aperture diameter A only needs to be smaller than the lens aperture D, and practically about ½ of the lens aperture D is preferable.

An Nb ₂ O ₅ film was formed by the same film forming method as in Example 1. The target was Nb of Φ5 inch, and the pressure during film formation was 1 Pa. At this time, the separation distance between the lens and the mask is H = R, the mask opening diameter A is Φ10 mm and no eccentricity, and the mask opening diameter A is Φ20 mm and the eccentric distance L is 10 mm and 15 mm. investigated. The result of measuring the film thickness distribution on the lens is shown in FIG. The vertical axis represents the film thickness ratio with respect to the lens center, and the horizontal axis represents the distance from the lens central axis. As can be seen from the graph of FIG. 3B, the difference in film thickness distribution is within about 2% in the region where the distance from the lens central axis is 15.3 mm (opening angle 50 ° from the lens central axis). This is the same even for a concave lens having the same curvature and a lens aperture of 30 mm or less. The film thickness ratio when the distance from the lens central axis is about 18.8 mm is centered on both the aperture diameter A of Φ10 mm and no eccentricity, and the aperture diameter A of Φ20 mm and the eccentric distance L of 10 mm. On the other hand, it is 90% or more. However, when the aperture diameter A is Φ20 mm and the eccentric distance L is 15 mm, that is, when the circular opening is not within the angle expected by the tangent at the lens end, the film thickness distribution is 75% with respect to the center. Became worse. From the viewpoint of film formation speed, it is more advantageous to make the opening diameter larger and decentered than to reduce the opening diameter of the circular opening.

An Nb ₂ O ₅ film was formed on a rotating concave lens 21 (material is S-BSL7, D = 1.6R) 21 having a radius of curvature R of 25 mm and a lens diameter D of Φ40 mm by the film forming method shown in FIG. The target (not shown) was Nb with Φ5 inches, and the pressure during film formation was 1 Pa. At this time, the distance H between the concave lens 21 and the mask 22 was set as H = D / 2, the opening diameter A of the opening 22a of the mask 22 was Φ30 mm, and the eccentric distance L was 0, 10, and 20 mm. In FIG. 4, when the eccentric distance L is 20 mm, the opening 22 a of the mask 22 partially deviates from the region of the corner that the tangent line T of the end of the concave lens 21 is expected.

  FIG. 5A shows the result of measuring the film thickness distribution on the lens. The film thickness distribution when the eccentric distance L is 0 mm is substantially equivalent to that without a mask (see Comparative Example 2). If the eccentric distance L increases, the peripheral film thickness becomes thicker than the lens center, and if the eccentric distance L is too large, the film thickness distribution tends to deteriorate. When the eccentric distance L is 20 mm, the film formation rate is halved with respect to no eccentricity.

Furthermore, in order to see the range of lens shapes in which the circular aperture mask is effective, a concave lens (material is S-BSL7, D = 1.33R) rotating in a similar manner with a radius of curvature R of 30 mm and a lens aperture D of Φ40 mm is used. An Nb ₂ O ₅ film was formed. A case was considered in which the distance H between the concave lens and the mask was H = D / 2, the aperture diameter A of the mask was Φ30 mm, and the eccentric distance L was 0, 10 and 20 mm. When the eccentric distance L is 20 mm, the opening of the mask is disposed in the region of the corner where the tangent of the lens end is expected. FIG. 5B shows the result of measuring the film thickness distribution on the lens. The peripheral film thickness distribution when the eccentric distance L is 0 mm is inferior to 95% when there is no mask. However, if the eccentric distance L increases, the peripheral film thickness becomes thicker than the lens center, and the film thickness distribution can be improved. However, when the eccentric distance L is 20 mm, the film formation rate is halved with respect to no eccentricity. Practically, the eccentric distance L is predicted to be about 12 mm. Therefore, when the circular opening is decentered, the lens shape is preferably a concave lens whose lens diameter D is 1.3 times or more the curvature radius R.

