JP2004268331A - Mold for molding optical element and method for manufacturing mold - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mold for molding an optical element having a fine structure having a complicated shape such as CGH, a conical shape or the like on a curved surface such as a lens surface or the like. <P>SOLUTION: The mold 1 is constituted of a mold matrix 2, a processing layer 3 and an etching layer 4. The processing layer 3 is laminated on the mold matrix 2 roughly processed into a shape corresponding to the lens surface. The processing layer 3 is processed into the shape corresponding to the lens surface with high precision. The etching layer 4 is formed on the processing layer 3 and subjected to etching treatment so as to have a fine structure. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００３０】
表１において、ＲＦは１３．５６ＭＨｚの高周波印加電力である。また、ガス種欄に／（スラッシュ）を挟んで２種類のガスが記載されている場合は、それらの混合ガスを意味する。流量欄において、／（スラッシュ）をはさんで記載されている値は、ガス種欄に記載されている、それぞれのガス種に対する流量である。
【００３１】
ＣＧＨ１６の高さは、次の２種類の方法によって制御できる。すなわち、
▲１▼エッチング層４を、ＣＧＨ１６の高さよりも厚く成膜し、エッチング時間を制御することにより、ＣＧＨ１６の高さを制御する。
▲２▼エッチング層４をＣＧＨ１６の高さと同じ膜厚で成膜し、加工層３が露出するまでエッチング層４をエッチングする。加工層３はエッチングされにくい材料で構成されているため、エッチング時間を厳しく制御する必要はない。
【００３２】
本実施形態では、上記▲２▼の方法により、加工層３が露出するまでエッチングを行っている。
（Ｉ）不要なレジスト５やドライエッチングによって変質したポリマーをアッシング（灰化）によって除去する。
【００３３】
このようにして作製された金型１を模式的に示す縦断面図を図１（ａ）に示す。金型１は、加工層３にレンズ面を加工し、加工層３上に形成したエッチング層４に、リソグラフィー技術を応用してＣＧＨ１６を形成しているため、レンズ面とＣＧＨを高精度にもつ光学素子面を、正確に転写することができる。なお図１（ｂ）に示す、エッチング時間を正確に制御しエッチング層４のみでＣＧＨ１６が構成される金型でも、光学素子面を正確に転写することができる。
【００３４】
以上説明した金型１により、ＣＧＨレンズ１５をＰＭＭＡを用いて射出成型したところ、所望の性能が得られた。なお、本実施形態のＣＧＨは図８に示すような矩形状であったが、階段形状をしていてもよい。このような形状は、レジスト塗布、パターン描画、現像処理、エッチング処理を複数回行うことで形成できる。
【００３５】
次に、工程（Ｆ）に関する、曲面上に回折格子パターンを露光する方法を説明する。本実施形態では、以下に示す露光法１によって行ったが、その他の方法、例えば露光法２、露光法３によっても良い。
【００３６】
（露光法１）電子線描画装置を使用する方法である。電子線描画装置は一般的に平面上に露光するものであるため、曲面上に露光する場合には、焦点位置と描画面が一致しないため、所望の微細な形状を描画することができない。そこで、曲面形状を補償するように、被露光物あるいは描画装置（光学系）を光軸方向に移動させながら露光を行う。この方法は電子線に限らずレーザ露光装置においても有効である。
【００３７】
（露光法２）密着露光法である。加工物の形状が大きく、回折格子パターンの格子周期が大きい場合に有効である。ただし、レンズ面の曲率が大きく平面とのずれが大きい場合には格子パターンはぼけてしまう。格子周期が５０ミクロン程度なら、平面から約２ｍｍ離れた面でも露光可能である。格子周期が大きく、加工物（金型）の面積が大きい場合には、密着露光法を用いたほうが簡単に作製できる。
