JP2004152464A

JP2004152464A - Optical recording medium

Info

Publication number: JP2004152464A
Application number: JP2003181610A
Authority: JP
Inventors: Kazunori Ito; 和典伊藤; Masato Harigai; 眞人針谷; Hiroko Tashiro; 浩子田代; Miki Mizutani; 未来水谷; Michiaki Shinozuka; 道明篠塚; Hiroyuki Iwasa; 博之岩佐
Original assignee: Ricoh Co Ltd
Current assignee: Ricoh Co Ltd
Priority date: 2002-09-04
Filing date: 2003-06-25
Publication date: 2004-05-27

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phase transition type optical recording medium with which a sufficient modulation can be assured even in a range of wide recording speeds from 1.0 times to ≥10 times in recording line speed with the same capacity as that of a DVD-ROM, preservation reliability is high and DOW (Direct Overwrite) characteristics are excellent. <P>SOLUTION: The optical recording medium is a rewritable optical recording medium which has at least a first protective layer, a recording layer, a second protective layer, and a reflection layer in this order on a transparent substrate and which gives rise to a reversible phase change in a recording medium material by irradiating the recording layer with a laser beam and performs recording and reproducing of information by utilizing the optical change thereof. The recording layer contains at least Ga and Sb, and the reflectivity Rg of an unrecorded area when the wavelength of the laser beam to be used is 630 to 700 nm satisfies the condition 12%≤Rg≤30%. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、光ビームを照射することにより記録層材料に光学的な変化を生じさせ、情報の記録、再生を行ない、かつ書換えが可能な相変化型光記録媒体に関するものである。
【０００２】
【従来技術】
特許文献１にはＧａとＳｂを含む記録層を有する相変化型光記録媒体が開示されている。
また、本発明者らは、特許文献２において、ＤＶＤ−ＲＯＭと同容量以上の高密度記録が可能で、更にこの２倍速以上（約７ｍ／ｓ以上）までをカバーできる、具体的には３．０ｍ／ｓから１４ｍ／ｓという高記録線速度までの範囲で記録可能な相変化型光記録媒体及びこの記録媒体にＣＡＶ記録或いは記録媒体の記録半径位置を複数に区切ってＣＬＶ記録を行う記録方法について開示した。
特許文献２において開示された、現在のＤＶＤ＋ＲＷで用いられている記録材料は、ＣＤに採用されているＡｇＩｎＳｂＴｅ系記録材料を改良し、高線速記録領域（記録線速度＝約８．５ｍ／ｓ）までの記録消去を可能にしたものである。
【０００３】
この材料系は、高線速記録領域の記録スピードに対応するためＳｂの含有量をＣＤ−ＲＷ対応の記録材料より多くしたものであるが、高Ｓｂ組成比の材料は結晶化スピードを促進するものの、結晶化温度が低下するという問題を有し、結晶化温度の低下は保存信頼性の悪化につながることが実験により確認されている。
ディスクの保存信頼性の問題は、記録材料中のＡｇの増量、或いはＧｅなどの第５元素の添加により、実用上問題にならない程度に抑えられているが、一層の高線速記録を達成するために、更にＳｂ量を増加すると、ついにはＳｂとその他の相に分相してしまい、記録層が相変化層として機能しなくなってしまう。
本発明者らは、このときの限界記録スピードは、ＤＶＤの記録密度において、２０ｍ／ｓ前後であると推定している。
【０００４】
【特許文献１】
特公平３−５５８９２号公報
【特許文献２】
特開２０００−３２２７４０号公報
【０００５】
【発明が解決しようとする課題】
本発明は、ＤＶＤ−ＲＯＭと同容量で記録線速が１．０倍速から１０倍速以上までの広い記録スピードの範囲においても、十分な変調度が確保され、かつ保存信頼性が高くＤＯＷ（ダイレクトオーバーライト）特性が良好な相変化型光記録媒体の提供を目的とする。
【０００６】
【課題を解決するための手段】
上記課題は、次の１）〜５）の発明（以下、本発明１〜５という）によって解決される。
