JP3543464B2

JP3543464B2 - Optical recording medium and information recording method

Info

Publication number: JP3543464B2
Application number: JP00464496A
Authority: JP
Inventors: 夕起鈴木; 通和堀江; 修一前田
Original assignee: Mitsubishi Chemical Corp
Current assignee: Mitsubishi Chemical Corp
Priority date: 1996-01-16
Filing date: 1996-01-16
Publication date: 2004-07-14
Anticipated expiration: 2016-01-16
Also published as: JPH09198714A

Description

【０００１】
【発明の属する技術分野】
本発明はレーザー光により記録できる光記録媒体及びそれを用いた情報記録方法に関する。
【０００２】
【従来の技術】
近年、高密度記録のため、レーザー光の発振波長の短波長化が注目され、７８０ｎｍ、８３０ｎｍよりも短波長のレーザー光で記録再生可能な光記録媒体が求められている。かかる状況においては、さまざまな記録媒体があるが、その中で、有機色素系光記録媒体はプロセスが比較的簡便であるため安価であるという特長を有する。
【０００３】
ところで、有機色素系光記録媒体としては、ＣＤとの互換性があるタイプの光ディスク（ＣＤ−Ｒ）が７８０ｎｍのレーザー光に対応するもので既に実用化されている。一方、短波長用途の有機色素系媒体として数々の提案があり、例えば、特開平７ー７６１６９号公報、特開平７ー１２５４４１号公報、等がある。しかしながら、これらは、７８０ｎｍでのＣＤーＲの知見を単に短波長用に適用したものであり、短波長化のメリットである、微小記録部形成による高容量化を実現するための、短波長化での特有の要請を満たす要件が明らかにされていない。
【０００４】
また、溝形状に関しては、特開平４ー３５８３３１号公報、特開平５ー１９８０１３号公報、特開平５ー２７７１号公報、特開平４ー１０９４４１号公報、特開平３ー２２２２４号公報等があり、７８０ｎｍ近傍の波長については知見が示されているが、短波長化については明らかにされていない。
【０００５】
【発明が解決しようとする課題】
上記の従来技術においては、記録時に、色素の分解のみか、基板の変形と色素の分解の両方により記録変調度を得ているが、かかる方法では記録部の変形が大きく、溝上記録の場合には隣接の溝間部に及ぶ大きなピットが形成されるため、高密度化のためにトラックピッチを１.６μｍから１μｍ以下に狭くした場合に、隣接する溝間で影響を与える、いわゆるクロストークが問題となる。また、従来の溝設計では、高密度化用に開口数（ＮＡ）の大きなレーザーヘッドを使用する際に、ビーム径よりも大きな記録ビットを形成するために、記録部と未記録部の反射率コントラストを有効に得ることが困難である。
【０００６】
【課題を解決するための手段】
本発明者らは、高密度記録を実現されるために良好な微小記録部を形成し、かつ、十分な記録変調度を有する波長６００〜７００ｎｍの短波長記録に好適な溝形状と記録媒体を鋭意検討した結果、特定の溝幅、特定の溝壁の傾斜角度により特に短波長記録に適した光記録媒体が得られることを見出し、本発明に到達した。
【０００７】
すなわち、本発明の目的は短波長のレーザー光を用いた高密度記録に適した光記録媒体を提供することにあり、かかる目的は、透明基板上に、少なくとも、有機色素を含有する記録層、金属反射層、保護層の順に積層し、下記（イ）〜（ハ）を満たす光記録媒体において、
（イ）記録層が波長６００ｎｍ〜７００ｎｍのレーザー光により光学的な変化を生じるものであること。
（ロ）基板の溝の壁の傾斜が７０〜８５度であること。
（ハ）基板の溝幅が０.３〜０.３７μｍであること。
基板の溝間部の平坦部（Ｌｐ）が、溝の両脇の壁の接線と平坦部を含む平面との交線間で表される平坦部外挿部（Ｌ）の４５〜９５％であることを特徴とする光記録媒体により達成される。
【０００８】
【発明の実施の形態】
以下、本発明の実施の形態について説明する。
本発明における記録層は、記録用のレーザー光を吸収することによる昇温で減量し、膜厚が減少し、光学特性が変化することにより、戻り光の位相が変化し、反射率が変化したところを記録部とするものである。
【０００９】
本発明において、透明基板としてはポリカーボネート、ポリメタクリレート、非晶質ポリオレフィン等の樹脂やガラス等の公知のものが用いられ、特にポリカーボネート樹脂が好ましい。また、透明基板はサーボ用の案内溝を有しており、その溝は、深さは、通常１００〜２００ｎｍ、好ましくは、１４０〜２００ｎｍである。溝の深さが１００ｎｍ未満の場合には、記録時に十分な変化がおきず、十分な記録変調度が得られない場合がある。２００ｎｍを越えると、溝部と溝間部の反射率差が大きすぎるため、溝上記録の場合には反射率が低くなりすぎるので好ましくない。
【００１０】