【００３８】
（露光法３）投影露光装置を使用する方法である。投影露光装置の光学系は、金型のレンズ面形状に対応した像面特性および歪曲特性を有している。
【００３９】
曲面上への回折格子パターンの露光は、上記３つの露光法のいずれかを用いることにより可能となる。
【００４０】
次に第２の実施の形態である、反射防止用微細構造をもつレンズ用金型について説明する。ガラスあるいはプラスチックス表面上に、錐形状を微細かつ緻密に形成することで、広い波長域を有する光に対して反射防止効果が得られることが知られている。本実施形態の金型は、このような錐形状と等価な作用を持つ微細形状（以下、略錐形状と呼ぶ）を反射防止構造として備えた、レンズを複製するためのものである。
【００４１】
本実施形態の金型で複製されるレンズは、携帯電話等に搭載される超小型レンズ系の、最も撮像素子に近い位置に配置されるレンズである。レンズの有効径は５ｍｍ、曲率半径が約５ｍｍと約２５０ｍｍの両凸レンズである。曲率半径が大きい面で表面反射が生じると、反射光が撮像素子に入射しゴーストが発生する。レンズが合成樹脂材料で成型されている場合には、一般的な誘電体膜では高性能な反射防止コートを行うことは困難であり、製造コストが上がる。上記略錐形状の反射防止構造をレンズ成型時に同時に成型することで、製造コストを大幅に削減できる。
【００４２】
図３は、第２の実施の形態の金型の製造工程を模式的に示す図である。以下、図３を参照しながら、金型の製造方法について説明する。
【００４３】
（Ａ）ステンレス鋼材からなる金型母材２を機械加工し、上記レンズのレンズ面に対応する形状に加工する。この加工は、レンズ面と同様の精度は必要なく、レンズ面に対応する概略形状を形成すればよい。なお、金型母材２はステンレス鋼材に限らず、金型としての強度、剛性を備えた材料であればよい。
【００４４】
（Ｂ）レンズ面の概略形状が形成された金型母材２の上に、電鋳によりニッケル材を厚さ５ｍｍに積層する（加工層３）。積層される材料は、ニッケルに限らず、例えば、ニッケル合金、銅、銅合金、黄銅であってもよい。加工層３は、上記材料のように、切削性が良く、エッチングされ難い材料で形成されることが好ましい。
【００４５】
（Ｃ）加工層３に、切削、研削あるいは研磨等の機械加工によって、レンズ面に対応する所望の光学鏡面を形成する。この形状は、複製される光学素子のレンズ面に必要な精度と同等以上の高精度に加工されている。
【００４６】
（Ｄ）工程（Ｃ）で形成された加工層３の加工面上に、スパッタリングにより、ＴｉＮ（窒化チタン）を成膜する（エッチング層４）。ＴｉＮの成膜条件は第１の実施の形態の場合と同様である。なお、エッチング層の厚さは３００ｎｍとした。
【００４７】
なお、膜材料は、ＴｉＮに限らず、Ｓｉ（シリコン）、ＳｉＯ_２（ニ酸化シリコン）、Ａｌ（アルミニウム）、ＳｉＮ（窒化シリコン）、Ｍｏ（モリブデン）、ＳｉＣ（シリコンカーバイド）、ＷＣ（タングステンカーバイド）であってもよい。エッチング層４は、上記材料のように、エッチングが容易な材料で形成されていることが望ましい。
【００４８】
（Ｅ）エッチング層４上に、レジスト５を、スピンコート法にて膜厚８００ｎｍに塗布する。
【００４９】
（Ｆ）錐形状を形成するためのパターンを露光する。図４に露光方法を示す。レーザ光を二光束に分離し（図示しない）、分離された二光束を、図４（ａ）に示すように互いに角度を成してレジスト５に入射させる。レジスト５には、干渉縞が生じ、この干渉縞形状が露光される。金型１１を９０°回転させ２回の露光を行うと、図４（ｂ）に示すような干渉縞が記録される。黒丸で表されている領域は２回の露光がされていることになる。二光束の角度を変化させることで、干渉縞の周期、すなわち錐形状のピッチを変えることができる。また、例えば１２０°づつ３回の露光を行うなど、金型の回転角および露光回数を変えることで錐形状の配列を変えることができる。
【００５０】
（Ｇ）レジスト５を現像する。図４（ｂ）で示す黒丸部は、他の領域よりも露光量が多いため、現像によって、エッチング層４に達する穴が生じる。
【００５１】
（Ｈ）現像後のレジスト５に、クロム膜６を真空蒸着法により成膜する。クロム膜６は、レジスト５表面と、エッチング層４の両方に蒸着されている。
【００５２】
（Ｉ）レジスト５を除去し、エッチング４層に形成されたクロム膜６（以下、金属マスクと呼ぶ）を表面に露出させる（リフトオフ）。
【００５３】