１）透明基板上に、少なくとも、第１保護層、記録層、第２保護層、反射層をこの順に有し、該記録層にレーザ光を照射することによって、記録材料に可逆的な相変化を生じさせ、その光学的変化を利用して、情報の記録再生を行う書換え可能な光記録媒体において、該記録層は少なくともＧａとＳｂを含み、用いるレーザ光の波長が６３０〜７００ｎｍのときに未記録部の反射率Ｒｇが、１２％≦Ｒｇ≦３０％、という条件を満足することを特徴とする光記録媒体。
２）用いるレーザ光の波長が６３０〜７００ｎｍ、用いる光学系のレンズＮＡが０．６０以上、記録線速度（Ｖ）の範囲が、３ｍ／ｓ＜Ｖ≦４０ｍ／ｓである時に、記録マークの反射率Ｒｂと未記録部の反射率Ｒｇの変調度Ｍ〔但し、Ｍ＝（Ｒｇ−Ｒｂ）／Ｒｇ〕が０．４以上の条件を満足することを特徴とする１）記載の光記録媒体。
３）第１保護層、記録層、第２保護層、反射層の各膜厚は、用いるレーザ光の波長をλとして、次の条件を満足することを特徴とする１）又は２）記載の光記録媒体。
第１保護層の膜厚ｔ_１：０．０７０≦ｔ_１／λ≦０．１６
記録層の膜厚ｔ_２：０．０１５≦ｔ_２／λ≦０．０３２
第２保護層の膜厚ｔ_３：０．０１１≦ｔ_３／λ≦０．０４０
反射層の膜厚ｔ_４：０．１０≦ｔ_４／λ
４）記録層の材料の組成式を、Ｍ_γＧａ_αＳｂ_β（α、β、γは原子比、ＭはＧａ、Ｓｂ以外の元素又はそれらの混合物）として、０．０５≦α≦０．２、０．８≦β≦０．９５、０＜γ≦０．１、α＋β＋γ＝１であることを特徴とする１）〜３）の何れかに記載の光記録媒体。
５）Ｍが、Ｇｅ、又はＧｅと、Ｉｎ、Ａｌ、Ａｇ、Ｍｎ、Ｃｕ、Ａｕ、Ｎよりなる群から選ばれた少なくとも１種の元素の混合物である事を特徴とする４）記載の光記録媒体。
【０００７】
以下、上記本発明について詳しく説明する。
現在のＤＶＤ系の記録材料に関しては、２．５Ｘのスピード（記録線速度＝約８．５ｍ／ｓ）において記録ができるシステムが既に市販され、更に高速記録の要求が高まってきている。
そこで、ＤＶＤ−ＲＯＭと同容量以上の高密度記録が可能で、更にこの１０倍速以上（約３５ｍ／ｓ以上）までをカバーできる相変化型光記録媒体に関して、本発明者らが記録材料の開発を行った結果、ＧａとＳｂの共晶組成近傍の組成が好適であることが分った。
この材料系は、１９８０年代半ばにＣＤ−ＲＯＭと反射率互換性を有する書換え型光ディスクの開発が行われた際に提案されたものであると考えられるが、別にＡｇＩｎＳｂＴｅ系の低反射媒体が提案されたため、この材料系は実用化されなかった経緯がある。
【０００８】
ＧａＳｂ系材料の特徴として挙げられるのは、前述したＡｇＩｎＳｂＴｅ系材料の融点が５００〜６００℃程度であるのに対し、７００℃前後の比較的高い融点を持つことである。そして、この材料系を用いたＣＤ−ＲＯＭと反射率互換性を有する書換え型光ディスクの反射率の目標値は約７０％以上であったため、記録材料が吸収可能な光エネルギーが小さく、充分な変調度を得ることが困難であったことが予想される。
更にまた、ＣＤ系の記録システムは、ＬＤ（レーザーダイオード）光の波長が７８０ｎｍ、レンズＮＡは０．４５である。このため記録に用いる光ビームのスポットは、現在ＤＶＤに用いられているものより約１．７倍大きく、光エネルギー密度は低い。従って、上記反射率条件と相俟って、記録材料の相変化に必要な融点近傍の温度領域までの昇温が困難であったことが考えられる。
【０００９】
しかしながら鋭意検討の結果、本発明者らは、前述のようにＡｇＩｎＳｂＴｅ系の限界記録スピードが２０ｍ／ｓ前後であるのに対し、ＧａＳｂ系においては、レンズＮＡ＝０．６５前後のＤＶＤ系の記録システムを用い、更に用いるレーザ光の波長が６３０〜７００ｎｍのときのディスクの未記録部の反射率をＲｇとして、１２％≦Ｒｇ≦３０％、という条件を満足させれば、この２元材料のみでも約３５ｍ／ｓまでの記録スピードにおいて充分な変調度が得られることを実験により確認した。
このような条件を満足させるためには、例えば第２保護層と反射層の膜厚を本発明３で限定する光学膜厚条件を満足する範囲内とし、主に記録層と第１保護層の膜厚をコントロールすればよい。特に波長が決められた場合には記録層と保護層の膜厚はごく限られた範囲からの選択となるため、これらの層の膜厚を設定することは容易である。
【００１０】
例えば、上記Ｒｇの範囲内で反射率を高くしようとする場合には、記録層と第１保護層の膜厚を厚く設定することにより解決できる。しかし、本発明３で限定する膜厚条件を外れた記録層の膜厚を設定すると、第１保護層による反射率制御が上記Ｒｇの範囲内では難しくなってしまうため、記録層の膜厚は、本発明３で限定する範囲が好ましい。なお、記録膜の膜厚を本発明３で限定する範囲に設定すると、第１保護層の膜厚は、本発明３で限定する第１の膜厚範囲以外に、それよりも膜厚が大きい第２の膜厚範囲、第３の膜厚範囲が存在するようになる。
しかし、第２、第３の膜厚範囲は、第１の場合よりも膜厚が厚く、１枚当りの製造時間が長くなるため、光ディスクの製造の観点からみると、第１の膜厚範囲が低コストのディスクを実現するには有利である。従って、本発明３で限定する膜厚範囲が好ましい。
【００１１】
反射率は、ディスク内の光エネルギーの吸収率を変化させるので、ディスクの記録特性を左右する重要な特性であり、その値によっては次のような現象を生ずる。即ち、Ｒｇが３０％を越える場合には、記録エネルギーが不足し、十分な変調度が得られず、１２％未満の場合には、記録システム上十分な信号強度が得られない。
第２保護層と反射層は上述したディスク内で吸収した光エネルギー（吸収の主体は記録層材料である）を効率よく反射層に伝播し、放熱させる役目があるため、第２保護層はあまり厚くしない方が好ましく、本発明３で限定する膜厚範囲が好ましい。これより厚い場合には、記録層に熱がこもって記録マークがぼやけてしまい、記録特性、特にジッタ特性が悪くなってしまう。また、これより薄いと、記録層で吸収した光エネルギーが蓄積されて記録層を溶融し記録マークを作るという相変化記録の原理を発揮できる熱量となる前に反射層に放熱されてしまうため、十分な記録特性が得られなくなってしまうという不具合を生ずる。
【００１２】
また、記録システムに用いる波長により光ビームのパワー密度が変わるため、第２保護層の膜厚を変える必要があるが、膜厚を前記光学膜厚の条件を満足する範囲内とすることで解決できる。このことは、他の層の膜厚条件にも当てはまる事項である。