また、本発明では、前記案内溝の溝幅を０.３〜０.３７μｍ、溝の壁の傾斜を７０〜８５度とすることが重要である。溝幅は、０．３μｍ未満には基板の溝転写率が低くなる上に、プッシュプル信号が小さいためトラッキングがかかりにくくなり、また、０．３７μｍを越える溝幅の場合には、記録層をスピンコートすることによっては十分狭い記録層の溝幅が得られなくなり、従って、光ビームスポットの半値全幅（＝０.５２λ/ＮＡ：λは波長。ＮＡは、レーザー光集束用レンズの開口数）に対して十分狭い記録層の溝を形成することが困難となる。一般に、ビーム径よりも十分狭い溝幅は、広くなることにより反射率が低下し、その変化率は、初期の溝幅が狭いほど大きい。すなわち、記録により、溝幅が大きくなるような記録媒体の場合には、記録による膜厚変化と光学定数の変化による位相差に由来する記録コントラストに加えて、未記録部の溝幅が狭いほど大きな記録変調度が期待されるということになる。また、基板の溝の壁の傾斜角は、７０〜８５度である。７０度よりも小さい場合には、相対的に溝幅が広くなるので、記録前後の溝幅変化の効果が小さくなるので好ましくない。また、８５度よりも大きい傾斜角で、１μｍ以下のトラックピッチの射出成形をプラスチック基板で実現することは技術的に困難である。なお、溝深さと溝幅は、トラックピッチが１μｍの場合にはＨｅーＣｄレーザーによる光学測定で求め、それよりもトラックピッチが狭い場合には、ＳＴＭにより求めることができる。その場合、基板の溝幅は溝深さの半分のところの溝幅であり、記録による基板の溝幅の変化量は、走査電子顕微鏡による基板の溝部の写真により求めた値である。また、溝の壁の傾斜角はＳＴＭ、ＡＦＭにより求められるが、本件に関しては、ＳＴＭでプロファイルを測定し、溝間部の水平線と溝の壁にひいた接線の成す角度を求める傾斜角（図１中のθ）とした。
【００１１】
さらに、基板の溝間部の平坦部（Ｌｐ）が、溝の両脇の壁の接線と平坦部を含む平面との交線間で表される平坦部外挿部（Ｌ）（図２にＬｐとＬの関係を示す）の４５〜９５％、特に５０〜９５％であることが好ましい。なお、図２にＬｐとＬの関係を示す。ＬｐがＬの４５％未満であると、スピンコート法によって記録層を形成すると、溝間部の塗布液が溝部に落ち込み、溝間部の記録層厚が著しく薄くなり、溝間部と溝部の膜厚のバランスが損なわれるため、良好な記録特性が得られない恐れがある。
【００１２】
記録層は、通常、有機色素等をエタノール、３ーヒドロキシー３ーメチルー２ーブタノン、ジアセトンアルコール、フッ素系アルコール等の溶媒に溶かした溶液をスピンコートして得られる。この溶媒としては、沸点が１００〜１８０℃である溶媒が特に好ましく用いられる。膜厚は溝部で１００〜２５０ｎｍ程度が好ましい。１００ｎｍ未満では薄すぎて良好な記録感度が得られにくい。また、２５０ｎｍを越えると、記録部の横方向への変形が大きくなるため、クロストークが大きくなり、好ましくない。基板の溝深さと膜の厚さの両方を含むパラメータとして、記録層の溝深さがある。これは、保護層のない状態では反射層の上から、また、保護層のある場合では、保護層を剥離し、記録層が残っている部分に金や、金ーパラジウムを５ｎｍ程度、イオンスパッタした層の上から、ＳＴＭ、ＡＦＭにより測定して得られる。この溝深さが、同じ方法で測定して得られた基板の溝深さの４０〜７５％であると、良好な特性が得られる。４０％未満の場合には、十分なトラッキングエラー信号が得られず、サーボがかかりにくくなるおそれがある。また、７５％を越える場合、膜厚が小さすぎて、十分な記録変調度が得られにくい。
【００１３】
記録層を構成する有機色素の熱的特性は記録特性に大きく影響する。短波長用途として充分な特性を得るためには、熱重量分析における、主減量過程での減量が、温度に対してシャープであることが必要であり、本発明では、主減量過程での減量の傾きが２％/℃以上、好ましくは、１０％/℃となる有機色素を用いることが好ましい。なお、ここでは、いくつかの減量過程のうちで減量が１８％以上の過程を主減量過程と呼ぶ。
【００１４】
本発明において、減量の傾きは、以下の如くして求める。（図３を参照。）
質量Ｍ₀の有機色素を窒素雰囲気で１０℃/分で昇温する。昇温に従って、質量は当初微量ずつ減少し、ほぼ直線ａーｂの減量線を描き、ついで急激に減量し始め、１８％以上の減量をほぼ直線ｄ₁ーｄ₂に沿って減量する。これが主減量過程であり、主減量開始温度は、Ｔ₁のことである。その後、ほぼ直線ｃ−ｃで示される減量過程におちつく。直線ｄ₁−ｄ₂と直線ｃ−ｃとの交点における温度をＴ₂、重量をｍ₂とし、初期重量をｍ₁とすれば、ここでいう減量の傾きとは、
【００１５】
【数１】
（ｍ₁−ｍ₂）（％）／（Ｔ₂−Ｔ₁）（℃）
で示される値である。この傾きが２％/℃未満である有機色素を用いると、記録部の横方向の広がりが大きくなり、また、短ビットの形成が困難となりやすく、高容量化を目的とする短波長用途に向かない。さらに、この主減量過程での総減量は当初質量Ｍ₀の２５％以上、好ましくは、３０％以上であることが好ましい。２５％未満であると、良好な記録変調度、記録感度が得られない恐れがある。なお、主減量開始温度は２５０℃〜３４０℃が好ましい。
【００１６】
有機色素としては、上記の要件を満足するものであればいずれでもよく、例えば、含金属アゾ系色素や、ジベンゾフラノン系、含金属インドアニリン系色素等がある。また、これらの有機色素を２種以上混ぜて使用してもよい。
また、記録再生波長±５ｎｍの波長領域の光に対する記録層単層の屈折率ｎが２〜３であり、消衰係数ｋが０.０３から０.１５であるものが好ましい。また、溶液でのモル吸光係数εが５万以上の吸収で最も長波長側の吸収極大に対応する、透明基板上の膜の状態での吸収極大が、記録再生波長よりも４０〜６０ｎｍ短波長側にあることが好ましい。溶液での吸収極大が、基板上のスピンコート膜の状態では長波長シフトすることが多いので、透明基板上での吸収極大で判断することが必要である。屈折率ｎが２よりも小さい場合には、十分な光学的変化が得られにくいため、記録変調度が低くなるので好ましくない。また、消衰係数ｋが０．０３未満では記録感度が悪くなる。消衰係数ｋが０.１５を越えると、５０％以上の反射率を得ることが困難となるので好ましくない。波長についても、上記範囲をはずれる場合には、十分な記録変調度と感度を得にくくなる。