（Ｊ）金型にドライエッチングを行う。エッチングの条件は、第１の実施の形態で説明した表１の条件である。ドライエッチングを開始すると、まずエッチング層上の金属マスク６がないところから、テーパーがついた形状でエッチングされ始める。そしてエッチングが進むにつれ、徐々に金属マスク６もエッチングされてその径が減少しつつエッチング層４がエッチングされる。さらに、金属マスク６がほぼ消失するまでエッチングを行うと、錘形状２０の突起が生成される。本実施形態では、周期２００ｎｍ、高さ２５０ｎｍの錘形状を作製した。
【００５４】
（Ｋ）エッチング層４に残っている不要なクロム膜６を、ウエットエッチングにて除去する。
【００５５】
このようにして作製された金型１１を用いて、レンズを成型した。成型されたレンズは波面収差１／１０λの光学性能を持ち、なおかつ所望の反射防止特性を有していた。
【００５６】
第２の実施形態の金型１１で成型されたレンズの略錐形状２５は、図５に示すように、その形状の先端部に平面部２６が形成されている。図５は、成型されたレンズが持つ略錐形状２５を模式的に示す断面図である。また、図６は作製された金型の錘形状２０を模式的に示している。図６（ａ）は金型の錘形状の斜視図、図６（ｂ）は錘形状の底部での横断面図である。図６（ａ）においては、金型のレンズ成型面の極一部を切り出して表示しているため、平面上に錘形状が形成されているように示されているが、実際は金型のレンズ形成面全面に錘形状２０が形成されている。図６（ｂ）に示すように、錘形状２０の底面部には平面部２１を有しているため、成型を行うと、この平面部２１が略錐形状２５の先端部に転写される。このように、先端部に平面部２６を有し、また錘形状２０と相補的な関係をなす略錐形状２５は、完全な錘形状ではないが、図６（ａ）に示すような錘形状をレンズ面に有する場合と比較して、反射防止構造としては、実用上性能に差はない。
【００５７】
なお、上記第１および第２の実施の形態では、レンズ面上に微細構造を有しているが、光学素子はレンズに限らず、例えば曲面を有するミラー面上にＣＧＨ等の微細構造を有してもよい。
【００５８】
【発明の効果】
請求項１の発明では、金型母材上に加工層を形成し、その加工層にレンズ面等に対応する曲面を機械加工し、さらにその上に積層されたエッチング層に微細構造を形成している。そのため光学素子用金型の面精度は機械加工によって確保され、また微細構造の形状精度はエッチング層のエッチングによって得られる。その結果、高精度な金型が得られ、そのような金型を用いることで、高精度な光学素子を容易に低コストで大量に生産することができる。
【００５９】
また、加工層に、曲面の機械加工を容易に行うことができる切削性のよい材料を使用するため、金型に剛性のある材料を用いることができる。さらに、エッチングの際に加工層が露出したとしても、エッチングされ難い材料のため、エッチングされることがなく、金型のレンズ面は良好な面精度、面粗さが得られる。また、エッチングされ難い特性を利用して、容易に微細構造の高さを制御できる。
【００６０】
請求項４の発明では、加工層をレンズ面等の曲面に機械加工し、その上に形成されたエッチング層に微細構造を形成するため、機械加工による面精度の確保と複雑な微細構造とが同時に得られる光学素子用金型を製造することができる。そのような光学素子用金型を使用することで、高精度な光学素子を容易に低コストで大量に生産することができる。
【図面の簡単な説明】
【図１】第１の実施の形態の金型を模式的に示す縦断面図である。
【図２】第１の実施の形態の金型の製造工程を模式的に示す縦断面図である。
【図３】第２の実施の形態の金型の製造工程を模式的に示す縦断面図である。
【図４】錐形状を露光するための方法を説明する図である。
【図５】第２の実施の形態の金型を用いて成型されたレンズの、反射防止構造を拡大して示す縦断面図である。
【図６】第２の実施の形態の金型の錘形状を拡大して示す図である。
【図７】ＣＧＨレンズの例を模式的に示す図である。
【図８】ＣＧＨを模式的に示す縦断面図である。
【符号の説明】
１、１１ 金型
２ 金型母材
３ 加工層
４ エッチング層
５ レジスト
６ クロム膜（金属マスク）
１５ ＣＧＨレンズ
１６ ＣＧＨ
１７ 光源
２０ 錐形状
２１ 平面部（錐形状底面）
２５ 略錐形状
２６ 平面部（略錐形状先端）[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a mold for an optical element, and more particularly to a mold for molding an optical element having a fine structure on a curved surface. The invention also relates to a method for manufacturing such a mold.