An Nb ₂ O ₅ film was formed by the film forming apparatus shown in FIG. Four concave lenses (material: S-BSL7) 31 having a radius of curvature R of 20 mm and a lens diameter D of Φ40 mm with respect to a Φ5 inch Nb target 40 were examined. The concave lens 31 rotates around the lens center axis O1, and the concave lens 31 and the mask 32 rotate around the revolution axis O2. That is, the concave lens 31 revolves and the mask 32 revolves. The mask 32 has a circular opening 32a, and the concave lens 31 and the opening 32a of the mask 32 are disposed substantially directly above the erosion track 40a of the target 40 symmetrically with respect to the revolution axis O2. The pressure during film formation at this time is 1 Pa, the separation distance H = R between the concave lens 31 and the mask 32, and the opening diameter A of the opening 32a of the mask 32 is Φ20 mm. Because of the self-revolution, the shape and thickness of the film thickness distribution in the radial direction between the lenses and between the lenses are substantially the same, and the same film thickness distribution as in the case of H = R in Example 1 was obtained.

  Even in the case of a large number of processes, the film thickness distribution of the antireflection film formed on the concave lens can be greatly improved by selecting the target size, the lens, and the arrangement of the circular aperture as compared with the conventional film forming method. it can.

A SiO ₂ film was formed by the same film forming method as in Example 6. Four concave lenses (material is S-LAH53) having a radius of curvature R of 20 mm and a lens diameter D of Φ40 mm with respect to one Φ5 inch Si target were studied. The pressure during film formation at this time is 0.3 Pa, 0.5 Pa, and 1 Pa, the separation distance H = R between the concave lens and the mask, and the opening diameter A of the mask is Φ20 mm. Because of the self-revolution, the shape and thickness of the film thickness distribution in the radial direction within the lens and between the lenses are substantially the same. The result of measuring the film thickness distribution is shown in FIG. The film thickness tends to be thicker around the lens than the Nb ₂ O ₅ film, which indicates that the film thickness distribution can be controlled by the pressure during film formation. Even with multiple processing, the film thickness distribution can be improved with respect to the concave lens by selecting the target size, the lens, and the arrangement of the circular aperture.

An antireflection film using Nb ₂ O ₅ and SiO ₂ was formed on a concave lens (material is S-LAH53) by the same film forming method as in Example 6 and Example 7. The pressure during Nb ₂ O ₅ deposition is 1 Pa, and the pressure during SiO ₂ deposition is 0.3 Pa. As an internal configuration, the concave lens rotates while rotating while maintaining the position and position of the mask attached to the lens holder, and moves to another target when switching the film. The next film was formed, and a multilayer film was formed by repeating this process. Table 1 shows the film structure of the eight-layer antireflection film according to this example.

In Table 1, the refractive index of the Nb ₂ O ₅ film at a wavelength of 588 nm is 2.29, and the refractive index of the SiO ₂ film is 1.46. The reflectance characteristics at this time are shown in FIG. In FIG. 7B, the horizontal axis represents wavelength, the vertical axis represents reflectance, and X = 0, 15.3, and 18.8 (mm) represent the distance from the lens central axis to the measurement point, and the lens central axis. It corresponds to the position of 50 and 70 degrees in the opening angle from The blue reflection color was slightly strong at the position of X = 18.8, but the reflectance distribution and color were significantly improved as compared with the case without the mask (Comparative Example 3).

(Comparative Example 1)
An Nb ₂ O ₅ film was formed in the same manner as in Example 1. However, the case where the distance between the target and the lens (material is S-BSL7) was 110 mm without using a mask and the pressure during film formation was 0.5 Pa and 1 Pa was examined. The result of measuring the film thickness distribution on the lens is shown in FIG. As can be seen from the graph of FIG. 8A, the film thickness decreases as the distance from the lens center increases, and is about 70% of the center film thickness at 1 Pa at a position of 18.8 mm. This is 20% worse than H = R in Example 1. In addition, the aperture diameter A of Example 3 is slightly worse than when the aperture diameter is 40 mm, and the effect of improving the film thickness distribution is small with an aperture diameter larger than the lens aperture.