【００１７】
なお、色素膜の屈折率ｎ、消衰係数ｋの正確な値を求めることはそれほど容易ではなく、むしろ色素膜の分光吸収スペクトルが容易に測定でき、かつ、反射率、記録変調度、記録感度に大きく影響を与えるパラメータとして有用である。有機色素の、ディスク基板上に膜として形成された状態での分光吸収スペクトルにおいて、上記要件を満たす吸収極大の吸光度に対して、記録再生光波長での吸光度が５〜１５％である場合、良好な特性を示す。５％未満の場合には十分な記録感度が得られにくい。１５％を越える場合、屈折率ｎが小さくなり、十分な記録変調度が得られない。なお、分光吸収スペクトル（Ａｂｓｏｒｂａｎｃｅ）は、参照サンプルを空気にし、透明基板上の案内溝のある面上に色素をスピンコートし、基板側から光を入射して測定することができる。なお、吸光度の比は、基板の吸収分に相当するベースラインを差し引いた値を吸光度として計算する。
【００１８】
金属反射層は、記録層を透過したレーザー光を効率良く反射する金属膜であり、６００ｎｍ〜７００ｎｍで反射率が低下しないために、記録再生波長±５ｎｍの波長領域の光の屈折率ｎが０.１〜０.２、消衰係数ｋが３〜５であるものが好ましい。好ましい金属反射膜として、金を主成分とした金属反射膜や、銀を主成分とした金属反射膜が例示できる。特に銀を主成分とした金属反射膜は、より高い反射率が得られ、安価であることから好ましい。また、対候性の向上のために、銀に、ロジウム、パラジウム、白金、チタン、モリブデン、ジルコニウム、タンタル、タングステン、バナジウム等の添加元素を５原子％以下の範囲で加えてもよい。金属反射層の膜厚は、好ましくは６０ｎｍ以上で、記録層の変形を抑制しすぎたり、記録感度を悪化させすぎない程度の膜厚が好ましい。
【００１９】
反射層の上に、保護層を積層し、記録部の金属反射層の穴の発生や、変形の非対称性を抑制する。保護層としては紫外線硬化樹脂が好ましい。また、通常は、１μｍ以上、好ましくは３μｍ以上の膜厚にして、酸素による硬化抑制等がおこらないようにする。
本発明では、記録により記録部の色素が分解し、減量することによって膜厚、屈折率ｎ、消衰係数ｋが変化することによる位相差の変化に加えて、基板の溝幅が広くなることによる反射率の変化が、記録変調度に寄与している。図４に一般的な光学計算結果を示す。図４は記録層の屈折率ｎを２．４、消衰係数ｋを０．０８、溝深さを１００ｎｍとしたときの溝幅と反射率との関係を計算で求めた相関関係を示すグラフである。記録層の屈折率ｎ、消衰係数ｋが変わらないと仮定した場合でも、溝幅が狭いほど、溝幅の変化による反射率変化が大きい。実際には、記録の前後で記録層の屈折率ｎ、消衰係数ｋが変化すること、さらに、スピンコート膜による溝の埋まり方に起因する実効的な溝幅の減少という２つの点から、計算以上に溝幅の変化の反射率変化への寄与が大きいと考えられる。さらに、この記録部の基板の溝幅増大という変化は、色素層の分解と十分な発熱がおこり、記録層の光学定数変化が起こり、かつ、色素層の膜厚が減少してピットができている場合に見られる。色素層の分解によるピットの生成により、再生光の位相差への寄与があり、即ち、記録部の反射率が位相差により低下して、より大きな記録変調度が得られる。このような記録部の位相差は、Ｊａｐ.Ｊ.Ａｐｐｌ.Ｐｈｙｓ.３１（１９９２）４８４ー４９３に示されるように、一般的には
【００２０】
【数２】
位相差＝２（ＮdyeーＮsub）ｘ（Ｄbump）＋２ｘ（Ｎdye）ｘ（Ｄpit）
（Ｎは、それぞれ、色素層、基板の屈折率：Ｄはそれぞれ、基板の溝深さの変化量、色素層ピット深さ）
で示され、この式からわかるように、ピット形成の寄与は大きい。基板の溝幅の変化は、それ自体の寄与効果（図４参照）とともに、この様な記録層の光学変化（位相変化）に大きく寄与するパラメータであり、それゆえ、記録変調度は溝幅の変化に大きく依存する。この溝幅の変化の割合は、本発明者らが検討した結果、未記録部の溝幅に対して２０〜４０％広くなると、良好な記録変調度が得られることが判明した。２０％未満では、十分な反射率変化が得られない恐れがある。４０％を越えると、記録前後のトラッキングエラー信号の変化が大きすぎるため、サーボがかからなくなることがある。なお、基板の溝幅の変化は、記録層を除去した形で、光学顕微鏡や電子顕微鏡で観察できる。記録層は、ディスク面に傷をつけ、両面テープで保護層ならびに反射層を剥離した後、エタノール等で色素を十分流しとることにより除去することができる。
【００２１】
また、記録層を除去せずに水銀ランプの光学顕微鏡により、焦点深度の長いＰＭＭＡレンズ等を対物レンズに使用して、基板側から溝幅の変化を観察してもよい。後述の実施例では、記録層を除去した後、電子顕微鏡により観察し、記録部と未記録部の溝幅の変化の割合を測定した。なお、後述の実施例では記録前後の溝幅は、半値溝幅ではなく、電子顕微鏡で明るく見えるランド部（溝間部）の内側の暗く見える部分（溝部）の幅を溝幅とした。
【００２２】
【実施例】
以下、本発明を実施例により更に詳細に説明するが、本発明は、その要旨を越えない限り実施例に限定されるものではない。
実施例１
溝深さ１８０ｎｍ、溝幅０．３７μｍ（１．０μｍピッチ）、溝の壁の傾斜角が７５〜８０度で、Ｌｐ／Ｌ＝４９％である（ＳＴＭによる測定）ポリカーボネート基板上に下記構造式［１］
【００２３】
【化１】