ここでジッタ特性は、チャネル周期Ｔｗに対するマークエッジのばらつきσ／Ｔｗで評価したものである。
反射層は上述したようにディスク内で吸収した光エネルギー（吸収の主体は記録層材料である）を放熱させる役割と光ディスクに入射した光を反射させる役割とを持つが、ある膜厚条件以上であればどちらの目的も達成される。
本発明者らの実験からは、反射層の膜厚は、６０ｎｍ程度以上であれば良い結果が得られた。厚い方の限界値は特に存在せず、ディスクの製造コストの観点から許容される範囲内の膜厚にすれば良いが、約３００ｎｍ以下が妥当であると考える。
【００１３】
記録層材料については、ＧａとＳｂの混合物であって、ＧａやＳｂの組成比が、本発明４に規定する条件を満足する場合には、繰り返しオーバーライトしてもジッタ特性が良く、保存安定性において問題のないディスクが得られるが、相変化媒体の記録再生システムを想定した場合、変調度が不足気味となる場合がある。
例えば、現在、記録型のＤＶＤにおいて用いられているような、ＮＡ０．６５、波長６６０ｎｍの評価システムで記録テストを行った際の、最大マーク長の記録マークの信号レベルＩ_Ｌと最大スペース長のスペース部の信号レベルＩ_Ｈの差から得られる変調度を
変調度Ｍ＝（Ｉ_Ｈ−Ｉ_Ｌ）／Ｉ_Ｈ
とすると、現在の光ディスクシステムの仕様では変調度が約０．５以上とされているのに対し、前述した各層の膜厚のうち、特に第１保護層の膜厚が大きい場合には変調度が０．４未満となってしまう場合がある。
【００１４】
このような場合には、第１保護層の膜厚範囲を変調度が０．４以上となるように設定し、第１保護層と記録層の膜厚をコントロールすることによって問題が解決できる。しかし、記録層に対し、Ｇｅ単体、或いは、Ｇｅと、Ｉｎ、Ａｌ、Ａｇ、Ｍｎ、Ｃｕ、Ａｕ、Ｎよりなる群から選ばれた少なくとも１種の元素の混合物を添加することによってもこの問題を解決できる。
この添加元素の組成比は、本発明４で限定する範囲であれば十分である。
例えば、本発明者等は、ＧａＳｂに対し、ＩｎやＧｅを原子比で３％程度添加することによって、変調度を約１０％程度向上できることを確認した。本発明５に示す元素も同様な効果を発揮する。更に、これらの元素の添加によりＧａＳｂのみの記録層の光ディスクより一層保存安定性が向上することや、記録に要する記録パワーが少なくて済むなど、記録システム上扱い易い記録媒体が得られることも確認した。各元素の添加は、記録システムの要求により、最適化すれば良い。
【００１５】
図１は本発明に係る光記録媒体（ディスク）の基本的な構成を示す概略図であり、図１（ａ）は、一部を切り欠いた状態の斜視図、図１（ｂ）は図１（ａ）の切り欠き部の概略断面図である。
透明基板の材料としては、ポリカーボネートの他に、ポリメタクリレート、ポリエチレンテレフタレート、或いはこれらの骨格を化学修飾した樹脂などを用いることができる。
また、本発明１の第１保護層（誘電体層）、記録層、第２保護層（誘電体層）、反射層の各層は、単層でもよいが、多層構成としても良い。
これらの層は、各種気相成長法、例えば真空蒸着法、スパッタリング法、電子ビーム蒸着法等により形成できる。
更に、オーバーコート層上に、必要に応じて上記透明基板と同じ材料からなる第２透明基板を貼り合わせてもよい。
【００１６】
【実施例】
以下、実施例及び比較例により本発明を更に具体的に説明するが、本発明はこれらの実施例により何ら限定されるものではなく、製造条件も適宜変更可能である。なお、各実施例及び比較例の光記録媒体の記録特性を表１に纏めて示す。
【００１７】
実施例１
ピッチサイズ０．７４μｍで、蛇行量が３５ｎｍ、グルーブ幅０．２５μｍの蛇行溝を有する厚さ０．６ｍｍのポリカーボネート基板上に、ＳｉＯ_２２０モル％のＺｎＳ・ＳｉＯ_２混合ターゲットを用いた厚さ７０、７３、７５ｎｍの第１誘電体層（第１保護層）、Ｇａ_０．１Ｓｂ_０．９からなる厚さ１７ｎｍの記録層、第１誘電体層と同じ材料を用いた厚さ１２ｎｍの第２誘電体層、ＳｉＣターゲットを用いた厚さ５ｎｍの第３誘電体層、Ａｇターゲットを用いた厚さ１２０ｎｍの反射層を、この順にスパッタ法で成膜し、反射層の上に紫外線硬化樹脂層（オーバーコート層）をスピンコーティングした後、厚さ０．６ｍｍの第２ポリカーボネート基板を紫外線硬化型の樹脂により貼り合わせ、次いで、１３０Ｗ・ｓｅｃ／ディスクのパワー条件にてレーザダイオード光でアニール（初期化）し、スパッタ直後にアモルファスであった記録材料を結晶化して光記録媒体を作成した（なお、本実施例では第２保護層を第２誘電体層と第３誘電体層の２層構成とした）。
スパッタ装置による各層の製膜条件は次の通りである。

上記のようにして得られた記録媒体に対して、書き込みレーザ光波長６５０〜６６５ｎｍ、レンズＮＡ０．６５の記録システムを用いてＤＶＤと同等の密度になるように記録を行ったところ、記録可能な線速範囲は６ｍ／ｓ〜４０ｍ／ｓという広い範囲であった。
なお、未記録部分の反射率Ｒｇと記録マークの反射率Ｒｂの変調度Ｍ〔Ｍ＝（Ｒｇ−Ｒｂ）／Ｒｇ〕が０．４以上の条件を満足する場合に記録可能であると判断した。
【００１８】
実施例２
記録層材料をＧｅ_０．０３（Ｇａ_０．１Ｓｂ_０．９）_０．９７に変えた点以外は、実施例１と同様にして記録媒体を作成した。
この記録媒体に対し、実施例１と同じシステムを用いて記録を行ったところ、記録可能な線速範囲は４ｍ／ｓ〜３７ｍ／ｓという広い範囲であった。
更に、実施例１の媒体と一緒に高温高湿（８０℃、８５％）下の保存信頼性試験を行った結果、記録マークのジッタ特性の上昇が、実施例１の媒体では３００時間経過後に増加率５％であったのに対し、実施例２の媒体では増加率１％以下であり、Ｇｅの添加は保存信頼性向上に効果があることが分った。
Ｇｅの添加量は、原子比で０．１を超えると効果がなく、更に添加量の増加に伴い記録可能な線速範囲が低くなってしまうことも同時に確認された。
目標とする線速範囲にもよるが、Ｇｅの添加量は原子比で０．１以下、好ましくは０．０７以下が良い。