[0002]
[Prior art]
[Patent Document 1]
JP-A-62-96902 [Patent Document 2]
Japanese Patent Application Laid-Open No. 2000-182024 [Patent Document 3]
JP 2002-326231 A [Patent Document 4]
JP-A-2002-274865
In recent years, it has been required to form a diffraction grating having a complicated structure typified by CGH (Computer Generated Hologram) or a fine structure for preventing reflection without using a deposited film on an optical element such as a lens surface. ing. For example, by integrating the CGH with the lens, one lens can realize the light-collecting (image-forming) function of the lens and another function by diffraction. Further, if a fine structure for performing anti-reflection is formed on the lens surface, higher-performance anti-reflection can be performed. That is, by forming these microstructures on the lens surface, it is possible to make the optical system highly functional, compact, and high-performance.
[0004]
On the other hand, in order to mass produce such an optical element at low cost, a mold is manufactured, and the mold shape is transferred to a synthetic resin material. In this case, it is necessary to process the shape of the lens surface and the fine structure in the mold. Molds having such a shape are described in Patent Documents 1, 2, and 3.
[0005]
The mold described in Patent Literature 1 is a mold for molding a lens surface having an antireflection structure, and has a saw-like structure having a depth of a wavelength or less on the surface. A saw-shaped structure is formed by forming an irradiation scar of 1/20 wavelength on the mold surface with an argon ion beam and corroding the surface with a nitric acid aqueous solution.
[0006]
The mold described in Patent Literature 2 is a mold for a lens on which a diffractive optical surface including a large number of concentric fine elements is formed. A photoresist layer is formed on a mold surface having a curved surface formed by machining, a diffractive optical surface is formed by machining on the surface of the resist layer, and an etching process is performed until the resist layer is removed by dry etching. It is made by doing.
[0007]
On the other hand, there is also widely used a method in which a required element shape is manufactured as a master mold, and the shape of the master mold is copied by electroforming to form a mold.
[0008]
[Problems to be solved by the invention]
However, since the mold described in Patent Literature 1 is manufactured by corroding an irradiation scratch formed by an argon ion beam with an aqueous nitric acid solution, a saw shape that spreads uniformly over the entire mold surface can be formed. , A fine structure having a complicated structure such as CGH cannot be formed with high precision.
[0009]
The mold described in Patent Literature 2 has a diffractive optical surface formed on a photoresist layer by machining. Such a machining method can be made with a one-dimensional structure or an axially symmetric structure. In the case where there are various and complicated fine structures in two dimensions as described above, it is practically impossible to manufacture. Also, very fine processing on the order of submicrons is very difficult by machining.
[0010]
The method of making a mold by electroforming makes it easy to duplicate the mold as long as a master mold is made. However, when high-performance imaging (light collecting) characteristics are required, it is not suitable for the following reasons. It is. In electroforming, as the thickness of the electroformed layer increases, a very large stress is generated inside. Therefore, the moment the electroformed layer is separated from the master mold, the mold surface is distorted. In a high-performance lens, since the surface accuracy of the lens surface is required to be equal to or less than the wavelength, an electroformed mold having a large distortion is not suitable for a high-performance lens.
[0011]
The present invention has been made in view of such a situation, and is an optical element for duplicating an optical element that achieves both high-performance optical action due to the curved surface of the optical element and optical action due to the fine structure. It is an object to provide a mold. Another object is to provide a method for manufacturing such a mold.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, an invention according to claim 1 is a mold for an optical element for molding an optical element having a fine structure on a curved surface. A processed layer having an appropriate curved surface and an etching layer made of an etching material having a fine structure are laminated, and the processed layer is made of a material that has better machinability than the mold base material and lower etching property than the etching layer.
[0013]
According to a second aspect of the present invention, in the mold for an optical element according to the first aspect, the processed layer is made of any one of nickel, a nickel alloy, copper, a copper alloy, and brass.