【００２４】
で示される含金属アゾ色素０.０３６ｇをオクタフルオロペンタノール（ＯＦＰ）３ｇに溶解し、８００ｒｐｍでスピンコートし、記録層とした。なお、溝の壁の傾斜角が７５〜８０度と幅を持っているのは、ＳＴＭの走査方向により、溝の左側の壁のθが７５度であり、右の壁のθが８０度となっているためである。この色素の減量特性は主減量過程（主減量開始温度は２８０℃）での総減量が２５.２％で、温度差が８.９℃で、減量の傾きは２.８％/℃であった。なお、熱重量分析はセイコー電子工業製の示差熱天秤（「ＳＳＣ５２００Ｈ」シリーズ「ＴＧ−ＤＴＡ−３２０」）を用いて測定した。
【００２５】
この記録層の上に金を６０ｎｍの厚さだけスパッタした。この状態でＳＴＭにより測定した記録層の溝深さは、同じくＳＴＭで測定した基板の溝深さの５７％であった（溝部膜厚は２１０ｎｍ）。その上にＵＶ硬化樹脂（大日本インキ製「ＳＤー３１８」）を約３μｍスピンコートして紫外線ランプで硬化してディスクを作製した。このディスクを６８０ｎｍの半導体レーザー評価機（ＮＡ＝０.６）で、線速３ｍ/ｓで溝上に記録したところ、再生パワー０.７ｍＷ、記録周波数３ＭＨｚ、ｄｕｔｙ比３０％、記録パワー６ｍＷで記録したところ、Ｃ／Ｎ５０ｄＢ、変調度４８％、クロストークは３０ｄＢであり、良好な記録部が形成されていた。なお、ここでのクロストークは、（グルーブ記録部再生Ｃ／Ｎー隣接グルーブでの再生Ｃ／Ｎ）（ｄＢ）である。このディスクの膜を除去し、ＳＥＭで記録部を観察したところ、記録部の溝幅が未記録記部よりも３３％広くなっていた。なお、この色素層の６８０ｎｍでの屈折率ｎ、消衰係数ｋはそれぞれ、２.４と０.１５であった。
【００２６】
実施例２
実施例１における色素量を０.０２ｇにした以外は全く同様にしてディスクを作製した。記録層の溝深さが、ＳＴＭにより、膜のない状態の基板の溝深さの７０％であった（溝部の膜厚は１６０ｎｍ）。このディスクを実施例１の評価機で記録パワーを９ｍＷにして同じ条件で溝上で記録したところ、Ｃ／Ｎ４５ｄＢで、記録変調度が４０％であり、クロストークは２５ｄＢであり、良好な特性が得られた。実施例と同様にして記録部の基板をみたところ、溝幅が、未記録部よりも２３％広くなっていた。
【００２７】
実施例３
実施例１において、溝深さ１８０ｎｍ、溝幅０.３３μｍ（トラックピッチ０.７４μｍ）、溝の壁の傾斜角が７７度〜８３度で、Ｌｐ／Ｌ＝４７％である案内溝を有するポリカーボネート基板にかえ、反射膜を銀６０ｎｍにかえた以外は全く同様にしてディスクを作製した。このディスクの記録層の溝深さは、基板の溝深さの５５％であった。このディスクを、実施例１と全く同じ条件で溝上で記録再生したところ、Ｃ／Ｎ５２ｄＢで、記録変調度４５％、クロストークが３２ｄＢであった。実施例１と同様にして記録部の基板の溝幅をＳＥＭでみたところ、未記録部より４０％広くなっていた。
【００２８】
比較例1
実施例１において、溝深さ９０ｎｍで溝幅０.４５μｍ（トラックピッチ１μｍ）溝の壁の傾斜角が６０度で、Ｌｐ／Ｌ＝４５％であるポリカーボネート基板にかえた以外は全く同様にしてディスクを作製した。記録層の溝深さは基板の溝深さの５０％であった。実施例１と同じ記録をおこなったところ、Ｃ／Ｎ３０ｄＢで記録変調度が１０％未満であった。記録部の基板の変形をしらべたところ、記録部の溝幅は未記録部の溝幅とほとんど変わらなかった。
【００２９】
比較例２
実施例１で、Ｌｐ／Ｌ＝３０％の基板に変えた以外は全く同様にしてディスクを作成し、６８０ｎｍの評価機で同様にして評価したところ、記録パワー６ｍＷではＣ／Ｎ３０ｄＢ、記録変調度１５％であり、記録パワー１３ｍＷでＣ／Ｎ４５ｄＢ、記録変調度２５％であった。
【００３０】
【発明の効果】
本発明により、溝幅の狭い溝上において、高いＣ／Ｎ、大きな記録変調度を有する、波長６００〜７００ｎｍの短波長記録に好適な光記録媒体を得ることができる。
【図面の簡単な説明】
【図１】溝の壁の傾斜角θを示す説明図
【図２】基板の溝間部の平坦部（Ｌｐ）と、溝の両脇の壁の接線と平坦部を含む平面との交線間で表される平坦部外挿部（Ｌ）との関係を説明する説明図
【図３】色素の主減量過程、主減量過程の総重量、減量の傾きを求める方法を説明するための示差熱天秤の模式チャート図
【図４】記録層の溝幅と反射率の相関関係を計算で求めたグラフ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical recording medium capable of recording with a laser beam and an information recording method using the same.
[0002]
[Prior art]
In recent years, attention has been paid to shortening the oscillation wavelength of laser light for high-density recording, and an optical recording medium capable of recording and reproducing with laser light having a wavelength shorter than 780 nm or 830 nm has been demanded. In such a situation, there are various recording media, and among them, the organic dye-based optical recording medium has a feature that the process is relatively simple and thus inexpensive.
[0003]
By the way, as an organic dye-based optical recording medium, an optical disk (CD-R) of a type compatible with a CD compatible with a 780 nm laser beam has already been put to practical use. On the other hand, there have been many proposals for organic dye-based media for short wavelength applications, for example, JP-A-7-76169 and JP-A-7-125441. However, in these methods, the knowledge of the CD-R at 780 nm is simply applied for short wavelengths, and the wavelength shortening for realizing high capacity by forming a minute recording portion, which is an advantage of the short wavelength, is considered. The requirements to meet the specific requirements of the country have not been disclosed.
[0004]
Regarding the groove shape, there are JP-A-4-358331, JP-A-5-198013, JP-A-5-2771, JP-A-4-109441, JP-A-3-22224, and the like. Although knowledge is shown about a wavelength near 780 nm, shortening of the wavelength is not disclosed.
[0005]
[Problems to be solved by the invention]
In the above-mentioned conventional technique, at the time of recording, only the decomposition of the dye or the recording modulation degree is obtained by both the deformation of the substrate and the decomposition of the dye. Since large pits are formed in adjacent groove portions, when the track pitch is reduced from 1.6 μm to 1 μm or less in order to increase the density, so-called crosstalk which affects between adjacent grooves is caused. It becomes a problem. Also, in the conventional groove design, when a laser head having a large numerical aperture (NA) is used for high density, the reflectance of a recorded portion and an unrecorded portion is formed in order to form a recording bit larger than a beam diameter. It is difficult to effectively obtain contrast.
[0006]
[Means for Solving the Problems]
The present inventors have formed a fine recording portion suitable for realizing high-density recording, and have a groove shape and a recording medium suitable for short-wavelength recording at a wavelength of 600 to 700 nm having a sufficient degree of recording modulation. As a result of intensive studies, they have found that an optical recording medium particularly suitable for short-wavelength recording can be obtained with a specific groove width and a specific inclination angle of a groove wall, and have reached the present invention.
[0007]
That is, an object of the present invention is to provide an optical recording medium suitable for high-density recording using short-wavelength laser light, such an object, on a transparent substrate, at least, a recording layer containing an organic dye, metal reflective layer, laminated in this order of the protective layer, the optical recording medium satisfies the following Symbol (a) to (c),
(A) The recording layer is to cause an optical change by laser light having a wavelength of 600 nm to 700 nm.
(B) The inclination of the wall of the groove of the substrate is 70 to 85 degrees.
(C) The groove width of the substrate is 0.3 to 0.37 μm.
The flat portion (Lp) of the inter-groove portion of the substrate is 45 to 95% of the flat portion extrapolated portion (L) expressed between the intersections of the tangents of the walls on both sides of the groove and the plane including the flat portion. This is achieved by an optical recording medium having the following features.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
The recording layer in the present invention is reduced in temperature by increasing the temperature by absorbing the recording laser light, the film thickness is reduced, the optical characteristics are changed, the phase of the return light is changed, and the reflectance is changed. However, the recording unit is used.
[0009]
In the present invention, as the transparent substrate, a known material such as a resin such as polycarbonate, polymethacrylate, and amorphous polyolefin, and glass is used, and a polycarbonate resin is particularly preferable. The transparent substrate has a servo guide groove, and the groove has a depth of usually 100 to 200 nm, preferably 140 to 200 nm. When the depth of the groove is less than 100 nm, a sufficient change does not occur during recording, and a sufficient recording modulation degree may not be obtained. If the thickness exceeds 200 nm, the reflectance difference between the groove portion and the groove portion is too large, and in the case of recording on a groove, the reflectance becomes too low, which is not preferable.
[0010]