(Comparative Example 2)
An Nb ₂ O ₅ film was formed in the same manner as in Example 1. However, a flat plate (material is S-BSL7) was used instead of the lens, and the case of no mask and with mask (opening diameter AΦ20) was examined. When there is no mask, the distance between the flat plate and the target is 130 mm, and when there is a mask, the separation distance between the flat plate and the mask is 40 mm (the distance between the mask and the target is 90 mm). The pressure during film formation is 1 Pa. The result of measuring the film thickness distribution on the flat plate is shown in FIG. In the case of a flat surface, when compared at a distance of 20 mm from the center, a film thickness of 90% with respect to the center can be obtained without a mask. However, the film thickness is reduced to 60% by using the mask. This is because the effect of the circular aperture mask depends on the lens shape.

In order to see the effective lens curvature of a circular aperture mask, a concave lens (material is S-BSL7, with a lens aperture D of Φ40 mm and a curvature radius R of 25 mm, with and without a mask (with an aperture diameter AΦ30 of no eccentricity)). D = 1.6R)), an Nb ₂ O ₅ film was formed in the same manner. When there is no mask and with a mask (opening diameter AΦ30), when there is no mask, the distance between the lens and the target is 110 mm, and when there is a mask, the separation distance H between the lens and the mask is 20 mm (the distance between the mask and the target is 90 mm). ). Further, the pressure during film formation is 1 Pa. The result of measuring the film thickness distribution at this time is shown in FIG. The film thickness distribution with no mask and with the mask (aperture diameter AΦ30) is substantially the same. Under these conditions, it can be seen that a concave lens having a radius of curvature R smaller than 25 mm has the effect of a circular aperture. When the radius of curvature R is larger than 25 mm, it is advantageous to have no mask because the film forming speed is large. Therefore, when there is no eccentricity, it is preferable that the lens shape is a concave lens having a lens diameter D of 1.5 times the radius of curvature R or more.

(Comparative Example 3)
In the same manner as in Example 8, an antireflection film using Nb ₂ O ₅ and SiO ₂ was formed on a concave lens (material is S-LAH53). However, without a mask, the distance between the target and the lens is 110 mm, the pressure during Nb ₂ O ₅ film formation is 1 Pa, and the pressure during SiO ₂ film formation is 0.3 Pa. The film configuration is the same as in Table 1. The reflectance characteristics at this time are shown in FIG. The redness is strong even at the position of the distance X = 15.3 from the center axis of the lens, but at the position of X = 18.8, the reflectance and color are not practically suitable.

1, 2, 31 Concave lens 2, 22, 32 Mask 2a, 22a, 32a Opening 10, 40 Target

Claims (4)

In a film forming method of forming a thin film with a uniform film thickness on the concave spherical surface of the concave lens by a sputtering method through a mask for adjusting the film thickness between the target and the target,
The mask includes a circular opening smaller than the lens diameter of the concave lens,
The film forming method, wherein the opening of the mask is disposed inside a tangential plane contacting the concave spherical surface at an edge of the concave lens.   The film forming method according to claim 1, wherein a distance between the edge of the concave lens and the mask is equal to or less than twice a radius of curvature of the concave spherical surface.   3. The film forming method according to claim 1, wherein a lens diameter of the concave lens is not less than 1.3 times and not more than 2 times a radius of curvature of the concave spherical surface.   The eccentric distance of the center of the opening of the mask with respect to the central axis of the concave lens and the pressure at the time of sputter deposition are adjusted according to the opening diameter of the opening of the mask. 4. The film forming method according to any one of 3 above.

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