[0014]
The invention according to claim 3 is the mold for an optical element according to claim 1, wherein the etching layer is any one of titanium nitride, silicon, silicon dioxide, aluminum, silicon nitride, molybdenum, silicon carbide, and tungsten carbide. Consisting of
[0015]
The invention according to claim 4 is a method for manufacturing a mold for an optical element for molding an optical element having a fine structure on a curved surface, comprising the steps of: forming a processed layer on a mold base material; Processing the processed layer into a desired optical mirror surface, forming an etched layer on the processed processed layer, applying a resist on the etched layer, drawing a fine structure pattern on the resist, and developing. The method includes a step of transferring a fine structure pattern and a step of forming a fine structure in an etching layer by an etching process, and integrally forms a curved surface and a fine structure.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. The mold according to the first embodiment is a mold for molding a lens in which a lens surface and CGH are integrally formed. The CGH is a hologram created by calculating the distribution of the amplitude and phase of light to be recorded based on diffraction by a virtual object, and drawing the distribution. As shown in FIG. 8, the CGH of the present embodiment includes a large number of rectangular diffraction gratings formed at various intervals on a lens surface. In FIG. 8, the diffraction grating is described only one-dimensionally, but the direction of the periodic arrangement of the diffraction grating is two-dimensionally arranged in various directions. Hereinafter, the diffraction grating is referred to as CGH, and the lens having the CGH formed on the lens surface is referred to as a CGH lens.
[0017]
First, the CGH lens 15 will be described. As shown in FIG. 7, the CGH lens 15 molded by the mold of the present embodiment has a lens function of converting light emitted from a light source 17 such as a light emitting diode (LED) or a laser diode (LD) into substantially parallel light. And a diffraction function of forming and irradiating a figure such as a cross chart. The effective diameter of the CGH lens 15 is 5 mm, the focal length is 7 mm, the minimum fringe period of the CGH 16 is 3 μm, and the height of the CGH 16 is 650 nm.
[0018]
Next, a method of manufacturing the mold 1 for molding the CGH lens 15 will be described. FIG. 2 is a vertical cross-sectional view schematically showing a manufacturing process of the mold. Hereinafter, description will be made with reference to FIG.
[0019]
(A) The mold base material 2 made of a stainless steel material is machined and processed into a shape corresponding to the lens surface of the CGH lens 15. This processing does not require the same precision as the lens surface, and it is sufficient to form a rough shape corresponding to the lens surface. The mold base material 2 is not limited to a stainless steel material, but may be any material having strength and rigidity as a mold.
[0020]
(B) A nickel material is laminated to a thickness of 5 mm by electroforming on the mold base material 2 having a rough shape corresponding to the lens surface. Hereinafter, this laminated layer is referred to as a processed layer 3. The material to be laminated is not limited to nickel, and may be, for example, a nickel alloy, copper, a copper alloy, or brass. The processing layer 3 is preferably formed of a material having good machinability and hard to be etched, such as the above-described materials. Note that a method other than electroforming may be used for laminating the processing layer 3.
[0021]
(C) A desired optical mirror surface corresponding to the lens surface is formed on the processing layer 3 by machining such as cutting, grinding, or polishing. This shape is the same as the surface shape of the lens surface, and the surface has the opposite surface irregularities, and is processed with high accuracy equal to or higher than the accuracy required for the lens surface of the optical element to be copied.
[0022]
As the mold base material, a material having high rigidity such as stainless steel is preferable, but it is difficult to form an optical mirror surface on an iron-based material such as stainless steel. It is for the following reasons. Generally, a diamond tool is used in mirror finishing, but if a diamond tool is used for iron-based material, an oxidation reaction occurs, and the diamond tool burns or excessively wears. . In this embodiment, an optical mirror surface is obtained by laminating a material having excellent machinability as a processing layer on a mold base material and processing the processing layer with high accuracy.
[0023]
(D) On the processed surface of the processed layer 3 formed in the step (C), a 650 nm-thick TiN (titanium nitride) film is formed by sputtering. This film is hereinafter referred to as an etching layer 4.
The conditions for forming the TiN film are as follows.
Set temperature: 300 ° C., pressure: 1.3 Pa, gas: Ar (argon),
Gas flow rate: 10 sccm, applied power: 500 W
It is.
[0024]
The material of the film is not limited to TiN, but Si (silicon), SiO ₂ (silicon dioxide), Al (aluminum), SiN (silicon nitride), Mo (molybdenum), SiC (silicon carbide), WC (tungsten carbide) ). The etching layer 4 is desirably formed of a material that can be easily etched, such as the above-described materials. The formation of the etching layer 4 is not limited to sputtering, and may be performed by another method such as vacuum evaporation, CVD (Chemical Vapor Deposition), and the like.
[0025]
(E) A resist 5 is applied on the etching layer 4 to a thickness of 800 nm by spin coating.
[0026]
(F) Exposing the diffraction grating pattern corresponding to CGH. In this embodiment, since exposure is performed on a curved surface, it is difficult to perform exposure using a conventional exposure apparatus. However, exposure is performed by a method using an electron beam lithography apparatus described later.
[0027]
(G) The resist 5 is developed. The exposed portions of the resist are removed by development, and the processed layer 4 is exposed.
[0028]
(H) The etching layer 4 (TiN) is etched by a dry etching method to form CGH 16 on the etching layer 4. Table 1 shows the etching conditions of various etching materials.
[0029]
[Table 1]

[0030]
In Table 1, RF is high frequency applied power of 13.56 MHz. In addition, when two types of gas are described in the gas type column with / (slash) therebetween, it means a mixed gas thereof. In the flow rate column, values described between / (slash) are the flow rates for each gas type described in the gas type column.
[0031]
The height of the CGH 16 can be controlled by the following two methods. That is,
(1) The height of the CGH 16 is controlled by forming the etching layer 4 thicker than the height of the CGH 16 and controlling the etching time.
(2) The etching layer 4 is formed with the same thickness as the height of the CGH 16, and the etching layer 4 is etched until the processing layer 3 is exposed. Since the processed layer 3 is made of a material that is difficult to be etched, it is not necessary to control the etching time strictly.
[0032]
In the present embodiment, etching is performed by the above method (2) until the processing layer 3 is exposed.
(I) Unnecessary resist 5 and polymer deteriorated by dry etching are removed by ashing (ashing).
[0033]
FIG. 1A is a longitudinal sectional view schematically showing the mold 1 thus manufactured. In the mold 1, the lens surface is processed into the processing layer 3 and the CGH 16 is formed on the etching layer 4 formed on the processing layer 3 by applying lithography technology, so that the lens surface and the CGH have high precision. The optical element surface can be accurately transferred. In addition, even in the mold shown in FIG. 1B in which the etching time is accurately controlled and the CGH 16 is formed only by the etching layer 4, the optical element surface can be transferred accurately.
[0034]
When the CGH lens 15 was injection-molded using PMMA using the mold 1 described above, desired performance was obtained. Note that the CGH of the present embodiment has a rectangular shape as shown in FIG. 8, but may have a stepped shape. Such a shape can be formed by performing resist application, pattern drawing, development processing, and etching processing a plurality of times.
[0035]
Next, a method of exposing a diffraction grating pattern on a curved surface regarding the step (F) will be described. In the present embodiment, the exposure method 1 is used, but other methods such as the exposure method 2 and the exposure method 3 may be used.
[0036]
(Exposure method 1) This is a method using an electron beam drawing apparatus. Since an electron beam lithography apparatus generally performs exposure on a flat surface, when exposing on a curved surface, a desired fine shape cannot be drawn because the focal position does not match the drawing surface. Therefore, exposure is performed while moving the object to be exposed or the drawing apparatus (optical system) in the optical axis direction so as to compensate for the curved surface shape. This method is effective not only for electron beams but also for laser exposure apparatuses.
[0037]
(Exposure method 2) A contact exposure method. This is effective when the shape of the workpiece is large and the grating period of the diffraction grating pattern is large. However, when the curvature of the lens surface is large and the deviation from the plane is large, the lattice pattern is blurred. If the grating period is about 50 microns, it is possible to expose even a plane about 2 mm away from a plane. When the lattice period is large and the area of the workpiece (mold) is large, it is easier to fabricate using the contact exposure method.
[0038]
(Exposure method 3) This is a method using a projection exposure apparatus. The optical system of the projection exposure apparatus has image surface characteristics and distortion characteristics corresponding to the lens surface shape of the mold.
[0039]
The exposure of the diffraction grating pattern on the curved surface can be performed by using any of the above three exposure methods.
[0040]
Next, a lens mold having an antireflection fine structure according to a second embodiment will be described. It is known that an antireflection effect can be obtained with respect to light having a wide wavelength range by forming a conical shape finely and densely on the surface of glass or plastics. The mold of the present embodiment is for duplicating a lens having a fine shape (hereinafter, referred to as a substantially conical shape) having an action equivalent to such a conical shape as an antireflection structure.
[0041]
The lens replicated by the mold of the present embodiment is a lens of a micro lens system mounted on a mobile phone or the like, which is arranged at a position closest to the image sensor. The lens is a biconvex lens having an effective diameter of 5 mm and a radius of curvature of about 5 mm and about 250 mm. When surface reflection occurs on a surface having a large radius of curvature, reflected light is incident on the image sensor and ghost occurs. When the lens is molded of a synthetic resin material, it is difficult to perform a high-performance antireflection coating with a general dielectric film, and the manufacturing cost increases. By molding the substantially cone-shaped antireflection structure at the same time as molding the lens, the manufacturing cost can be significantly reduced.
[0042]
FIG. 3 is a diagram schematically illustrating a manufacturing process of the mold according to the second embodiment. Hereinafter, the method of manufacturing the mold will be described with reference to FIG.
[0043]
(A) A mold base material 2 made of a stainless steel material is machined and processed into a shape corresponding to the lens surface of the lens. This processing does not require the same precision as the lens surface, and it is sufficient to form a rough shape corresponding to the lens surface. The mold base material 2 is not limited to a stainless steel material, but may be any material having strength and rigidity as a mold.
[0044]
(B) A nickel material is laminated to a thickness of 5 mm by electroforming on the mold base material 2 on which the approximate shape of the lens surface is formed (working layer 3). The material to be laminated is not limited to nickel, and may be, for example, a nickel alloy, copper, a copper alloy, or brass. The processing layer 3 is preferably formed of a material having good machinability and hard to be etched, such as the above-described materials.
[0045]
(C) A desired optical mirror surface corresponding to the lens surface is formed on the processing layer 3 by machining such as cutting, grinding, or polishing. This shape is processed with high accuracy equal to or higher than the accuracy required for the lens surface of the optical element to be copied.
[0046]
(D) A TiN (titanium nitride) film is formed on the processed surface of the processed layer 3 formed in the step (C) by sputtering (etching layer 4). The TiN film forming conditions are the same as those in the first embodiment. Note that the thickness of the etching layer was 300 nm.
[0047]
The material of the film is not limited to TiN, but Si (silicon), SiO ₂ (silicon dioxide), Al (aluminum), SiN (silicon nitride), Mo (molybdenum), SiC (silicon carbide), WC (tungsten carbide) ). The etching layer 4 is desirably formed of a material that can be easily etched, such as the above-described materials.
[0048]
(E) A resist 5 is applied on the etching layer 4 to a thickness of 800 nm by spin coating.
[0049]
(F) Exposure of a pattern for forming a conical shape. FIG. 4 shows an exposure method. The laser beam is separated into two light beams (not shown), and the separated two light beams are incident on the resist 5 at an angle to each other as shown in FIG. Interference fringes are generated on the resist 5, and the interference fringe shape is exposed. When the mold 11 is rotated 90 ° and two exposures are performed, interference fringes as shown in FIG. 4B are recorded. The region represented by the black circle has been exposed twice. By changing the angle of the two light beams, the period of the interference fringes, that is, the pitch of the conical shape can be changed. Further, the arrangement of the cones can be changed by changing the rotation angle of the mold and the number of exposures, for example, performing three exposures at 120 °.
[0050]
(G) The resist 5 is developed. The black circle shown in FIG. 4B has a larger exposure amount than the other regions, so that a hole reaching the etching layer 4 is generated by development.
[0051]
(H) A chromium film 6 is formed on the developed resist 5 by a vacuum evaporation method. The chromium film 6 is deposited on both the surface of the resist 5 and the etching layer 4.
[0052]
(I) The resist 5 is removed, and the chromium film 6 (hereinafter, referred to as a metal mask) formed on the four etching layers is exposed on the surface (lift-off).
[0053]
(J) Perform dry etching on the mold. The etching conditions are the conditions shown in Table 1 described in the first embodiment. When dry etching is started, first, etching is started in a tapered shape from a position where the metal mask 6 on the etching layer is not present. Then, as the etching proceeds, the metal mask 6 is also gradually etched, and the etching layer 4 is etched while its diameter decreases. Further, when the etching is performed until the metal mask 6 is almost eliminated, a protrusion having a weight shape 20 is generated. In the present embodiment, a cone shape with a period of 200 nm and a height of 250 nm was manufactured.
[0054]
(K) The unnecessary chromium film 6 remaining on the etching layer 4 is removed by wet etching.
[0055]
Using the mold 11 thus manufactured, a lens was molded. The molded lens had an optical performance of a wavefront aberration of 1 / 10λ and also had desired antireflection characteristics.
[0056]
As shown in FIG. 5, a substantially conical shape 25 of a lens molded by the mold 11 of the second embodiment has a flat portion 26 at the tip of the shape. FIG. 5 is a cross-sectional view schematically showing the substantially conical shape 25 of the molded lens. FIG. 6 schematically shows a weight shape 20 of the manufactured mold. FIG. 6A is a perspective view of the weight of the mold, and FIG. 6B is a cross-sectional view of the bottom of the weight. In FIG. 6A, since a very small part of the lens molding surface of the mold is cut out and displayed, it is shown as if a weight shape is formed on a plane, but actually, the lens of the mold is formed. A weight shape 20 is formed on the entire formation surface. As shown in FIG. 6B, since the bottom surface of the weight 20 has a flat surface 21, when the molding is performed, the flat surface 21 is transferred to the tip of the substantially conical shape 25. As described above, the substantially conical shape 25 having the flat portion 26 at the tip end and complementary to the weight shape 20 is not a complete weight shape, but has a weight shape as shown in FIG. There is no practical difference in performance as an anti-reflection structure as compared with the case where is provided on the lens surface.
[0057]
In the first and second embodiments, the fine structure is provided on the lens surface. However, the optical element is not limited to the lens. For example, the fine structure such as CGH is provided on the mirror surface having a curved surface. May be.
[0058]
【The invention's effect】
According to the first aspect of the present invention, a processed layer is formed on a mold base material, a curved surface corresponding to a lens surface or the like is machined on the processed layer, and a fine structure is formed on an etching layer laminated thereon. ing. Therefore, the surface accuracy of the optical element mold is ensured by machining, and the fine structure shape accuracy is obtained by etching the etching layer. As a result, a high-precision mold is obtained, and by using such a mold, high-precision optical elements can be easily mass-produced at low cost.
[0059]
In addition, since a material having good machinability, which can easily perform machining of a curved surface, is used for the processing layer, a rigid material can be used for the mold. Furthermore, even if the processed layer is exposed during the etching, the material is difficult to be etched, so that the material is not etched, and the lens surface of the mold can have good surface accuracy and surface roughness. In addition, the height of the fine structure can be easily controlled by using the characteristic that is hardly etched.
[0060]
According to the fourth aspect of the present invention, the processing layer is machined into a curved surface such as a lens surface, and a fine structure is formed on the etching layer formed thereon. A mold for an optical element obtained at the same time can be manufactured. By using such a mold for an optical element, a high-precision optical element can be easily mass-produced at low cost.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view schematically showing a mold according to a first embodiment.
FIG. 2 is a longitudinal sectional view schematically showing a manufacturing process of the mold according to the first embodiment.
FIG. 3 is a longitudinal sectional view schematically showing a manufacturing process of a mold according to a second embodiment.
FIG. 4 is a diagram illustrating a method for exposing a conical shape.
FIG. 5 is an enlarged longitudinal sectional view of an anti-reflection structure of a lens molded using a mold according to a second embodiment.
FIG. 6 is an enlarged view showing a weight shape of a mold according to the second embodiment.
FIG. 7 is a diagram schematically illustrating an example of a CGH lens.
FIG. 8 is a longitudinal sectional view schematically showing a CGH.
[Explanation of symbols]
1, 11 Mold 2 Mold base material 3 Processing layer 4 Etching layer 5 Resist 6 Chromium film (metal mask)
15 CGH lens 16 CGH
17 light source 20 cone 21 flat part (cone bottom)
25 Substantially conical shape 26 Flat part (substantially conical tip)

Claims (4)

An optical element mold for molding an optical element having a fine structure on a curved surface, in which a working layer having a curved surface corresponding to the optical element curved surface, and an etching having a fine structure are sequentially formed on a mold base material. An optical element mold, wherein an etching layer made of a material is laminated, and the processed layer is made of a material having a better cutting property than the mold base material and a lower etching property than the etching layer. The mold for an optical element according to claim 1, wherein the processed layer is made of any one of nickel, a nickel alloy, copper, a copper alloy, and brass. The mold for an optical element according to claim 1, wherein the etching layer is made of any of titanium nitride, silicon, silicon dioxide, aluminum, silicon nitride, molybdenum, silicon carbide, and tungsten carbide. A method for manufacturing a mold for an optical element for molding an optical element having a fine structure on a curved surface,
Forming a working layer on the mold base material;
A step of processing the processing layer into a desired optical mirror surface and a step of forming an etching layer on the processed processing layer,
A step of applying a resist on the etching layer,
A step of drawing a fine structure pattern on a resist and transferring the fine structure pattern by a development process;
Forming a fine structure in the etching layer by an etching process;
Wherein the curved surface and the fine structure are integrally formed.

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