In the present invention, it is important that the width of the guide groove is 0.3 to 0.37 μm and the inclination of the wall of the groove is 70 to 85 degrees. When the groove width is less than 0.3 μm, the groove transfer rate of the substrate is low, and since the push-pull signal is small, tracking is difficult to be performed. When the groove width exceeds 0.37 μm, the recording layer is By spin coating, a sufficiently narrow groove width of the recording layer cannot be obtained. Therefore, the full width at half maximum of the light beam spot (= 0.52λ / NA: λ is a wavelength; NA is the numerical aperture of a laser beam focusing lens) It is difficult to form a groove in the recording layer that is sufficiently narrow. In general, a groove width sufficiently smaller than the beam diameter is reduced in reflectivity as it is widened, and the change rate is larger as the initial groove width is smaller. That is, in the case of a recording medium in which the groove width is increased by recording, in addition to the recording contrast derived from the phase difference due to the change in film thickness and the change in optical constant due to recording, the smaller the groove width of the unrecorded portion, the smaller the groove width. This means that a large recording modulation degree is expected. The inclination angle of the wall of the groove of the substrate is 70 to 85 degrees. If the angle is smaller than 70 degrees, the groove width becomes relatively large, and the effect of changing the groove width before and after recording becomes small, which is not preferable. Also, it is technically difficult to achieve injection molding with a tilt angle greater than 85 degrees and a track pitch of 1 μm or less on a plastic substrate. The groove depth and groove width can be determined by optical measurement using a He—Cd laser when the track pitch is 1 μm, and can be determined by STM when the track pitch is smaller than that. In that case, the groove width of the substrate is the groove width at half the groove depth, and the amount of change in the groove width of the substrate due to recording is a value obtained from a photograph of the groove of the substrate by a scanning electron microscope. The inclination angle of the groove wall is obtained by STM and AFM. In this case, the profile is measured by STM, and the inclination angle between the horizontal line between the grooves and the tangent drawn to the groove wall is obtained (see FIG. Θ in 1).
[0011]
Further, a flat portion extrapolation portion (L) (see FIG. 2) in which the flat portion (Lp) of the inter-groove portion of the substrate is expressed between the intersections of the tangents of the walls on both sides of the groove and the plane including the flat portion. (Indicating the relationship between Lp and L) is 45 to 95%, preferably 50 to 95%. FIG. 2 shows the relationship between Lp and L. If the recording layer is formed by spin coating when Lp is less than 45% of L, the coating liquid in the inter-groove portion falls into the groove portion, the recording layer thickness in the inter-groove portion becomes extremely thin, and the gap between the inter-groove portion and the groove portion is reduced. Since the balance of the film thickness is lost, good recording characteristics may not be obtained.
[0012]
The recording layer is usually obtained by spin coating a solution in which an organic dye or the like is dissolved in a solvent such as ethanol, 3-hydroxy-3-methyl-2-butanone, diacetone alcohol, or fluorinated alcohol. As the solvent, a solvent having a boiling point of 100 to 180 ° C is particularly preferably used. The film thickness in the groove is preferably about 100 to 250 nm. If it is less than 100 nm, it is too thin to obtain good recording sensitivity. On the other hand, if the thickness exceeds 250 nm, the lateral deformation of the recording portion becomes large, so that the crosstalk increases, which is not preferable. A parameter including both the groove depth of the substrate and the thickness of the film is the groove depth of the recording layer. This is because the protective layer was peeled off from above the reflective layer when there was no protective layer, or when the protective layer was present, and gold or gold-palladium was ion-sputtered on the portion where the recording layer remained to about 5 nm. It is obtained by measuring STM and AFM from above the layer. If the groove depth is 40 to 75% of the groove depth of the substrate obtained by the same method, good characteristics can be obtained. If it is less than 40%, a sufficient tracking error signal cannot be obtained, and it may be difficult to operate the servo. On the other hand, if it exceeds 75%, the film thickness is too small, and it is difficult to obtain a sufficient recording modulation degree.
[0013]
The thermal characteristics of the organic dye constituting the recording layer greatly affect the recording characteristics. In order to obtain sufficient properties for short wavelength applications, the thermogravimetric analysis requires that the weight loss in the main weight loss process be sharp with respect to the temperature. It is preferable to use an organic dye having a gradient of 2% / ° C. or more, preferably 10% / ° C. Here, a process in which the weight loss is 18% or more among several weight loss processes is referred to as a main weight loss process.
[0014]
In the present invention, the slope of the weight loss is obtained as follows. (See FIG. 3)
The temperature of the organic dye having a mass of M ₀ is increased at a rate of 10 ° C./min in a nitrogen atmosphere. As the temperature rises, the mass initially decreases by a very small amount, draws a weight loss line substantially in a straight line ab, then starts to decrease sharply, and the weight loss of 18% or more is reduced substantially along a straight line d ₁ -d ₂ . This is the main reduction process, the main weight reduction initiation temperature is that of the T _1. After that, the weight loss process is substantially indicated by a straight line cc. If the temperature at the intersection of the straight line d ₁ -d ₂ and the straight line cc is T ₂ , the weight is m ₂ , and the initial weight is m ₁ , the slope of the weight loss is
[0015]
(Equation 1)
(M ₁ −m ₂ ) (%) / (T ₂ −T ₁ ) (° C.)
Is the value indicated by. When an organic dye having a slope of less than 2% / ° C. is used, the spread of the recording portion in the horizontal direction becomes large, and it becomes difficult to form a short bit. No Further, the main reduction process total loss initially mass M ₀ of more than 25% in, preferably, is preferably 30% or more. If it is less than 25%, good recording modulation and recording sensitivity may not be obtained. Note that the main weight loss start temperature is preferably from 250C to 340C.
[0016]
Any organic dye may be used as long as it satisfies the above requirements, and examples thereof include metal-containing azo dyes, dibenzofuranone-based, and metal-containing indoaniline-based dyes. Further, two or more of these organic dyes may be used as a mixture.
Further, it is preferable that the single layer of the recording layer has a refractive index n of 2 to 3 and a extinction coefficient k of 0.03 to 0.15 with respect to the light in the wavelength range of ± 5 nm. In addition, the absorption maximum in the state of a film on a transparent substrate corresponding to the absorption maximum on the longest wavelength side with a molar absorption coefficient ε of 50,000 or more in a solution is 40 to 60 nm shorter than the recording / reproducing wavelength. Preferably on the side. Since the absorption maximum in the solution often shifts by a long wavelength in the state of the spin coat film on the substrate, it is necessary to determine the maximum absorption on the transparent substrate. When the refractive index n is smaller than 2, it is difficult to obtain a sufficient optical change, so that the recording modulation degree becomes low, which is not preferable. If the extinction coefficient k is less than 0.03, the recording sensitivity becomes poor. If the extinction coefficient k exceeds 0.15, it is difficult to obtain a reflectance of 50% or more, which is not preferable. If the wavelength is out of the above range, it becomes difficult to obtain a sufficient recording modulation degree and sensitivity.