【００１９】
実施例３
記録層材料をＧｅ_０．０３Ｉｎ_０．０５（Ｇａ_０．１Ｓｂ_０．９）_０．９２に変えた点以外は、実施例１と同様にして記録媒体を作成した。
この記録媒体に対し、実施例１と同じシステムを用いて記録を行ったところ、記録可能な線速範囲は６ｍ／ｓ〜４３ｍ／ｓという広い範囲であった。
実施例２の媒体よりも記録可能な線速範囲が広くなり、Ｉｎの添加は記録線速向上に効果があることが確認された。また、Ｉｎの代りにＭｎやＡｌを添加しても同等の効果が得られた。これらの元素の添加量は、目的とする線速にもよるが、あまり多く添加すると、連続再生の際にマークの結晶化がパワーの低い再生光でも起ってしまうため、原子比で０．１５以下、好ましくは０．１以下が良い。
【００２０】
実施例４
記録層材料をＧｅ_０．０３Ａｇ_０．０２（Ｇａ_０．１Ｓｂ_０．９）_０．９５に変えた点以外は、実施例１と同様にして記録媒体を作成した。
この記録媒体に対し、実施例１と同じシステムを用いて記録を行ったところ、記録可能な線速範囲は４ｍ／ｓ〜３５ｍ／ｓであったが、実施例２の媒体よりも記録用レーザ光のパワーが約１０％小さい場合でも同等の変調度を得ることが可能となった。また、Ａｇの代りにＣｕ、Ａｕ又はＮを添加しても同等の効果が得られた。しかしこれらの元素の添加量は、目的とする線速にもよるが、あまり多く添加すると記録可能な線速範囲が狭くなってしまうため、原子比で０．０５以下、好ましくは０．０３以下が良い。
【００２１】
上記実施例１〜４と同じ層構成の光記録媒体について、基準クロックに対する記録マークエッジのばらつきを表すジッタ特性の記録線速依存性を評価した。
結果を図２に示す。図中の各ポイントの対象となる媒体は、実施例毎に反射率Ｒｇがなるべく同じになるように第１誘電体層の膜厚条件を変更して作成した。
記録可能か否かの判断ポイントの閾値をジッタ１０％とすると、３ｍ／ｓ＜Ｖ≦４０ｍ／ｓの線速範囲で記録可能であることが分る。
【００２２】
実施例５、比較例１
第１誘電体層の膜厚を１０５ｎｍに変えて反射率を上昇させた点以外は、実施例１と同様にして本実施例の光記録媒体を作成した。また、第１誘電体層の膜厚を１２０ｎｍとした点以外は、実施例１と同様にして比較例１の光記録媒体を作成した。
これらの光記録媒体に対し、記録線速３５ｍ／ｓ、記録波長６６０ｎｍで記録を行い評価した結果、本実施例の媒体の反射率Ｒｇは２７％台であった。更に、変調度Ｍが０．４以上という本発明２の条件を満たしていることが分った。この場合のジッタは閾値とした１０％以内であり、記録可能な範囲であった。
一方、比較例１の光記録媒体では、反射率Ｒｇが３０％を超え、更に変調度Ｍは０．４未満となってしまった。
これらの結果を図３に示す。図３は反射率Ｒｇと変調度Ｍに関する第１誘電体層の膜厚依存性を評価した結果を示したものである。
図から分るように、第１誘電体層の膜厚が１０８ｎｍ前後で本発明１のＲｇの上限値３０％を超え、本発明２のＭが０．４以上という条件を満足できなくなり、記録特性が悪化することが分る。更に、反射率Ｒｇや変調度Ｍには本発明３の第１保護層（誘電体層）の膜厚条件が大きく関係することが分る。
記録波長を６６０ｎｍとした場合の、本発明３の第１保護層の膜厚範囲の上限値は約１０６ｎｍであるから、これらの結果と整合している。
【００２３】
実施例６、比較例２
記録層の膜厚を１２〜２０ｎｍの間で変化させ、第１誘電体層の膜厚を６０ｎｍに変えた点以外は、実施例１と同様にして光記録媒体を作成し、記録波長６６０ｎｍでのＲｇを評価した。結果を図３に示すが、第１保護層の膜厚が約６０ｎｍのときに反射率Ｒｇは極小値を持ち、このときの反射率は１６％台であった。また、これらの条件では変調度Ｍは大きい値となり、ジッタ特性も良好であった。
一方、記録層の膜厚を８ｎｍとし、第１誘電体層の膜厚を６０ｎｍに変更した点以外は、実施例１と同様にして比較例２の光記録媒体を作成し、同様の評価を行った。この媒体は記録層膜厚を薄くしたことにより更に反射率Ｒｇが低下した。この様子を図４に示す。図から分るように、Ｒｇを１２％以上とするには記録層膜厚を約１０ｎｍ以上とする必要があるが、これは本発明３の記録層膜厚の下限条件とほぼ一致する。
また、図５に示すように、記録層膜厚が薄くなるに従い、ジッタ特性が大きくなり充分な記録特性が得られない。
一方、現状のＤＶＤなどの記録システムにおいては、記録パワーの点で記録層膜厚が２５ｎｍ程度以下までの条件でしか記録が出来ないため、本発明３で限定する記録層膜厚が好ましい上限値となる。
【００２４】
【表１】

【００２５】
【発明の効果】
本発明１〜４によれば、ＤＶＤの１Ｘから１０Ｘ以上の広い範囲の記録スピードに対応する光記録媒体を実現できる。その結果、ユーザーがコンテンツの入力を行う際に短時間の記録が可能となり、またシステムによってはリアルタイムに再生しながら記録を行うことが可能となり、記録システムの利便性が向上する。
本発明５によれば、更に記録マークの変調度と保存安定性が向上し、記録に要する記録パワーが少なくて済む光記録媒体を実現できる。
【図面の簡単な説明】
【図１】本発明に係る光記録媒体（ディスク）の基本的な構成を示す概略図。
（ａ）一部を切り欠いた状態の斜視図
（ｂ）（ａ）の切り欠き部の概略断面図
【図２】実施例１〜４の光ディスクの記録可能な線速範囲を示す図。
【図３】第１誘電体層膜厚と反射率Ｒｇ及び変調度Ｍの関係を示す図。
【図４】記録層膜厚と反射率Ｒｇの関係を示す図。
【図５】記録層膜厚とジッタの関係を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a phase-change optical recording medium in which a recording layer material is optically changed by irradiating a light beam to record and reproduce information, and is rewritable.
[0002]
[Prior art]
Patent Document 1 discloses a phase-change optical recording medium having a recording layer containing Ga and Sb.
In addition, the present inventors have disclosed in Patent Document 2 that high-density recording with the same capacity or more as that of a DVD-ROM is possible, and can cover up to 2 × speed (about 7 m / s or more). Phase change type optical recording medium capable of recording in a range from 0.0 m / s to a high recording linear velocity of 14 m / s, and recording for performing CAV recording or CLV recording by dividing a recording radius position of the recording medium into a plurality of recording media on this recording medium A method was disclosed.
The recording material used in the current DVD + RW disclosed in Patent Literature 2 is an improvement over the AgInSbTe-based recording material used for CDs, and has a high linear velocity recording area (recording linear velocity = about 8.5 m / s). ) Can be erased.
[0003]
In this material system, the content of Sb is made larger than that of a recording material compatible with CD-RW in order to correspond to the recording speed of a high linear velocity recording region, but a material having a high Sb composition ratio promotes the crystallization speed. However, there is a problem that the crystallization temperature is lowered, and it has been experimentally confirmed that a reduction in the crystallization temperature leads to a deterioration in storage reliability.
The problem of the storage reliability of the disc is suppressed to a level that does not cause a practical problem by increasing the amount of Ag in the recording material or adding a fifth element such as Ge, but achieves higher linear velocity recording. Therefore, if the amount of Sb is further increased, the phase will eventually be separated into Sb and other phases, and the recording layer will not function as a phase change layer.
The present inventors estimate that the critical recording speed at this time is about 20 m / s in the recording density of DVD.
[0004]
[Patent Document 1]
Japanese Patent Publication No. 3-55892 [Patent Document 2]
JP 2000-322740 A
[Problems to be solved by the invention]
The present invention provides a DOW (direct recording) with a sufficient degree of modulation, high storage reliability and a high storage reliability even in a wide recording speed range from 1.0 × to 10 × or higher recording speed with the same capacity as a DVD-ROM. Overwrite) An object of the present invention is to provide a phase-change optical recording medium having good characteristics.
[0006]
[Means for Solving the Problems]
The above object is achieved by the following inventions 1) to 5) (hereinafter, referred to as inventions 1 to 5).
1) At least a first protective layer, a recording layer, a second protective layer, and a reflective layer are provided in this order on a transparent substrate, and the recording material is irradiated with a laser beam so that the recording material undergoes a reversible phase change. In a rewritable optical recording medium that records and reproduces information by utilizing the optical change, the recording layer contains at least Ga and Sb, and the wavelength of the laser beam used is 630 to 700 nm. An optical recording medium, wherein the reflectance Rg of an unrecorded portion satisfies a condition of 12% ≦ Rg ≦ 30%.
2) When the wavelength of the laser beam used is 630 to 700 nm, the lens NA of the optical system used is 0.60 or more, and the range of the recording linear velocity (V) is 3 m / s <V ≦ 40 m / s, the recording mark (1) The optical recording medium according to (1), wherein the degree of modulation M between the reflectance Rb and the reflectance Rg of the unrecorded portion satisfies a condition of 0.4 or more, where M = (Rg-Rb) / Rg. .
3) The film thickness of each of the first protective layer, the recording layer, the second protective layer, and the reflective layer satisfies the following condition, where λ is the wavelength of a laser beam to be used. Optical recording medium.
Film thickness t ₁ of first protective layer: 0.070 ≦ t ₁ /λ≦0.16
Film thickness t _{2 of} recording layer: 0.015 ≦ t ₂ /λ≦0.032
Film thickness t ₃ of second protective layer: 0.011 ≦ t ₃ /λ≦0.040
Reflective layer thickness t ₄ : 0.10 ≦ t ₄ / λ
4) the composition formula of the material of the recording _{_{_{layer, M γ Ga α Sb β (}}} α, β, γ are atomic ratio, M as the Ga, elements or mixtures thereof other than Sb), 0.05 ≦ α ≦ 0 . 2. The optical recording medium according to any one of 1) to 3), wherein 0.8 ≦ β ≦ 0.95, 0 <γ ≦ 0.1, and α + β + γ = 1.
5) The light according to 4), wherein M is Ge or a mixture of Ge and at least one element selected from the group consisting of In, Al, Ag, Mn, Cu, Au, and N. recoding media.
[0007]
Hereinafter, the present invention will be described in detail.
With respect to current DVD-based recording materials, systems capable of recording at a speed of 2.5X (recording linear velocity = about 8.5 m / s) have already been marketed, and the demand for high-speed recording has been increasing.
Accordingly, the present inventors have developed a recording material for a phase-change type optical recording medium capable of recording at a high density equal to or higher than that of a DVD-ROM and capable of covering up to 10 × speed (about 35 m / s or more). As a result, it was found that a composition near the eutectic composition of Ga and Sb was preferable.
This material system is considered to have been proposed when a rewritable optical disk having reflectivity compatibility with a CD-ROM was developed in the mid-1980s, but a low reflective medium of AgInSbTe system was proposed separately. As a result, this material system has not been put to practical use.
[0008]
A feature of the GaSb-based material is that the AgInSbTe-based material described above has a relatively high melting point of about 700 ° C., whereas the melting point is about 500 to 600 ° C. Since the target value of the reflectivity of the rewritable optical disk having the reflectivity compatibility with the CD-ROM using this material is about 70% or more, the light energy that can be absorbed by the recording material is small, and sufficient modulation is performed. It is expected that it was difficult to obtain the degree.
Furthermore, in a CD-based recording system, the wavelength of LD (laser diode) light is 780 nm, and the lens NA is 0.45. Therefore, the spot of the light beam used for recording is about 1.7 times larger than that currently used for DVD, and the light energy density is low. Therefore, it is considered that it was difficult to raise the temperature to a temperature region near the melting point necessary for the phase change of the recording material, in combination with the above-mentioned reflectance condition.
[0009]
However, as a result of diligent studies, the present inventors have found that the limit recording speed of the AgInSbTe system is about 20 m / s as described above, whereas the GaSb system has the DVD-based lens with a lens NA of about 0.65. If the system satisfies the condition of 12% ≦ Rg ≦ 30%, where Rg is the reflectance of the unrecorded portion of the disk when the wavelength of the laser beam to be used is 630 to 700 nm, only this binary material is used. However, it was confirmed by experiments that a sufficient degree of modulation could be obtained at a recording speed up to about 35 m / s.
In order to satisfy such conditions, for example, the thicknesses of the second protective layer and the reflective layer are set to be within a range that satisfies the optical film thickness condition defined in the third aspect of the present invention, and the recording layer and the first protective layer are mainly What is necessary is just to control a film thickness. In particular, when the wavelength is determined, the thicknesses of the recording layer and the protective layer are selected from a very limited range, so that it is easy to set the thicknesses of these layers.
[0010]
For example, when the reflectance is to be increased within the range of Rg, it can be solved by setting the thicknesses of the recording layer and the first protective layer to be large. However, if the film thickness of the recording layer deviates from the film thickness condition defined in the third aspect of the present invention, it becomes difficult to control the reflectance by the first protective layer within the above range of Rg. The range limited by the present invention 3 is preferable. When the thickness of the recording film is set within the range defined by the third aspect of the invention, the thickness of the first protective layer is larger than the first thickness range defined by the third aspect of the invention. A second thickness range and a third thickness range exist.
However, the second and third film thickness ranges are thicker than in the first case and require a longer manufacturing time per sheet. Therefore, from the viewpoint of optical disk manufacturing, the first and second film thickness ranges are different. However, it is advantageous to realize a low-cost disk. Therefore, the thickness range limited by the third aspect of the present invention is preferable.
[0011]
The reflectance changes the absorptivity of light energy in the disk, and is an important characteristic that affects the recording characteristics of the disk. Depending on the value, the following phenomenon occurs. That is, when Rg exceeds 30%, the recording energy is insufficient and a sufficient degree of modulation cannot be obtained. When Rg is less than 12%, a sufficient signal intensity cannot be obtained in the recording system.
Since the second protective layer and the reflective layer have a function of efficiently transmitting the light energy absorbed in the above-described disk (the main component of the absorption is the recording layer material) to the reflective layer and dissipating the heat, the second protective layer is not so much used. It is preferable not to increase the thickness, and the thickness range limited in the present invention 3 is preferable. If the thickness is larger than this, the recording marks are blurred due to the heat stored in the recording layer, and the recording characteristics, particularly the jitter characteristics, deteriorate. On the other hand, if the thickness is smaller than this, the light energy absorbed by the recording layer is accumulated and is radiated to the reflective layer before the amount of heat that can demonstrate the principle of phase change recording that melts the recording layer and creates a recording mark, This causes a problem that sufficient recording characteristics cannot be obtained.
[0012]
In addition, since the power density of the light beam changes depending on the wavelength used in the recording system, it is necessary to change the thickness of the second protective layer. it can. This is a matter that also applies to the thickness conditions of other layers.
Here, the jitter characteristic is obtained by evaluating the mark edge variation σ / Tw with respect to the channel period Tw.
As described above, the reflective layer has a role of dissipating the light energy absorbed in the disc (the main component of absorption is the recording layer material) and a role of reflecting the light incident on the optical disc. With that, both objectives are achieved.
From the experiments by the present inventors, good results were obtained when the thickness of the reflective layer was about 60 nm or more. There is no particular limit value for the thicker one, and the film thickness may be set within a range allowable from the viewpoint of the manufacturing cost of the disk. However, it is considered that about 300 nm or less is appropriate.
[0013]
The recording layer material is a mixture of Ga and Sb, and when the composition ratio of Ga and Sb satisfies the condition defined in the fourth aspect of the present invention, the jitter characteristics are good even after repeated overwriting, and the storage stability is good. Although a disk having no problem in performance can be obtained, the degree of modulation may be insufficient when a recording / reproducing system for a phase change medium is assumed.
For example, currently, as used in DVD recordable, NA 0.65, when performing the recording tests in the evaluation system of wavelength 660 nm, the signal level I _L and the maximum space length of the recording marks of the maximum mark length space portion of the signal level _I modulation factor obtained from the difference between the modulation of the _{_{_{H M = (I H -I L}}} ) / I H
Then, according to the specifications of the current optical disk system, the modulation degree is about 0.5 or more, but the modulation degree is particularly large when the thickness of the first protective layer is large among the above-mentioned film thicknesses of the respective layers. May be less than 0.4.
[0014]
In such a case, the problem can be solved by setting the thickness range of the first protective layer so that the degree of modulation is 0.4 or more and controlling the thicknesses of the first protective layer and the recording layer. However, this problem is also caused by adding Ge alone or a mixture of Ge and at least one element selected from the group consisting of In, Al, Ag, Mn, Cu, Au and N to the recording layer. Can be solved.
It is sufficient that the composition ratio of the additional element is within the range defined in the fourth aspect of the invention.
For example, the present inventors have confirmed that the degree of modulation can be improved by about 10% by adding about 3% by atomic ratio of In or Ge to GaSb. The element shown in the fifth aspect of the present invention also exerts a similar effect. Furthermore, it has been confirmed that the addition of these elements further improves the storage stability compared to an optical disk having a recording layer containing only GaSb, and that a recording medium that is easy to handle in a recording system, such as requiring less recording power for recording, can be obtained. did. The addition of each element may be optimized according to the requirements of the recording system.
[0015]
FIG. 1 is a schematic diagram showing a basic configuration of an optical recording medium (disk) according to the present invention. FIG. 1 (a) is a perspective view of a partially cut-out state, and FIG. It is a schematic sectional drawing of the notch part of 1 (a).
As a material for the transparent substrate, besides polycarbonate, polymethacrylate, polyethylene terephthalate, or a resin in which these skeletons are chemically modified can be used.
Each of the first protective layer (dielectric layer), the recording layer, the second protective layer (dielectric layer), and the reflective layer according to the first embodiment of the present invention may be a single layer, or may have a multilayer structure.
These layers can be formed by various vapor deposition methods, for example, a vacuum evaporation method, a sputtering method, an electron beam evaporation method, or the like.
Further, a second transparent substrate made of the same material as the transparent substrate may be bonded on the overcoat layer, if necessary.
[0016]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. However, the present invention is not limited to these Examples, and the manufacturing conditions can be appropriately changed. Table 1 summarizes the recording characteristics of the optical recording media of Examples and Comparative Examples.
[0017]
Example 1
A thickness using a ZnS / SiO ₂ mixed target of 20 mol% of SiO ₂ on a 0.6 mm thick polycarbonate substrate having a pitch size of 0.74 μm, a meandering amount of 35 nm, and a groove width of 0.25 μm and a meandering groove. 70, 73, 75 nm first dielectric layer (first protective layer), Ga _0.1 Sb _0.9, 17 nm thick recording layer, 12 nm thick, using the same material as the first dielectric layer A second dielectric layer, a third dielectric layer with a thickness of 5 nm using a SiC target, and a reflective layer with a thickness of 120 nm using an Ag target are formed in this order by a sputtering method, and ultraviolet curing is performed on the reflective layer. After spin coating the resin layer (overcoat layer), a second polycarbonate substrate having a thickness of 0.6 mm is bonded with an ultraviolet curing resin, and then a 130 W · sec / disk An optical recording medium was produced by annealing (initializing) with laser diode light under power conditions and crystallizing an amorphous recording material immediately after sputtering (in this embodiment, the second protective layer is a second dielectric material). Layer and a third dielectric layer).
The film forming conditions of each layer by the sputtering device are as follows.

On the recording medium obtained as described above, recording was performed using a recording system having a writing laser light wavelength of 650 to 665 nm and a lens NA of 0.65 so as to have a density equivalent to that of a DVD. The linear velocity range was as wide as 6 m / s to 40 m / s.
It is determined that recording is possible when the degree of modulation M [M = (Rg−Rb) / Rg] of the reflectance Rg of the unrecorded portion and the reflectance Rb of the recording mark satisfies the condition of 0.4 or more. .
[0018]
Example 2
A recording medium was prepared in the same manner as in Example 1, except that the material of the recording layer was changed to Ge _0.03 (Ga _0.1 Sb _0.9 ) _0.97 .
When recording was performed on this recording medium using the same system as in Example 1, the recordable linear velocity range was as wide as 4 m / s to 37 m / s.
Further, as a result of performing a storage reliability test under high temperature and high humidity (80 ° C., 85%) together with the medium of Example 1, the jitter characteristic of the recording mark was increased after 300 hours in the medium of Example 1. While the increase rate was 5%, the increase rate of the medium of Example 2 was 1% or less, indicating that the addition of Ge was effective in improving the storage reliability.
It was also confirmed that the addition amount of Ge had no effect if the atomic ratio exceeded 0.1, and that the linear velocity range in which recording could be performed was reduced as the addition amount increased.
Although depending on the target linear velocity range, the amount of Ge added is preferably 0.1 or less, preferably 0.07 or less in atomic ratio.
[0019]
Example 3
A recording medium was prepared in the same manner as in Example 1, except that the material of the recording layer was changed to Ge _0.03 In _0.05 (Ga _0.1 Sb _0.9 ) _0.92 .
When recording was performed on this recording medium using the same system as in Example 1, the recordable linear velocity range was as wide as 6 m / s to 43 m / s.
The recordable linear velocity range was wider than that of the medium of Example 2, and it was confirmed that the addition of In was effective in improving the recording linear velocity. The same effect was obtained by adding Mn or Al instead of In. The addition amount of these elements depends on the target linear velocity, but if added too much, crystallization of the mark will occur even with low-power reproduction light during continuous reproduction. It is 15 or less, preferably 0.1 or less.
[0020]
Example 4
A recording medium was prepared in the same manner as in Example 1, except that the material of the recording layer was changed to Ge _0.03 Ag _0.02 (Ga _0.1 Sb _0.9 ) _0.95 .
When recording was performed on this recording medium using the same system as in Example 1, the linear velocity range in which recording was possible was 4 m / s to 35 m / s. Even when the light power is about 10% smaller, the same degree of modulation can be obtained. The same effect was obtained by adding Cu, Au or N instead of Ag. However, the addition amount of these elements depends on the target linear velocity, but if it is added too much, the range of recordable linear velocity becomes narrow, so that the atomic ratio is 0.05 or less, preferably 0.03 or less. Is good.
[0021]
With respect to the optical recording medium having the same layer configuration as in Examples 1 to 4, the dependency of the jitter characteristic indicating the variation of the recording mark edge with respect to the reference clock on the recording linear velocity was evaluated.
FIG. 2 shows the results. The target medium at each point in the drawing was prepared by changing the film thickness condition of the first dielectric layer so that the reflectance Rg became the same as much as possible for each example.
Assuming that the threshold value of the determination point for determining whether or not recording is possible is 10% jitter, it can be seen that recording is possible in the linear velocity range of 3 m / s <V ≦ 40 m / s.
[0022]
Example 5, Comparative Example 1
An optical recording medium of this example was produced in the same manner as in Example 1 except that the reflectance was increased by changing the thickness of the first dielectric layer to 105 nm. An optical recording medium of Comparative Example 1 was prepared in the same manner as in Example 1, except that the thickness of the first dielectric layer was changed to 120 nm.
As a result of recording on these optical recording media at a recording linear velocity of 35 m / s and a recording wavelength of 660 nm, the reflectance Rg of the medium of this example was on the order of 27%. Further, it was found that the condition of the present invention 2 that the modulation factor M was 0.4 or more was satisfied. The jitter in this case was within 10% of the threshold value, and was within the recordable range.
On the other hand, in the optical recording medium of Comparative Example 1, the reflectance Rg exceeded 30%, and the degree of modulation M was less than 0.4.
These results are shown in FIG. FIG. 3 shows the result of evaluating the dependency of the reflectance Rg and the degree of modulation M on the thickness of the first dielectric layer.
As can be seen from the figure, when the thickness of the first dielectric layer is about 108 nm, the upper limit of Rg of the present invention 1 exceeds 30%, and the condition of M of the present invention 2 that M is 0.4 or more cannot be satisfied. It can be seen that the characteristics deteriorate. Further, it can be seen that the film thickness condition of the first protective layer (dielectric layer) according to the third aspect of the present invention is greatly related to the reflectance Rg and the modulation factor M.
When the recording wavelength is 660 nm, the upper limit of the thickness range of the first protective layer of the third invention is about 106 nm, which is consistent with these results.
[0023]
Example 6, Comparative Example 2
An optical recording medium was prepared in the same manner as in Example 1, except that the thickness of the recording layer was changed between 12 and 20 nm, and the thickness of the first dielectric layer was changed to 60 nm. Was evaluated for Rg. The results are shown in FIG. 3. When the thickness of the first protective layer was about 60 nm, the reflectance Rg had a minimum value, and the reflectance at this time was on the order of 16%. Further, under these conditions, the modulation factor M was a large value, and the jitter characteristics were good.
On the other hand, an optical recording medium of Comparative Example 2 was prepared in the same manner as in Example 1 except that the thickness of the recording layer was changed to 8 nm and the thickness of the first dielectric layer was changed to 60 nm. went. In this medium, the reflectance Rg was further reduced by reducing the thickness of the recording layer. This is shown in FIG. As can be seen from the figure, the recording layer thickness needs to be about 10 nm or more in order to make Rg 12% or more, which almost coincides with the lower limit condition of the recording layer thickness of the third embodiment.
Further, as shown in FIG. 5, as the thickness of the recording layer becomes thinner, the jitter characteristic becomes larger and sufficient recording characteristics cannot be obtained.
On the other hand, in the current recording system such as a DVD, recording can be performed only under the condition that the recording layer film thickness is about 25 nm or less in terms of recording power. It becomes.
[0024]
[Table 1]

[0025]
【The invention's effect】
According to the first to fourth aspects of the invention, it is possible to realize an optical recording medium that supports a wide range of recording speeds of DVD from 1X to 10X or more. As a result, it is possible to record for a short time when the user inputs the content, and it is possible to perform recording while reproducing in real time depending on the system, thereby improving the convenience of the recording system.
According to the fifth aspect of the invention, it is possible to realize an optical recording medium in which the modulation degree of a recording mark and the storage stability are further improved, and the recording power required for recording is small.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a basic configuration of an optical recording medium (disk) according to the present invention.
FIG. 2A is a perspective view of a partially cut-out state. FIG. 2B is a schematic cross-sectional view of a cut-away portion of FIG. 2A.
FIG. 3 is a diagram showing the relationship between the thickness of a first dielectric layer, the reflectance Rg, and the degree of modulation M.
FIG. 4 is a diagram showing a relationship between a recording layer thickness and a reflectance Rg.
FIG. 5 is a diagram showing a relationship between recording layer thickness and jitter.

Claims

On a transparent substrate, at least a first protective layer, a recording layer, a second protective layer, and a reflective layer are provided in this order, and by irradiating the recording layer with laser light, a reversible phase change occurs in the recording material. In a rewritable optical recording medium for recording and reproducing information by utilizing the optical change, the recording layer contains at least Ga and Sb, and unrecorded when the wavelength of the laser beam used is 630 to 700 nm. An optical recording medium, wherein the reflectance Rg of the portion satisfies the condition of 12% ≦ Rg ≦ 30%.

When the wavelength of the laser beam used is 630 to 700 nm, the lens NA of the optical system used is 0.60 or more, and the range of the recording linear velocity (V) is 3 m / s <V ≦ 40 m / s, the reflectance of the recording mark 2. The optical recording medium according to claim 1, wherein a modulation degree M of Rb and a reflectance Rg of an unrecorded portion (where M = (Rg-Rb) / Rg) satisfies a condition of 0.4 or more.

3. The optical recording according to claim 1, wherein the thickness of each of the first protective layer, the recording layer, the second protective layer, and the reflective layer satisfies the following condition, where λ is the wavelength of the laser beam to be used. Medium.
Film thickness t ₁ of first protective layer: 0.070 ≦ t ₁ /λ≦0.16
Film thickness t _{2 of} recording layer: 0.015 ≦ t ₂ /λ≦0.032
Film thickness t ₃ of second protective layer: 0.011 ≦ t ₃ /λ≦0.040
Reflective layer thickness t ₄ : 0.10 ≦ t ₄ / λ

The composition formula of the material of the recording layer is expressed as M _γ Ga _α Sb _β (α, β, γ is an atomic ratio, M is an element other than Ga and Sb or a mixture thereof), and 0.05 ≦ α ≦ 0.2; 4. The optical recording medium according to claim 1, wherein 0.8 ≦ β ≦ 0.95, 0 <γ ≦ 0.1, and α + β + γ = 1.

The optical recording according to claim 4, wherein M is Ge or a mixture of Ge and at least one element selected from the group consisting of In, Al, Ag, Mn, Cu, Au, and N. Medium.