[0017]
It is not so easy to obtain accurate values of the refractive index n and the extinction coefficient k of the dye film. Rather, the spectral absorption spectrum of the dye film can be easily measured, and the reflectance, recording modulation degree, and recording sensitivity can be easily measured. This is useful as a parameter that greatly affects the In the spectral absorption spectrum of an organic dye formed as a film on a disk substrate, it is preferable that the absorbance at the recording / reproducing light wavelength is 5 to 15% with respect to the absorbance at the absorption maximum satisfying the above requirements. Characteristic. If it is less than 5%, it is difficult to obtain sufficient recording sensitivity. If it exceeds 15%, the refractive index n becomes small, and a sufficient recording modulation degree cannot be obtained. Note that the spectral absorption spectrum (Absorbance) can be measured by using a reference sample as air, spin-coating a dye on a surface of a transparent substrate having a guide groove, and entering light from the substrate side. The absorbance ratio is calculated as a value obtained by subtracting a baseline corresponding to the amount absorbed by the substrate.
[0018]
The metal reflective layer is a metal film that efficiently reflects the laser light transmitted through the recording layer. Since the reflectance does not decrease between 600 nm and 700 nm, the refractive index n of the light in the wavelength region of the recording / reproducing wavelength ± 5 nm is 0. Those having an extinction coefficient k of 3 to 5 are preferred. Preferred examples of the metal reflection film include a metal reflection film containing gold as a main component and a metal reflection film containing silver as a main component. In particular, a metal reflection film containing silver as a main component is preferable because a higher reflectance can be obtained and the cost is lower. Further, in order to improve weatherability, additional elements such as rhodium, palladium, platinum, titanium, molybdenum, zirconium, tantalum, tungsten, and vanadium may be added to silver in a range of 5 atomic% or less. The thickness of the metal reflection layer is preferably 60 nm or more, and is preferably such a thickness that the deformation of the recording layer is not excessively suppressed or the recording sensitivity is not excessively deteriorated.
[0019]
A protective layer is laminated on the reflective layer to suppress the generation of holes in the metal reflective layer of the recording section and the asymmetry of deformation. As the protective layer, an ultraviolet curable resin is preferable. In addition, the thickness is usually set to 1 μm or more, preferably 3 μm or more so that the suppression of curing by oxygen or the like does not occur.
In the present invention, in addition to the change in the phase difference due to the change in the film thickness, the refractive index n, and the extinction coefficient k by decomposing and reducing the dye in the recording portion by recording, the groove width of the substrate is increased. Changes in reflectivity contribute to the recording modulation degree. FIG. 4 shows general optical calculation results. FIG. 4 is a graph showing the calculated correlation between the groove width and the reflectance when the refractive index n of the recording layer is 2.4, the extinction coefficient k is 0.08, and the groove depth is 100 nm. It is. Even if it is assumed that the refractive index n and the extinction coefficient k of the recording layer do not change, the smaller the groove width, the greater the change in reflectance due to the change in groove width. Actually, from the two points that the refractive index n and the extinction coefficient k of the recording layer change before and after recording, and that the effective groove width is reduced due to how the groove is filled by the spin-coat film. It is considered that the change in the groove width contributes more to the change in the reflectance than the calculation. Further, the change of the groove width of the substrate in the recording portion is caused by the decomposition of the dye layer and sufficient heat generation, a change in the optical constant of the recording layer, and the reduction in the thickness of the dye layer to form pits. Seen if there is. The generation of pits due to the decomposition of the dye layer contributes to the phase difference of the reproduction light, that is, the reflectivity of the recording portion is reduced by the phase difference, and a larger recording modulation is obtained. The phase difference of such a recording section is generally represented by the following expression, as shown in Jap. J. Appl. Phys. 31 (1992) 484-493.
(Equation 2)
Phase difference = 2 (Ndye−Nsub) × (Dbump) + 2 × (Ndye) × (Dpit)
(N is the refractive index of the dye layer and the refractive index of the substrate, respectively; D is the amount of change in the groove depth of the substrate and the depth of the dye layer pit, respectively)
As can be seen from this equation, the contribution of pit formation is large. The change in the groove width of the substrate is a parameter that greatly contributes to such an optical change (phase change) of the recording layer, together with its own contribution effect (see FIG. 4). Depends heavily on change. As a result of an examination by the present inventors, it has been found that when the ratio of the change in the groove width is 20 to 40% wider than the groove width of the unrecorded portion, a good recording modulation degree can be obtained. If it is less than 20%, a sufficient change in reflectance may not be obtained. If it exceeds 40%, the servo may not work because the change of the tracking error signal before and after recording is too large. The change in the groove width of the substrate can be observed with an optical microscope or an electron microscope with the recording layer removed. The recording layer can be removed by damaging the disk surface, peeling off the protective layer and the reflective layer with a double-sided tape, and then thoroughly removing the dye with ethanol or the like.
[0021]
Further, the change in the groove width may be observed from the substrate side by using a PMMA lens or the like having a long depth of focus as an objective lens using an optical microscope of a mercury lamp without removing the recording layer. In the examples described below, after the recording layer was removed, observation was performed with an electron microscope, and the ratio of the change in the groove width between the recorded portion and the unrecorded portion was measured. In the examples described later, the groove width before and after recording is not the half-value groove width, but the width of a dark portion (groove portion) inside a land portion (inter-groove portion) that looks bright with an electron microscope is defined as the groove width.
[0022]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples unless it exceeds the gist.
Example 1
A groove depth of 180 nm, a groove width of 0.37 μm (1.0 μm pitch), an inclination angle of a groove wall of 75 to 80 degrees, and Lp / L = 49% (measured by STM). [1]
[0023]
Embedded image

[0024]
Was dissolved in 3 g of octafluoropentanol (OFP) and spin-coated at 800 rpm to form a recording layer. Note that the inclination angle of the wall of the groove has a width of 75 to 80 degrees because the θ of the left wall of the groove is 75 degrees and the θ of the right wall is 80 degrees according to the scanning direction of the STM. It is because it has become. The weight loss characteristics of this dye were as follows: the total weight loss in the main weight loss process (main weight loss starting temperature was 280 ° C) was 25.2%, the temperature difference was 8.9 ° C, and the slope of the weight loss was 2.8% / ° C. Was. In addition, the thermogravimetric analysis was measured using a differential thermal balance (“SSC5200H” series “TG-DTA-320”) manufactured by Seiko Denshi Kogyo.
[0025]
Gold was sputtered on this recording layer to a thickness of 60 nm. In this state, the groove depth of the recording layer measured by STM was 57% of the groove depth of the substrate also measured by STM (the groove film thickness was 210 nm). A UV-curable resin (“SD-318” manufactured by Dainippon Ink) was spin-coated thereon at about 3 μm and cured with an ultraviolet lamp to produce a disk. This disc was recorded on a groove at a linear velocity of 3 m / s with a 680 nm semiconductor laser evaluator (NA = 0.6). The recording power was 0.7 mW, the recording frequency was 3 MHz, the duty ratio was 30%, and the recording power was 6 mW. As a result, the C / N was 50 dB, the modulation was 48%, the crosstalk was 30 dB, and a good recording portion was formed. Here, the crosstalk is (groove recording portion reproduction C / N-reproduction C / N in adjacent groove) (dB). When the film of the disk was removed and the recorded portion was observed by SEM, the groove width of the recorded portion was 33% wider than that of the unrecorded portion. The refractive index n and the extinction coefficient k at 680 nm of this dye layer were 2.4 and 0.15, respectively.
[0026]
Example 2
A disk was prepared in exactly the same manner as in Example 1 except that the amount of the dye was 0.02 g. According to the STM, the groove depth of the recording layer was 70% of the groove depth of the substrate without a film (the film thickness of the groove portion was 160 nm). When this disc was recorded on the groove under the same conditions with the recording power of 9 mW by the evaluation machine of Example 1, the recording modulation degree was 40%, the crosstalk was 25 dB, the C / N was 45 dB, and the good characteristics were good. Obtained. When the substrate of the recording portion was viewed in the same manner as in the example, the groove width was 23% wider than the unrecorded portion.
[0027]
Example 3
In Example 1, polycarbonate having a groove depth of 180 nm, a groove width of 0.33 μm (track pitch of 0.74 μm), an inclination angle of a groove wall of 77 ° to 83 °, and a guide groove with Lp / L = 47% A disk was prepared in exactly the same manner except that the substrate was replaced with a reflective film of silver of 60 nm. The groove depth of the recording layer of this disk was 55% of the groove depth of the substrate. When this disk was recorded and reproduced on the groove under the same conditions as in Example 1, the C / N was 52 dB, the recording modulation was 45%, and the crosstalk was 32 dB. When the groove width of the substrate of the recording portion was examined by SEM in the same manner as in Example 1, it was 40% wider than the unrecorded portion.
[0028]
Comparative Example 1
Example 1 is exactly the same as in Example 1, except that the polycarbonate substrate was replaced with a groove depth of 90 nm, a groove width of 0.45 μm (track pitch of 1 μm), an inclination angle of the groove wall of 60 °, and Lp / L = 45%. A disk was made. The groove depth of the recording layer was 50% of the groove depth of the substrate. When the same recording as in Example 1 was performed, the recording modulation degree was less than 10% at C / N 30 dB. When the deformation of the substrate of the recording portion was examined, the groove width of the recording portion was almost the same as the groove width of the unrecorded portion.
[0029]
Comparative Example 2
A disk was prepared in exactly the same manner as in Example 1 except that the substrate was changed to Lp / L = 30%, and evaluated in the same manner using a 680 nm evaluator. When the recording power was 6 mW, the C / N was 30 dB, and the recording modulation factor was The recording power was 13 mW, the C / N was 45 dB, and the recording modulation was 25%.
[0030]
【The invention's effect】
According to the present invention, it is possible to obtain an optical recording medium having a high C / N ratio and a large recording modulation degree on a groove having a small groove width and suitable for recording at a short wavelength of 600 to 700 nm.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an inclination angle θ of a wall of a groove; FIG. FIG. 3 is an explanatory diagram for explaining a relationship between a flat portion extrapolated portion (L) and a difference between the main weight loss process of the pigment, the total weight of the main weight loss process, and a slope of weight loss. FIG. 4 is a schematic chart of a thermal balance. FIG. 4 is a graph showing a correlation between a groove width of a recording layer and a reflectance.

Claims

On a transparent substrate having a guide groove track pitch is 0.7～1.0Myuemu, at least, a recording layer containing an organic dye, a metal reflective layer, laminated in this order of the protective layer, the lower Symbol (a) to (c Optical recording medium satisfying
(A) The recording layer is to be changed optically by laser light having a wavelength of 600 nm to 700 nm.
(B) The inclination of the wall of the groove of the substrate is 70 to 85 degrees.
(C) The groove width of the substrate is 0.3 to 0.37 μm.
The flat portion (Lp) of the inter-groove portion of the substrate is 45 to 95% of the flat portion extrapolated portion (L) expressed between the intersections of the tangents of the walls on both sides of the groove and the plane including the flat portion. An optical recording medium, comprising:

2. The light according to claim 1, wherein when a recording portion is formed by irradiating a laser beam having a wavelength of 600 nm to 700 nm, a groove width of the substrate is increased by 20 to 40% with respect to an unrecorded portion. recoding media.

3. The optical recording medium according to claim 1, wherein the groove depth of the recording layer is 40 to 75% of the groove depth of the substrate.

Refractive index n of the recording layer alone with respect to light in a wavelength range of the recording and reproducing wavelength ± 5 nm is 2 to 3, any one of claims 1-3 extinction coefficient k is 0.03 to 0.15 3. The optical recording medium according to claim 1.

The absorption maximum of the organic dye formed as a film on the disk substrate, which corresponds to the absorption maximum on the longest wavelength side among the absorptions in which the molar extinction coefficient ε in the solution is 50,000 or more, is recorded / reproduced. located 40~60nm shorter wavelength side than the wavelength, and, according to relative absorbance of the absorption maximum, any one of claims 1-4 absorbance is 5% to 15% in the recording and reproducing light wavelength Optical recording medium.

The optical recording medium according to any one of claims 1 to 5 , wherein the metal reflection layer contains silver as a main component.

The optical recording medium according to any one of claims 1 to 6 , wherein the protective layer is an ultraviolet curable resin.

The optical recording medium according to any one of claims 1 to 7 , which is irradiated with laser light having a wavelength of 600 to 700 nm so that the groove width of the substrate of the recording portion is 20 to 40% wider than the groove width of the unrecorded portion. An information recording method, comprising: