JP3708446B2 - Focus servo system and optical disk master exposure device - Google Patents

Description

【００５８】
また、フォーカスセンサ２１０においては、パラメータｄが考慮される。ｄは、第１集光レンズ１４２と対物レンズ１２４との間隔の所定の設計値（基準値）からのずれを表し、これは、対物レンズ１２４の基準位置に対する実位置のずれに対応する量である。第１集光レンズ１４２のＺ位置をＺL1、第１集光レンズ１４２と対物レンズ１２４の間隔の基準値をＤ０とすると、ｄは次式で表すことができる。
【００５９】

ここで、εは無視しうる微少量である。
【００６０】
フォーカスセンサ２１０は、これらの入力パラメータに基づいて補助光のフォーカス誤差信号ＦＥ２を出力する。フォーカスセンサのＤＣ感度ゲインをａ、 オフセット係数をｋとすると、ＦＥ２は、次式で表すことができる。
【００６１】

【００６２】
オフセット係数ｋについては、後に図５及び図６を参照しつつ詳述する。
ＦＥシフト補正器２２０には、ＦＥ２とともに、センサ１２８の出力ｄsも入力される。ここで、センサ１２８のＤＣ感度ゲインをｂとすると、ｄsは次式により表すことができる。
【００６３】
ｄs≒ｂ・Ｚ
【００６４】
ＦＥシフト補正器２２０は、ＦＥ２とｄsとに基づいて、記録光のフォーカス誤差を推定し、そのフォーカス誤差に相当する出力ＦＥを生成する。ここで、ＦＥシフト補正器２２０が推定した記録光のフォーカス誤差量をｄｆ0とすると、その出力ＦＥは次式により表すことができる。
【００６５】

【００６６】
すなわち、ＦＥは、ｄｆ（ガラス原盤面における記録光のフォーカス誤差）に感度ゲインａを乗じた値に近似することができる。
【００６７】
図５は、記録光のフォーカス誤差量ｄｆとＦＥ２との関係を例示するグラフである。同図においては、第１集光レンズ１４２と対物レンズ１２４との間隔の所定の設計値（基準値）からのずれｄが、−２０μｍ、−１０μｍ、０、＋１０μｍ、＋２０μｍのそれぞれの場合について両者の関係が示されている。
【００６８】
図５から分かるように、第１集光レンズ１４２と対物レンズ１２４との間隔が設計位置からずれると、ｄｆとＦＥ２との間にはオフセットが生ずる。従って、ガラス原盤の上下動に追従させるように対物レンズ１２４をサーボさせると、集光レンズ１４２と対物レンズ１２４との光軸方向の間隔が原盤上下動に伴って変化し、オフセット量が変化する。このオフセット量を考慮しなければ、正確なフォーカンシングはできない。
【００６９】
仮に、基準位置（ｄ＝０）の関係のみに基づいて、ＦＥ２からｄｆを決定した場合には、原盤の上下動に従って対物レンズが＋１μｍ程度変位しただけでも、記録光に対するフォーカス誤差が約２５ｎｍも発生してしまう。
【００７０】
図６は、代表的な条件における補助光の光路を図解した概念図である。すなわち、同図（ａ）〜（ｅ）においては、第１集光レンズ（色収差補正レンズ）１４２、対物レンズ１２４、原盤１１４の表面における補助光の入射・反射光路をそれぞれ表す。そして、原盤面での記録光のフォーカス誤差ｄｆ、第１集光レンズ１４２と対物レンズ１２４との間隔ｄ、及びＦＥ２に関して場合分けして表した。また、これらの概念図においては、便宜上、レンズ光軸の左側に入射光を表し、右側に反射光を表したが、実際には、左右対称の光路が形成される。
【００７１】
図６（ａ）〜（ｅ）に表したように、補助光は、第１集光レンズ１４２に対しては、平行光として入射する。
【００７２】
そして、図６（ｂ）に表したように、記録光の合焦条件より原盤が遠い（ｄｆ＜０）と、補助光の戻り光は若干集束光となり、非点収差光学系の差信号によりＦＥ２＜０のフォーカス誤差信号が得られる。
逆に、図６（ｄ）に表したように、記録光の合焦条件よりも原盤が近い場合（ｄｆ＞０）は、若干発散光となり、ＦＥ２＞０のフォーカス誤差信号が得られる。
【００７３】
しかし、合焦状態（ｄｆ＝０）であっても、第１集光レンズ／対物レンズ間距離が設計位置からずれると、図６（ａ）及び（ｅ）に示したように、戻り光は集束光／発散光となり、見かけ上フォーカス誤差が生じている状態となる。
【００７４】
このような、第１集光レンズ／対物レンズ間距離の設計位置からの光軸方向ズレ量ｄをパラメータに、ＦＥ２出力と記録光のフォーカス誤差量ｄｆとの関係を表したものが図５である。
【００７５】
すなわち、ＦＥ２とｄｆとの間にはオフセットがあり、ＦＥ２は、上述したようにオフセット係数ｋを用いて次式により表すことができる。
【００７６】

【００７７】
図５に例示した具体例の場合、オフセット係数ｋ＝０．０２５７となる。
【００７８】
図４に表したＦＥシフト補正器２２０は、図５に例示したようなＦＥ２とｄｆとの間の関係と、変位センサからの出力ｄｓとに基づいて、記録光のフォーカス誤差ｄｆを推定する。
この推定に基づく出力ＦＥは、制御器２３０に入力され、その出力に基づいてＶＣＭ１２６が駆動され、対物レンズ１２４が記録光の合焦位置に調節される。
【００７９】
本具体例においては、ＶＣＭ１２６が駆動基準位置（中立位置）にある場合に、第１集光レンズ１４２と対物レンズ１２４との間隔が色収差補正の設計位置に一致するように調整され、変位センサ１２８の出力ｄsも０となる様に初期化してある。
【００８０】
ここで、一般に、原盤記録装置は、ＶＣＭフォーカス系の中立点でサーボが可能なように、フォーカス駆動系をＺ方向に上下動作させる粗いフォーカス駆動系を持つ。これに対して、本具体例においては、原盤側をスピンドルモータごと上下させる駆動系を備え、これにより粗フォーカスを調節すれば、基本的に第１集光レンズ／対物レンズ間距離のズレ量ｄは零のまま、フォーカス引込みを実施できる。
【００８１】
つまり、スピンドル回転に伴う原盤上合焦点スポット位置の上下変動がなければ、真のフォーカス誤差ｄｆが、そのままセンサ感度ゲイン倍されてＦＥ２として出力される。しかし、実際には、原盤１１４の厚さのムラなどによって１μｍ程度の原盤上下動があるため、ＶＣＭ１２６が追従駆動して変位する。上記したように、ほぼ対物レンズ中立点に粗フォーカスしておけば、ＶＣＭ１２６の変位Ｚは第１集光レンズ／対物レンズ間距離のズレ量ｄと等価になり、フォーカスセンサ出力ＦＥ２は、（１）式に従ってオフセット誤差項を含んだ出力となる。
【００８２】
また、ズレ量ｄと変位センサ出力ｄSとはほぼ同じ意味を有し、変位センサ１２８の感度係数に応じた比例関係を有するものと近似できる。すなわち、上記したように、変位センサ１２８のＤＣ感度ゲインｂを用いて、次式により表すことができる。
【００８３】
ｄs≒ｂ・Ｚ （２）
【００８４】
ＦＥシフト補正器２２０は、変位センサ出力ｄS、及びフォーカスセンサ出力ＦＥ２を基に、以下のようなオフセット誤差の除去処理を行う。
【００８５】
ＦＥ＝ａ・ｄｆ＝ＦＥ２−ｇ・ｄs （３）
但し、ｇ＝ｋ・ａ／ｂ
【００８６】
これにより、次式が得られる。
【００８７】

【００８８】
すなわち、記録光のフォーカス誤差でサーボする制御系に近似できる。
【００８９】
なお、本実施形態において、補助光側のフォーカス誤差検出系に非点収差光学系を用いたが、例えば、光テコ式のように公知のフォーカス誤差検出系を用いても良い。
【００９０】
以上説明したように、本実施形態によれば、原盤の上下動に追従して補助光に対する色収差条件が変動した場合でも、これを補正することができる。その結果として、回転する原盤全周での記録光のフォーカス誤差を極めて小さく抑えることが可能となり、高密度の露光記録を安定して実施することが可能となる。
【００９１】
なお、対物レンズの上下変動量が小さいと、光学設計ズレ量ｄによる検出オフセット影響は、記録光学系の焦点深度程度に収まる場合もあるが、このような場合でも、実フォーカス誤差振動の残留振動を焦点深度より十分小さくしておかなければ、トラック全周の十分な記録安定性を確保できない。
【００９２】
（第２の実施の形態）
次に、本発明の第２の実施の形態として光ディスク原盤１１４の厚みの個体差に応じた粗フォーカス処理を組み合わせたフォーカスサーボ方式について説明する。
【００９３】
図７は、本実施形態にかかる光ディスク原盤露光装置の要部構成を表す概念図である。同図については、図１乃至図６に関して前述した要素と同様のものには同一の符号を付して詳細な説明は省略する。
【００９４】
前述した第１実施形態においては、スピンドルモータ１１０側が上下して粗フォーカス処理を実施する場合を例示したが、本実施形態においては、図７に表したように、フォーカス駆動系をＺ方向に上下動作させる粗フォーカス処理を行う。
【００９５】
粗フォーカス上下機構３００のベース側は、トラック送りを実施するスライダ３２０であり、この上部に図１に例示した光学系に相当するものが固定配置されている。記録光は第１レーザ導入・検出系３３０により供給・検出され、補助光は第２レーザ導入・検出系３４０により供給・検出される。
ただし、本実施形態においては、第１集光レンズ（色収差補正レンズ）１４２と対物レンズ１２４との間に、スライダ上固定配置された立ち下げミラー１３９が設けられ、この反射光が対物レンズ１２４に入射する。
【００９６】
また、可動部はフォーカス駆動系（ＶＣＭ１２６、対物レンズ１２４）であり、ＶＣＭフォーカス駆動変位を零にしたまま、対物レンズ１２４を数ｍｍ範囲で上下移動可能としている。
【００９７】
つまり、本実施形態においては、粗フォーカス駆動系３００を駆動させると集光レンズ１４２／対物レンズ１２４間の距離が変動することになる。集光レンズ１４２の位置は、所定の基準厚さのガラス原盤に対して粗フォーカス処理を行った際に、集光レンズ１４２／対物レンズ１２４間距離が設計値になるように固定されている。
【００９８】
以下に本実施形態における、粗フォーカス処理について説明する。粗フォーカス処理は、図４に関して前述したように、補助光の戻り光量の和信号と差信号とをモニタし、差信号であるＦＥ２がＳ字カーブ中心のゼロ交差位置になるように、粗フォーカス上下機構３００で、フォーカス駆動系（対物レンズ１２４など）を上下して位置決めする。その後、図示しないロック機構により、粗フォーカス上下機構３００をスライダ３２０に固定する。光ディスクの原盤露光などの用途においては、ナノメータ精度というきわめて精密なトラック送り精度が要求される。このため、トラック送り方向の高剛性化が必要であり、フォーカス上下機構の固定ロックは不可欠である。
【００９９】
この際、ガラス原盤１１４の厚さが、光学系調整時に採用した所定の基準厚さと同一であれば、集光レンズ１４２／対物レンズ１２４間距離のズレ量ｄは零となり、第１実施形態の場合と全く同じになる。しかし、実際にはガラス原盤１１４の厚さの個体差が１０μｍ程度ある。このため、この厚さバラツキに応じたズレ量ｄが発生し、ＦＥ２＝０に粗フォーカス処理した状態でも、記録光に対しての真のフォーカス誤差量は、（１）式に表したズレ量ｄに依存した程度存在することになる。この点が、第１実施形態との大きな違いであり、粗フォーカス後から信号記録までの間に新たな調節処理が必要とされる。
【０１００】
図８は、このオフセット誤差を除去するためのフォーカスセンサキャリブレーション処理を表す概念図である。
本実施形態におけるセンサキャリブレーションは、第１レーザ導入・検出系３３０において、記録光の戻り光によるフォーカス誤差信号ＦＥ１でフォーカスサーボする。このため、センサキャリブレーションは、原盤１１４の信号非記録領域において実施する。以下に、その零調整処理を図８を参照しつつ説明する。
【０１０１】
図８に表した第１フォーカスセンサ２１０Ａは、記録光の戻り光によるフォーカスセンサであり、図２乃至図４に関して前述したような非点収差光学系のＰＤ（フォトダイオード）差信号からＦＥ１信号を得るものである。また、第２フォーカスセンサ２１０Ｂは、補助光の戻り光によるフォーカスセンサであり、図２に例示したものと同様の構成を有し、図４に例示したような出力特性を有するものである。
【０１０２】
まず、原盤１１４の厚みに合わせた粗フォーカス処理の直後は、フォーカスサーボオフ状態であるが、ＦＥ２出力はほぼ零にあり、ＶＣＭ１２６の変位も零となっている。しかし、原盤１１４の厚みの基準値からずれに応じたオフセット影響があるため、実際のフォーカス誤差量はゼロではない。この状態で、記録光を原盤１１４に照射し、その戻り光によるフォーカス誤差信号ＦＥ１をモニタすると、実際のフォーカス誤差量がＦＥ１出力に反映され、ＦＥ１がオフセットした状態であると確認できる。
【０１０３】
次に、ＦＥ１を使い記録光に基づくフォーカスサーボを掛けると、ＦＥ１は零となるが、ＦＥ２出力が逆方向にオフセットする。このＦＥ２のＤＣオフセット成分を第２フォーカスセンサ２１０Ｂに出力オフセットＦＥ２_０として与えれば、第２フォーカスセンサ２１０Ｂの零調整が実現できるが、実際には原盤１１４の回転に伴う原盤上下動Ｒが発生するため、ＦＥ１とＦＥ２との誤差Ｅを求め、低ゲインのＰＩ（比例積分）動作とローパスフィルタとを組合せたオフセット調整器２１５により、ＦＥ２がＦＥ１の残留振動と一致するようにして、このＤＣオフセット成分を抽出する。
【０１０４】
第２フォーカスセンサ２１０Ｂの零調整後は、ＦＥ２_０はこの値にホールドされ、図１及び図３に例示した変位センサ１２８の零調整も実施される。以下に、原盤１１４の厚さの基準厚さに対する誤差をＺ０とし、原盤回転による原盤位置の上下変動がないとした場合の各フェーズでの状態変動関係を示す。
【０１０５】
粗フォーカス機構固定直後（サーボオフ）：
ｄ＝−Ｚ０／（１＋ｋ）
ｄｆ＝−Ｚ０・ｋ／（１＋ｋ）
ｄs＝０
ＦＥ２＝０
【０１０６】
精引込み直後の零調整前（第１サーボオン）：
ｄ＝−Ｚ０
ｄｆ＝０
ｄs＝−Ｚ０・ｋ／（１＋ｋ）
ＦＥ２＝ａ・ｋ・Ｚ０
【０１０７】
ＶＣＭ変位センサを含めた零調整後（第１サーボオン）：
ｄ＝−Ｚ０
ｄｆ＝０
ｄs＝０
ＦＥ２＝０
ｄsｏ＝Ｚ０・ｋ／（１＋ｋ）
ＦＥ２０＝−ａ・ｋ・Ｚ０
【０１０８】
センサキャリブレーション処理は以上であるが、その後、図４に関して前述したようにＦＥ２信号によるフォーカスサーボへの切替え処理、記録光の遮光処理が順次行われ、補助光フォーカスサーボ（ＦＥ２信号ＦＢ）のまま、スライダ３２０を記録開始位置に移動して、信号露光記録処理へと移行する。信号露光記録時のサーボに関しては、図４に関して前述したものと同様に実施できるので省略する。
【０１０９】
以上、本発明の第２の実施の形態として、原盤１１４の厚みの個体差に対応するための粗フォーカス調節について説明した。図７及び図８に関して前述した構成とは別に、図９に表したように、粗フォーカスのための可動部を立ち下げミラー１３８から対物レンズ１２４及びＶＣＭ１２６までの一体構成とする方法も考えられる。このようにすれば、粗フォーカスにより第１集光レンズ１４２と対物レンズ１４２との間隔がずれることを防ぐことができる。しかし、図９の構成の場合、粗フォーカスのための可動部の質量が大きく、機械的な負荷に対処した機構が必要とされる。この観点からは、図７及び図８に例示した構成の方が有利である。
【０１１０】
（第３の実施の形態）
次に、本発明の第３の実施の形態として、変位センサの出力などに含まれるノイズを除去するフィルタを設けた構成について説明する。
【０１１１】
図１０は、本実施形態にかかるフォーカスサーボの制御構成を表す概念図であるる。同図については、図１乃至図９に関して前述したものと同様の要素には同一の符号を付して詳細な説明は省略する。
【０１１２】
本実施形態においては、具体的な装置構成は第１実施形態のものでも第２実施形態のものでも良いが、図１０に例示した如く、色収差補正するフォーカスサーボ構成にハイパスフィルタ２４０が設けられている。
【０１１３】
第１実施形態においては、図４に関して前述したように変位センサ１２８でＶＣＭ１２６の変位Ｚが（２）式の如く検出されるとした。しかし、実際には、センサ１２８の出力は、センサ自体のローパス特性やセンサノイズなどを含んだものである場合が多く、必要帯域で近似しても、次式のようにセンサノイズｗを含んだ出力となることが多い。
【０１１４】
ｄs＝ｂ・Ｚ＋ｗ （４）
【０１１５】
特に、静電容量式変位計を変位センサに用いる場合、原盤１１４の回転周波数以下の一般に「ドリフト」と呼ばれる低周波ノイズが発生することがある。このドリフトレベルは、変位に換算して０．２μｍ程度と極めて小さいため、（１）式の係数から見て、フォーカス誤差にさほど影響するものではないが、原盤記録部の内外周差を生じ、記録ピットの全面安定性を低下させる要因になる。
【０１１６】
そこで、図１０に例示したように、低周波遮断のためのハイパスフィルタ２４０を設けて、このようなドリフトの除去を行う。ここで、原盤回転に伴う上下動変動の周波数は、ＳＰＭ１１０の回転周波数およびその高調波振動で構成されるので、ハイパスフィルタ２４０の遮断周波数は、ＳＰＭ１１０の回転周波数未満とすればよい。
【０１１７】
このようにハイパスフィルタ２４０を設けることにより、（４）式に例示した如くセンサ１２８の出力にノイズ成分が含まれていても、効果的に除去することができ、初期に十分なフォーカスセンサの零調整がなされていれば、（３）式に表した如く真のフォーカス誤差を推定でき、原盤１１４の全面に亘って高い精度で安定した露光記録を確保できる。
【０１１８】
（第４の実施の形態）
次に、本発明の第４の実施の形態として、対物レンズの変位をセンサによらずに推定するフォーカスサーボ方式について説明する。
【０１１９】
図１１は、本実施形態のフォーカスサーボ制御系を表す概念図である。同図についても、図１乃至図１０に関して前述したものと同様の要素には同一の符号を付して詳細な説明は省略する。
【０１２０】
本実施形態においては、図４などに例示したＶＣＭ駆動変位センサ１２８を不要としている点に特徴がある。ここで、フォーカスセンサ２１０が検出するのはフォーカス誤差量ｄｆであり、基本的には図３に例示したような変位センサ１２８を設けなければ、ＶＣＭ１２６の駆動変位Ｚを真に測定することはできない。
【０１２１】
しかし、ＶＣＭ１２６のコイルに電流を流していない初期状態が既知であり、レンズホルダ１２５のスライダ部に加わる外乱影響は一般的には無視できる程小さいので、ＶＣＭ１２６の変位Ｚは、その駆動電流から推定可能である。
【０１２２】
フォーカス駆動系の周波数特性は、高周波域に副共振や第２共振等の高次影響を持つが、原理的には、次式のような２次状態関数で近似できる。
【０１２３】
【数１】

【０１２４】
状態変数の初期状態は既知であるので、これをノミナルモデルとして、フォーカス駆動指令に対するＶＣＭ駆動変位Ｚの推定値を次式の様にオープン推定する。
【０１２５】
【数２】

【０１２６】
このＶＣＭ駆動変位推定値をノミナルＶＣＭモデル２５０によって演算し、ＦＥシフト補正器２２０に与えることにより、（３）式に関して前述した場合と同様に、記録光の真のフォーカス誤差を推定できる。その結果として、第１実施形態と同様の色収差によるオフセット影響を受けない高性能フォーカスサーボを、センサ１２８を用いることなく実現できる。
【０１２７】
ここで、ノミナルＶＣＭモデルは、実際には、計算機によるデジタル演算処理などにより実現する場合もある。そこで、ＦＥシフト補正器２２０、制御２３０、ノミナルＶＣＭモデル２５０をまとめて、高次ＩＩＲフィルタ表現に書き直し、ＦＥ２のＡ／Ｄ入力をＩＩＲフィルタに掛けて、この出力を制御器２３０の出力ｕとしてＤ／Ａ出力することもできる。
【０１２８】
以上、具体例を参照しつつ本発明の実施の形態について説明した。しかし、本発明は、これらの具体例に限定されるものではない。例えば、記録光や補助光の波長や光源の種類、光学系の具体的な構成、原盤露光装置の具体的な構成などは、当業者が適宜選択し設計変更して本発明の効果を同様に得ることができる。
【０１２９】
また、本発明は、２種類の波長光を用いるフォーカスサーボに広く適用することが可能であり、その対象は光ディスク原盤露光装置に限定されず、追加型や書き換え可能型の高密度光ディスクの記録装置にも同様に適用可能である。
【０１３０】
また、複数の波長光を有する読みとり装置においても、同様に適用可能である。
【０１３１】
【発明の効果】
以上詳述したように、本発明によれば、色収差を有する対物レンズを用いた原盤露光装置などのフォーカスサーボ方式において、レジスト露光感度の低い補助光を用いてフォーカスサーボしても、対物レンズの原盤追従動作によるフォーカス誤差信号へのオフセット誤差影響がなくなり、記録光にとっての真のフォーカス誤差を小さくする事が可能になる。これにより、記録ピットの原盤回転位相による影響がなくなり、高い記録密度で記録安定性を十分に確保できるようになり産業上のメリットは多大である。
【図面の簡単な説明】
【図１】本発明の第１の実施形態にかかる光ディスク原盤露光装置の要部構成を例示する概念図である。
【図２】光検出器１５４の出力信号特性を表すグラフ図である。
【図３】第１実施形態のフォーカス駆動系の構成を例示する概念図である。
【図４】第１実施形態にかかるフォーカスサーボ系の制御ブロックを例示する概念図である。
【図５】 記録光のフォーカス誤差量ｄｆとＦＥ２との関係を例示するグラフである。
【図６】代表的な条件における補助光の光路を図解した概念図である。す
【図７】本発明の第２の実施の形態にかかる光ディスク原盤露光装置の要部構成を表す概念図である。
【図８】オフセット誤差を除去するためのフォーカスセンサキャリブレーション処理を表す概念図である。
【図９】粗フォーカス調節のための別の構成を表す概念図である。
【図１０】本発明の第３の実施の形態にかかるフォーカスサーボの制御構成を表す概念図である。
【図１１】本発明の第４の実施の形態のフォーカスサーボ制御系を表す概念図である。
【図１２】本発明者が本発明に至る過程で検討した原盤露光装置の要部構成を表す概念図である。
【図１３】色収差補正光学系全体を移動させる方法を採用した場合の原盤露光装置の要部構成を例示する概念図である。
【符号の説明】
１１０ スピンドルモータ
１１２ ターンテーブル
１１４ 原盤
１２０ 記録光
１２２ ダイクロイックプリズム
１２４ 対物レンズ
１２５ レンズホルダ
１２６ ボイスコイルモータ（ＶＣＭ）
１２７ カウンタウエイト
１２８ 変位センサ
１２９ 平行渦巻きバネ
１３０ 第２レーザ光源
１３２ 拡大光学系
１３４ 偏光ビームスプリッタ（ＰＢＳ）
１３６ １／４波長板
１３８ ミラー
１４０ 開口絞り
１４２ 第１集光レンズ（色収差補正レンズ）
１５０ 第２集光レンズ
１５２ シリンドリカルレンズ
１５４ 光検出器
２１０ フォーカスセンサ
２１５ オフセット調整器
２２０ ＦＥシフト補正器
２３０ 制御器
２４０ ハイパスフィルタ
２５０ ノミナルＶＣＭモデル[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a focus servo system and an optical disk master exposure apparatus, and more specifically, to a glass resist master in a mastering process of an optical disk manufacturing process such as a next-generation high-density CD (Compact Disk) or DVD (Digital Versatile Disk). The present invention relates to a focus servo system and an optical disc master exposure apparatus suitable for use in exposure recording.
[0002]
[Prior art]
In mass production of optical discs such as CDs and DVDs, a process of first exposing a master coated with a resist on a glass substrate to form recording pits, producing a predetermined mold from this master and mass-producing it with a stamper. Generally adopted.
[0003]
In order to always irradiate recording light (exposure beam) with optimized focus and tracking on the master disk coated with the photosensitive resist, “auxiliary light” having a wavelength different from that of the recording light is required. . In other words, it is necessary to dynamically adjust the focus of the recording light during writing using auxiliary light having a wavelength with low resist sensitivity.
For this reason, in the conventional optical disc master exposure apparatus, a recording light focusing servo is performed based on a focus error signal (hereinafter abbreviated as “FE2”) due to return light from the master of the second laser (auxiliary light). Was taken.
[0004]
However, if the wavelengths of the first laser (recording light) and the second laser (auxiliary light) are different, the objective lens of the exposure apparatus may cause aberration. On the other hand, conventionally, the configuration of the objective lens has been devised so that the aberration (chromatic aberration) depending on the wavelength becomes as small as possible even when laser beams having different wavelengths are incident on the objective lens. In this way, the focus error signal (FE1) and FE2 due to the return light of the first laser (recording light) and the FE2 can be obtained in the vicinity of the focal position by using a slight optical system.
[0005]
[Problems to be solved by the invention]
However, as the recording density of the optical disc increases, it is necessary to reduce the wavelength of the recording light. Then, there arises a problem that the design of a high NA objective lens without chromatic aberration becomes very difficult. For example, conventionally, when recording light having a wavelength of 351 nanometers is used, it has been possible to suppress chromatic aberration of the objective lens. However, if recording light having a wavelength of 266 nanometers is to be employed for higher density recording. Therefore, it is difficult to obtain an objective lens that suppresses chromatic aberration. Therefore, in this case, an optical design for focus error detection in consideration of chromatic aberration is required.
[0006]
FIG. 12 is a conceptual diagram showing a main part configuration of a master exposure apparatus examined by the inventor in the course of reaching the present invention.
[0007]
The configuration will be briefly described as follows. That is, the master 114 is installed on the turntable 112 that is rotated by the spindle motor 110. The recording light 120 is applied to the master 114 via the dichroic prism 122 and the objective lens 124. On the other hand, the auxiliary light is transmitted from the second laser light source 130 to the magnifying optical system 132, the polarizing beam splitter (PBS) 134, the quarter wavelength plate 136, the mirror 138, the aperture stop 140, the first condenser lens 142, the dichroic prism 122, The master 114 is irradiated through the objective lens 124, and the reflected light is condensed by the second condenser lens 150 and the cylindrical lens 152 through the objective lens 124 to the PBS 134 through the reverse path and is detected by the photodetector 154. Is detected.
[0008]
In the configuration outlined above, the recording light 120 is incident on the objective lens 124 as an infinite system, that is, substantially parallel light. On the other hand, as shown in the figure, the auxiliary light is incident on the objective lens as a finite system, that is, convergent light.
[0009]
For this reason, the distance from the 1st condensing lens 142 to the objective lens 124 becomes very important. That is, if the distance from the first condenser lens 142 to the village lens 124 is always the same as the design value, complete chromatic aberration correction is possible, and the focus error signal FE2 of the auxiliary light is in the focusing condition of the recording light. Should always produce an output that is zero.
[0010]
However, because the glass master 114 has “thickness unevenness” or the like, vertical movement occurs when it is rotated. For this reason, it is necessary to cause the objective lens 124 to follow the vertical movement of the glass master 114 by the focus servo. However, when the objective lens 124 is made to follow in this way, the distance between the objective lens 124 and the first condenser lens 142 changes, and FE2 becomes an output signal having an error corresponding to this displacement. That is, if the FE2 is used as it is, the recording light 120 may not be in focus.
[0011]
Conversely, if the objective lens 124 is focus-servoed using FE2 as a feedback signal, a large focus error amount synchronized with the vertical movement of the master 114 occurs for the recording light.
In order to prevent this, a method of causing the entire optical system to follow the vertical movement of the disk is also conceivable.
[0012]
FIG. 13 is a conceptual diagram illustrating the main configuration of a master exposure apparatus when this method is employed. In other words, in the apparatus shown in the figure, the optical system from the objective lens 124 to the mirror 138 moves integrally to follow the vertical movement of the master 114. However, in this configuration, there is a problem in that the weight of the movable part of the focus mechanism is increased, the mechanism becomes complicated, and the followability is deteriorated.
[0013]
As described above, when focus servo with auxiliary light is performed on an objective lens having chromatic aberration, the amount of focus error with respect to the original recording light increases, and the error cannot be suppressed within the depth of focus. Will occur.
[0014]
The present invention has been made based on recognition of such problems. That is, the object is to provide a focus servo system and an optical disc master capable of sufficiently reducing a focus error with respect to recording light even if the objective lens is focus-servoed based on a focus error signal of auxiliary light in an optical system having chromatic aberration. It is to provide an exposure apparatus.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, the VCM displacement is measured based on the optical relationship between the focus servo actuator (VCM) displacement of the objective lens and the focusing conditions of the recording light and the auxiliary light. A servo system that electrically compensates for the offset is configured. The main points are as follows.
[0016]
(I) A means for measuring the VCM displacement amount is provided, and the focus error amount of the recording light is estimated from the sensor value.
(Ii) A high-pass filter is provided in consideration of the drift effect of the VCM displacement sensor.
(Iii) The state of the VCM displacement is estimated from the simple motion model formula of the VCM objective lens system and the VCM current command value instead of the VCM displacement sensor.
[0017]
  More specifically, the focus servo system of the present invention has a first light having a first wavelength and a second wavelength different from the first wavelength.Movable with respect to irradiated objectThe second light collected through the condenser lens and,TheIt is movable with respect to the condensing lens and the irradiated object and has chromatic aberrationIncident to the irradiated surface of the irradiated object through the objective lensAnd
[0018]
  After positioning the condenser lens with respect to the irradiated object,Obtained by the return light of the second light from the irradiated surfaceSecondFocus error informationFocus error information corresponding to the focus error of the first light, generated from the relative position shift of the condenser lens and the objective lensOn the basis of the, So that the first light is focused on the irradiated surfaceThe objective lensofDisplacementIs used to correct the deviation of the focusing conditions of the first and second lights.It is characterized by that.
[0019]
Here, the shift of the focusing condition between the first and second lights may be an offset caused by the displacement of the objective lens.
[0020]
In addition, the displacement of the objective lens may be detected by a sensor to estimate a shift in focusing conditions between the first and second lights.
[0021]
Further, by applying a high-pass filter process to the output of the sensor, low frequency noise mixed in the sensor output can be removed.
[0022]
Further, the displacement can be estimated based on a drive signal given to a drive source for displacing the objective lens.
[0023]
  On the other hand, the optical disc master exposure apparatus of the present invention has a recording light having an exposure wavelength for recording on the optical disc master and a wavelength different from the exposure wavelength.Movable with respect to the optical disc masterAuxiliary light collected through a condenser lens and,TheIt is movable with respect to the condensing lens and the optical disc master and has chromatic aberrationThe light enters the irradiated surface of the master optical disc through an objective lens., DrivingAn optical disc master exposure apparatus that performs exposure recording on the optical disc master while displacing the objective lens by a moving source,
[0024]
  After positioning the condensing lens with respect to the optical disc master, the second focus error information obtained by the return light of the auxiliary light from the irradiated surface and the deviation of the relative positions of the condensing lens and the objective lens Based on the first focus error information corresponding to the focus error of the recording light generated by the driving source so that the recording light is focused on the irradiated surface.Control the displacement of the objective lensThus, the deviation of the focusing condition of the recording light and the auxiliary light is corrected.Control means is provided.
[0025]
Here, the shift in the focusing conditions of the recording light and the auxiliary light may be an offset caused by the displacement of the objective lens.
[0026]
In addition, a sensor for detecting the displacement of the objective lens may be further provided, and a deviation in focusing conditions of the recording and auxiliary light may be estimated based on the output of the sensor.
[0027]
Here, low-frequency noise included in the sensor output can be removed by performing high-pass filter processing on the output of the sensor.
[0028]
Further, the displacement of the objective lens may be estimated based on a drive signal given to the drive source.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to specific examples.
[0030]
(First embodiment)
First, as a first embodiment of the present invention, a focus servo system that measures a displacement amount of an objective lens and estimates a focus error amount from the measured value will be described.
[0031]
FIG. 1 is a conceptual diagram illustrating the main configuration of an optical disc master exposure apparatus according to this embodiment. In this figure, the same elements as those described above with reference to FIG.
[0032]
The second laser light source 130 emits a linearly polarized laser beam having a low intensity below the threshold sensitivity at a wavelength where the sensitivity of the resist applied to the master 114 is low. The beam width is expanded by the magnifying optical system 132, transmitted through the polarization beam splitter (PBS) 134 whose polarization phase angle is adjusted to be transmitted, and is incident on the quarter wavelength plate 136. The laser light is changed from linearly polarized light to circularly polarized light by the quarter wavelength plate 136 and is incident on the objective lens 124 while being condensed by the first condenser lens 142. The objective lens 124 can follow the vertical movement of the master 114 by a voice coil motor (VCM) 126, and at the same time, the displacement amount is measured by a displacement sensor 128.
[0033]
In addition, the apparatus of this embodiment includes a drive system (not shown) that moves the master disk side up and down together with the spindle motor 110, and adjusts the coarse focus in the Z direction so that servo is possible at the neutral point of the focus system of the VCM 126. be able to.
[0034]
The aperture stop 140 is for removing the scattered component of the laser light. The dichroic prism (dichroic mirror) 122 has a characteristic of reflecting the light beam having the wavelength of the recording light 120 and transmitting the beam having the auxiliary light wavelength, and combining the recording light and the auxiliary light into the objective lens 124. Used for incidence.
[0035]
The first condenser lens 142 is used as a chromatic aberration correction lens for correcting the difference in refractive index depending on the wavelength in the objective lens 124. That is, the refractive index in the objective lens 124 is different between the recording light and the auxiliary light. For this reason, the recording light is supplied as parallel light, whereas the auxiliary light is supplied as convergent light by the first condenser lens 142. In this way, when the distance between the first condenser lens 142 and the objective lens 124 is equal to a predetermined design value, the auxiliary light is also emitted at the focal position of the recording laser light 120 incident on the objective lens 124 as parallel light. It will be in focus.
[0036]
In addition, the focal length of the first condenser lens 142 and the distance between the first condenser lens 142 and the objective lens 124 are set in consideration of conditions for maintaining a sufficient detection range and detection sensitivity in the astigmatism focus detection system. are doing.
[0037]
As described above, when the distance between the first condenser lens 142 and the objective lens 124 is the same as a predetermined design value, the auxiliary light and the recording light are focused at the same distance from the objective lens 124. The auxiliary light reflected on the master disk 114 enters the PBS 134 as return light again through the objective lens 124, the dichroic prism 120, the first condenser lens 142, and the quarter wavelength plate 136.
[0038]
In this process, the return light becomes a linearly polarized beam again at the quarter-wave plate 136, but has a phase difference of 90 degrees with respect to the light emitted from the second laser light source 130, and is reflected by the PBS 134. The light enters the photodetector 154 through the condenser lens 150 and the cylindrical lens 152. The photodetector 154 is an astigmatism focus detection system composed of a four-divided photodetector and converts an optical signal into an electrical signal.
[0039]
FIG. 2 is a graph showing output signal characteristics of the photodetector 154. That is, the photodetector 154 has a light receiving portion that is divided into four parts in the vertical and horizontal directions, and the sum signal (sum signal) from these four light receiving portions and the difference output (difference signal) of the diagonal sum are respectively The relationship shown in the figure is related to the focus error amount of the auxiliary light.
[0040]
The difference signal illustrated in FIG. 2 is a signal corresponding to “FE2” and has an output characteristic called “S-curve characteristic”. In the example in the figure, the focus error detection range from the in-focus point is the ± 4 μm range, and the focus error can be detected within this range.
[0041]
Next, the configuration of the focus drive system around the objective lens 124 will be described.
[0042]
FIG. 3 is a conceptual diagram illustrating the configuration of the focus drive system. This focus drive system includes an internal sensor 128 for detecting the amount of displacement of the objective lens 124 in addition to the focus VCM 126. The focus drive system may include a current amplifier (not shown) for the VCM 126 and a sensor amplifier (not shown) for the interior sensor 128.
[0043]
The objective lens 124 is bonded and fixed to the lens holder 125, forms a movable part together with the lens holder 125, and the lens holder 125 is joined to the fixed side of the VCM 126 by a parallel spiral spring 129.
[0044]
The counterweight 127 is provided to balance the power point of the movable part. The objective lens 124 moves up and down with a generated force proportional to the energization current of the coil of the VCM 126. Further, as will be described in detail later, the energization current to the coil of the VCM 126 is supplied via a current amplifier (not shown) according to a command value from the focus controller.
[0045]
The internal sensor 128 can be, for example, a capacitance-type high-precision displacement sensor provided on the upper part of the lens holder, detects the amount of displacement of the objective lens 124, and outputs an electrical output as a sensor amplifier output signal (not shown). It is possible to do.
[0046]
However, specific configurations of the VCM 126, the sensor 128, the lens holder 125, and the like are not limited to those illustrated in FIG. 3, and those skilled in the art can appropriately select and change the design to obtain the effects of the present invention in the same manner. .
[0047]
In the configuration described above, in the present embodiment, the focusing condition of the recording light is adjusted by adding an offset to the focus error signal (FE2) of the auxiliary light according to the amount of displacement of the objective lens 124 detected by the sensor 128.
[0048]
FIG. 4 is a conceptual diagram illustrating a control block of the focus servo system according to this embodiment.
[0049]
That is, the control block in FIG. 3 includes a focus sensor 210, a focus error (FE) shift corrector 220, a controller 230, a VCM 126, and a displacement sensor 128. Here, the focus sensor 210 corresponds to the auxiliary light return light optical processing system shown in FIG.
[0050]
In FIG. 4, the parameter R input to the control system represents the height fluctuation with respect to the reference height at the writing position of the glass master 114. For example, the reference height at the writing position of the master is Zdisk0, and the actual height is In the case of Zdisk, it can be expressed by the following formula.
[0051]
R = Zdisk-Zdisk0
[0052]
The parameter Z output from the control system represents the drive displacement with respect to the reference position of the objective lens 124. When the reference position of the objective lens is ZLens0 and the target position is ZLens, it can be expressed by the following equation.
[0053]
Z = ZLens−ZLens0
[0054]
A difference df between these parameters R and Z is input to the focus sensor 210. That is, df corresponds to the focus error of the recording light on the glass master surface and can be expressed by the following equation.
[0055]
df = R−Z
[0056]
Here, if the difference between the reference position of the objective lens and the master disk height is ZWD, df can also be expressed by the following equation.
[0057]

[0058]
In the focus sensor 210, the parameter d is considered. d represents the deviation of the distance between the first condenser lens 142 and the objective lens 124 from a predetermined design value (reference value), which is an amount corresponding to the deviation of the actual position with respect to the reference position of the objective lens 124. is there. If the Z position of the first condenser lens 142 is ZL1 and the reference value of the distance between the first condenser lens 142 and the objective lens 124 is D0, d can be expressed by the following equation.
[0059]

Here, ε is a negligible small amount.
[0060]
The focus sensor 210 outputs a focus error signal FE2 of auxiliary light based on these input parameters. If the DC sensitivity gain of the focus sensor is a and the offset coefficient is k, FE2 can be expressed by the following equation.
[0061]

[0062]
The offset coefficient k will be described in detail later with reference to FIGS.
The output ds of the sensor 128 is input to the FE shift corrector 220 together with FE2. Here, when the DC sensitivity gain of the sensor 128 is b, ds can be expressed by the following equation.
[0063]
ds≈b · Z
[0064]
The FE shift corrector 220 estimates a focus error of the recording light based on FE2 and ds, and generates an output FE corresponding to the focus error. Here, when the focus error amount of the recording light estimated by the FE shift corrector 220 is df0, the output FE can be expressed by the following equation.
[0065]

[0066]
That is, FE can be approximated to a value obtained by multiplying df (a recording light focus error on the glass master surface) by a sensitivity gain a.
[0067]
FIG. 5 is a graph illustrating the relationship between the focus error amount df of recording light and FE2. In the figure, both of the cases where the deviation d from the predetermined design value (reference value) of the distance between the first condenser lens 142 and the objective lens 124 is −20 μm, −10 μm, 0, +10 μm, and +20 μm. The relationship is shown.
[0068]
As can be seen from FIG. 5, when the distance between the first condenser lens 142 and the objective lens 124 deviates from the design position, an offset occurs between df and FE2. Therefore, when the objective lens 124 is servoed so as to follow the vertical movement of the glass master, the distance in the optical axis direction between the condenser lens 142 and the objective lens 124 changes with the vertical movement of the master, and the offset amount changes. . If this offset amount is not taken into account, accurate focusing cannot be performed.
[0069]
If df is determined from FE2 based only on the relationship of the reference position (d = 0), the focus error with respect to the recording light is about 25 nm even if the objective lens is displaced about +1 μm according to the vertical movement of the master. Will occur.
[0070]
FIG. 6 is a conceptual diagram illustrating the optical path of auxiliary light under typical conditions. That is, FIGS. 9A to 9E show incident and reflected light paths of auxiliary light on the surfaces of the first condenser lens (chromatic aberration correction lens) 142, the objective lens 124, and the master disk 114, respectively. The focus error df of the recording light on the master surface, the distance d between the first condenser lens 142 and the objective lens 124, and FE2 are shown separately. In these conceptual diagrams, incident light is shown on the left side of the lens optical axis and reflected light is shown on the right side for the sake of convenience, but in reality, a symmetrical optical path is formed.
[0071]
As shown in FIGS. 6A to 6E, the auxiliary light is incident on the first condenser lens 142 as parallel light.
[0072]
Then, as shown in FIG. 6B, when the master is farther than the focusing condition of the recording light (df <0), the return light of the auxiliary light becomes slightly focused light, which is caused by the difference signal of the astigmatism optical system. A focus error signal with FE2 <0 is obtained.
Conversely, as shown in FIG. 6D, when the master is closer than the focusing condition of the recording light (df> 0), the light is slightly divergent and a focus error signal of FE2> 0 is obtained.
[0073]
However, even in the in-focus state (df = 0), if the distance between the first condenser lens and the objective lens deviates from the design position, as shown in FIGS. Focused light / divergent light is generated, and a focus error is apparently generated.
[0074]
FIG. 5 shows the relationship between the FE2 output and the focus error amount df of the recording light, using the deviation d in the optical axis direction from the design position of the distance between the first condenser lens and the objective lens as a parameter. is there.
[0075]
That is, there is an offset between FE2 and df, and FE2 can be expressed by the following equation using the offset coefficient k as described above.
[0076]

[0077]
In the specific example illustrated in FIG. 5, the offset coefficient k = 0.0257.
[0078]
The FE shift corrector 220 shown in FIG. 4 estimates the focus error df of the recording light based on the relationship between FE2 and df exemplified in FIG. 5 and the output ds from the displacement sensor.
The output FE based on this estimation is input to the controller 230, and the VCM 126 is driven based on the output, and the objective lens 124 is adjusted to the focus position of the recording light.
[0079]
In this specific example, when the VCM 126 is at the drive reference position (neutral position), the distance between the first condenser lens 142 and the objective lens 124 is adjusted to coincide with the design position for correcting chromatic aberration, and the displacement sensor 128 is adjusted. The output ds is also initialized to zero.
[0080]
Here, generally, the master recording apparatus has a rough focus drive system that moves the focus drive system up and down in the Z direction so that servo can be performed at the neutral point of the VCM focus system. On the other hand, in this specific example, a drive system that moves the master disk side up and down together with the spindle motor is provided, so that by adjusting the coarse focus, basically, the shift amount d between the first condenser lens / objective lens distances. Focus pull-in can be carried out with 0 remaining zero.
[0081]
That is, if there is no vertical fluctuation of the focal spot position on the master due to the spindle rotation, the true focus error df is multiplied by the sensor sensitivity gain as it is and output as FE2. However, in reality, since the master disk is moved up and down by about 1 μm due to the unevenness of the thickness of the master disk 114, the VCM 126 is driven and displaced. As described above, if the focus is roughly focused on the objective lens neutral point, the displacement Z of the VCM 126 is equivalent to the shift amount d of the distance between the first condenser lens and the objective lens, and the focus sensor output FE2 is (1). The output includes an offset error term according to the equation (1).
[0082]
Further, the shift amount d and the displacement sensor output dS have substantially the same meaning, and can be approximated as having a proportional relationship according to the sensitivity coefficient of the displacement sensor 128. That is, as described above, it can be expressed by the following equation using the DC sensitivity gain b of the displacement sensor 128.
[0083]
ds≈b · Z (2)
[0084]
The FE shift corrector 220 performs the following offset error removal processing based on the displacement sensor output dS and the focus sensor output FE2.
[0085]
FE = a.df = FE2-g.ds (3)
However, g = k · a / b
[0086]
As a result, the following equation is obtained.
[0087]

[0088]
That is, it can be approximated to a control system that servos with a focus error of recording light.
[0089]
In this embodiment, the astigmatism optical system is used as the focus error detection system on the auxiliary light side. However, for example, a known focus error detection system such as an optical lever system may be used.
[0090]
As described above, according to the present embodiment, even when the chromatic aberration condition for the auxiliary light fluctuates following the vertical movement of the master, this can be corrected. As a result, the focus error of the recording light on the entire circumference of the rotating master can be kept extremely small, and high-density exposure recording can be stably performed.
[0091]
If the amount of vertical fluctuation of the objective lens is small, the detection offset effect due to the optical design deviation amount d may fall within the focal depth of the recording optical system. Even in such a case, the residual vibration of the actual focus error vibration is present. Is not sufficiently smaller than the depth of focus, sufficient recording stability cannot be ensured for the entire track circumference.
[0092]
(Second Embodiment)
Next, as a second embodiment of the present invention, a focus servo system that combines rough focus processing according to individual differences in the thickness of the optical disc master 114 will be described.
[0093]
FIG. 7 is a conceptual diagram showing the main configuration of the optical disc master exposure apparatus according to the present embodiment. In this figure, the same elements as those described above with reference to FIGS. 1 to 6 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0094]
In the first embodiment described above, the case where the spindle motor 110 side is moved up and down to perform the coarse focus processing is illustrated, but in this embodiment, as shown in FIG. 7, the focus drive system is moved up and down in the Z direction. Perform coarse focus processing.
[0095]
The base side of the coarse focus up-and-down mechanism 300 is a slider 320 that performs track feeding, and an upper portion corresponding to the optical system illustrated in FIG. The recording light is supplied / detected by the first laser introduction / detection system 330, and the auxiliary light is supplied / detected by the second laser introduction / detection system 340.
However, in the present embodiment, a falling mirror 139 fixed on the slider is provided between the first condenser lens (chromatic aberration correction lens) 142 and the objective lens 124, and this reflected light is applied to the objective lens 124. Incident.
[0096]
The movable part is a focus drive system (VCM 126, objective lens 124), and the objective lens 124 can be moved up and down within a range of several mm while the VCM focus drive displacement is kept at zero.
[0097]
That is, in the present embodiment, when the coarse focus drive system 300 is driven, the distance between the condenser lens 142 and the objective lens 124 varies. The position of the condensing lens 142 is fixed so that the distance between the condensing lens 142 and the objective lens 124 becomes a design value when rough focusing is performed on a glass master having a predetermined reference thickness.
[0098]
Hereinafter, the rough focus process in the present embodiment will be described. In the coarse focus process, as described above with reference to FIG. 4, the sum signal and the difference signal of the return light amount of the auxiliary light are monitored, and the coarse signal is adjusted so that the difference signal FE2 becomes the zero crossing position of the S-curve center. The vertical drive mechanism 300 positions the focus drive system (such as the objective lens 124) up and down. Thereafter, the coarse focus up-and-down mechanism 300 is fixed to the slider 320 by a lock mechanism (not shown). In applications such as optical disc master exposure, extremely precise track feed accuracy of nanometer accuracy is required. For this reason, it is necessary to increase the rigidity in the track feeding direction, and a fixed lock for the focus up-and-down mechanism is indispensable.
[0099]
At this time, if the thickness of the glass master plate 114 is the same as the predetermined reference thickness adopted at the time of adjusting the optical system, the shift amount d of the distance between the condenser lens 142 and the objective lens 124 becomes zero, which is the first embodiment. It will be exactly the same as the case. However, the individual difference in thickness of the glass master 114 is actually about 10 μm. For this reason, a deviation amount d corresponding to the thickness variation is generated, and the true focus error amount with respect to the recording light is the deviation amount expressed by the equation (1) even in the state where the coarse focus processing is performed with FE2 = 0. It exists to some extent depending on d. This is a significant difference from the first embodiment, and a new adjustment process is required between the coarse focus and the signal recording.
[0100]
FIG. 8 is a conceptual diagram showing a focus sensor calibration process for removing the offset error.
In the sensor calibration in the present embodiment, the first laser introduction / detection system 330 performs focus servo with the focus error signal FE1 generated by the return light of the recording light. Therefore, the sensor calibration is performed in the signal non-recording area of the master 114. The zero adjustment process will be described below with reference to FIG.
[0101]
The first focus sensor 210A shown in FIG. 8 is a focus sensor based on the return light of the recording light, and the FE1 signal is obtained from the PD (photodiode) difference signal of the astigmatism optical system as described above with reference to FIGS. To get. The second focus sensor 210B is a focus sensor using return light of auxiliary light, has the same configuration as that illustrated in FIG. 2, and has output characteristics as illustrated in FIG.
[0102]
First, immediately after the coarse focus processing in accordance with the thickness of the master disk 114, the focus servo is off, but the FE2 output is almost zero and the displacement of the VCM 126 is also zero. However, since there is an offset effect corresponding to the deviation from the reference value of the thickness of the master 114, the actual focus error amount is not zero. In this state, when the recording light is irradiated to the master 114 and the focus error signal FE1 due to the return light is monitored, the actual focus error amount is reflected in the FE1 output, and it can be confirmed that the FE1 is in an offset state.
[0103]
Next, when focus servo based on recording light is applied using FE1, FE1 becomes zero, but the FE2 output is offset in the reverse direction. The DC offset component of FE2 is output to the second focus sensor 210B as an output offset FE2.₀, The second focus sensor 210B can be zero-adjusted. However, since the master disk vertical movement R actually occurs as the master disk 114 rotates, the error E between FE1 and FE2 is obtained, and a low gain PI ( This DC offset component is extracted by an offset adjuster 215 combining a proportional integration operation and a low-pass filter so that FE2 matches the residual vibration of FE1.
[0104]
After zero adjustment of the second focus sensor 210B, FE2₀Is held at this value, and zero adjustment of the displacement sensor 128 illustrated in FIGS. 1 and 3 is also performed. The following shows the state variation relationship in each phase when the error of the thickness of the master disk 114 is Z0 and there is no vertical fluctuation of the master disk position due to the master disk rotation.
[0105]
Immediately after fixing the coarse focus mechanism (servo off):
d = −Z0 / (1 + k)
df = −Z0 · k / (1 + k)
ds = 0
FE2 = 0
[0106]
Before zero adjustment immediately after fine pull-in (first servo on):
d = −Z0
df = 0
ds = −Z0 · k / (1 + k)
FE2 = a ・ k ・ Z0
[0107]
After zero adjustment including the VCM displacement sensor (first servo on):
d = −Z0
df = 0
ds = 0
FE2 = 0
dso = Z0 · k / (1 + k)
FE20 = -a · k · Z0
[0108]
The sensor calibration processing is as described above. Thereafter, as described above with reference to FIG. 4, the switching processing to the focus servo by the FE2 signal and the recording light shielding processing are sequentially performed, and the auxiliary light focus servo (FE2 signal FB) remains. Then, the slider 320 is moved to the recording start position, and the process proceeds to the signal exposure recording process. The servo at the time of signal exposure recording can be performed in the same manner as described above with reference to FIG.
[0109]
As described above, as the second embodiment of the present invention, the coarse focus adjustment for dealing with the individual difference in the thickness of the master 114 has been described. In addition to the configuration described above with reference to FIGS. 7 and 8, as shown in FIG. 9, a method may be considered in which the movable portion for coarse focusing is integrated from the falling mirror 138 to the objective lens 124 and the VCM 126. In this way, it is possible to prevent the gap between the first condenser lens 142 and the objective lens 142 from being shifted due to the coarse focus. However, in the case of the configuration of FIG. 9, the mass of the movable part for coarse focusing is large, and a mechanism that copes with the mechanical load is required. From this viewpoint, the configuration illustrated in FIGS. 7 and 8 is more advantageous.
[0110]
(Third embodiment)
Next, as a third embodiment of the present invention, a configuration provided with a filter for removing noise included in the output of the displacement sensor and the like will be described.
[0111]
FIG. 10 is a conceptual diagram illustrating a control configuration of the focus servo according to the present embodiment. In the figure, the same elements as those described above with reference to FIGS. 1 to 9 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0112]
In this embodiment, the specific device configuration may be that of the first embodiment or the second embodiment, but as illustrated in FIG. 10, a high-pass filter 240 is provided in a focus servo configuration for correcting chromatic aberration. Yes.
[0113]
In the first embodiment, as described above with reference to FIG. 4, the displacement Z of the VCM 126 is detected by the displacement sensor 128 as expressed by the equation (2). However, in reality, the output of the sensor 128 often includes the low-pass characteristics of the sensor itself, sensor noise, and the like, and even if it approximates the necessary band, it includes the sensor noise w as shown in the following equation. Often output.
[0114]
ds = b · Z + w (4)
[0115]
In particular, when a capacitive displacement meter is used as a displacement sensor, low-frequency noise generally called “drift” that is equal to or lower than the rotational frequency of the master 114 may occur. Since this drift level is extremely small, about 0.2 μm in terms of displacement, it does not affect the focus error so much as seen from the coefficient of the equation (1), but it causes a difference between the inner and outer circumferences of the master recording part, This is a factor that reduces the overall stability of the recording pits.
[0116]
Therefore, as illustrated in FIG. 10, a high-pass filter 240 for cutting off the low frequency is provided to remove such drift. Here, since the frequency of the vertical movement fluctuation accompanying the rotation of the master disk is composed of the rotation frequency of the SPM 110 and its harmonic vibration, the cutoff frequency of the high-pass filter 240 may be less than the rotation frequency of the SPM 110.
[0117]
By providing the high-pass filter 240 in this way, even if a noise component is included in the output of the sensor 128 as illustrated in the equation (4), it can be effectively removed, and the focus sensor zero is sufficient in the initial stage. If the adjustment is made, the true focus error can be estimated as shown in the equation (3), and stable exposure recording can be ensured over the entire surface of the master 114 with high accuracy.
[0118]
(Fourth embodiment)
Next, a focus servo system that estimates the displacement of the objective lens without using a sensor will be described as a fourth embodiment of the present invention.
[0119]
FIG. 11 is a conceptual diagram showing the focus servo control system of this embodiment. Also in this figure, the same elements as those described above with reference to FIGS. 1 to 10 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0120]
The present embodiment is characterized in that the VCM drive displacement sensor 128 illustrated in FIG. Here, the focus sensor 210 detects the focus error amount df. Basically, the drive displacement Z of the VCM 126 cannot be truly measured unless the displacement sensor 128 illustrated in FIG. 3 is provided. .
[0121]
However, since the initial state in which no current flows through the coil of the VCM 126 is known and the influence of the disturbance applied to the slider portion of the lens holder 125 is generally negligibly small, the displacement Z of the VCM 126 is estimated from the drive current. Is possible.
[0122]
The frequency characteristics of the focus drive system have higher-order effects such as sub-resonance and second resonance in the high-frequency range, but in principle can be approximated by a quadratic state function such as the following equation.
[0123]
[Expression 1]

[0124]
Since the initial state of the state variable is known, using this as a nominal model, the estimated value of the VCM drive displacement Z with respect to the focus drive command is open-estimated as in the following equation.
[0125]
[Expression 2]

[0126]
By calculating this VCM drive displacement estimated value using the nominal VCM model 250 and giving it to the FE shift corrector 220, the true focus error of the recording light can be estimated as in the case described above with respect to the equation (3). As a result, a high-performance focus servo that is not affected by the offset caused by chromatic aberration similar to the first embodiment can be realized without using the sensor 128.
[0127]
Here, the nominal VCM model may actually be realized by digital arithmetic processing by a computer. Therefore, the FE shift corrector 220, the control 230, and the nominal VCM model 250 are collectively rewritten into a high-order IIR filter expression, the A / D input of FE2 is applied to the IIR filter, and this output is used as the output u of the controller 230. D / A output is also possible.
[0128]
The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. For example, the wavelength of the recording light or auxiliary light, the type of light source, the specific configuration of the optical system, the specific configuration of the master exposure apparatus, etc. are appropriately selected and changed by those skilled in the art, and the effects of the present invention are similarly achieved. Obtainable.
[0129]
The present invention can be widely applied to a focus servo using two types of wavelength light, and the object is not limited to an optical disk master exposure apparatus, but an additional type or rewritable type high density optical disk recording apparatus. The same applies to the above.
[0130]
Further, the present invention can be similarly applied to a reading device having a plurality of wavelength lights.
[0131]
【The invention's effect】
As described above in detail, according to the present invention, even if focus servo is performed using auxiliary light with low resist exposure sensitivity in a focus servo system such as a master exposure apparatus using an objective lens having chromatic aberration, the objective lens The influence of the offset error on the focus error signal due to the master follow-up operation is eliminated, and the true focus error for the recording light can be reduced. As a result, the influence of the recording pit master rotation phase is eliminated, and sufficient recording stability can be ensured at a high recording density, resulting in a great industrial advantage.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram illustrating the main configuration of an optical disc master exposure apparatus according to a first embodiment of the invention.
FIG. 2 is a graph showing output signal characteristics of the photodetector 154;
FIG. 3 is a conceptual diagram illustrating the configuration of a focus drive system of the first embodiment.
FIG. 4 is a conceptual diagram illustrating a control block of a focus servo system according to the first embodiment.
FIG. 5 is a graph illustrating the relationship between a focus error amount df of recording light and FE2.
FIG. 6 is a conceptual diagram illustrating an optical path of auxiliary light under typical conditions. You
FIG. 7 is a conceptual diagram showing a main configuration of an optical disc master exposure apparatus according to a second embodiment of the present invention.
FIG. 8 is a conceptual diagram illustrating a focus sensor calibration process for removing an offset error.
FIG. 9 is a conceptual diagram showing another configuration for coarse focus adjustment.
FIG. 10 is a conceptual diagram showing a control configuration of a focus servo according to a third embodiment of the present invention.
FIG. 11 is a conceptual diagram illustrating a focus servo control system according to a fourth embodiment of the present invention.
FIG. 12 is a conceptual diagram showing a configuration of a main part of a master exposure apparatus studied by the inventor in the course of reaching the present invention.
FIG. 13 is a conceptual diagram illustrating the configuration of a main part of a master exposure apparatus when a method of moving the entire chromatic aberration correcting optical system is employed.
[Explanation of symbols]
110 Spindle motor
112 turntable
114 Master
120 Recording light
122 Dichroic prism
124 Objective lens
125 Lens holder
126 Voice coil motor (VCM)
127 counterweight
128 Displacement sensor
129 Parallel spiral spring
130 Second laser light source
132 Magnifying optical system
134 Polarizing beam splitter (PBS)
136 1/4 wave plate
138 mirror
140 Aperture stop
142 First condenser lens (chromatic aberration correction lens)
150 Second condenser lens
152 Cylindrical lens
154 photodetector
210 Focus sensor
215 Offset adjuster
220 FE shift corrector
230 Controller
240 High-pass filter
250 Nominal VCM model

Claims (10)

A first light having a first wavelength and a second light condensed via a condenser lens having a second wavelength different from the first wavelength and movable with respect to the irradiated object ; the incident on the irradiated surface of the irradiated body through an objective lens having and chromatic aberration movable with respect to said condenser lens and the irradiation object,
After positioning the condenser lens with respect to the irradiated object, the second focus error information obtained from the return light of the second light from the irradiated surface and the relative relationship between the condenser lens and the objective lens Based on the first focus error information corresponding to the focus error of the first light generated from the position shift, the displacement of the objective lens so that the first light is focused on the irradiated surface. focus servo system, characterized that you correct the deviation of the focusing condition of the first and second light by controlling the   2. The focus servo system according to claim 1, wherein the shift of the focusing condition between the first and second lights is an offset caused by the displacement of the objective lens.   3. The focus servo system according to claim 1, wherein the displacement of the objective lens is detected by a sensor to estimate a shift in focusing condition between the first and second lights.   4. The focus servo system according to claim 3, wherein high-pass filter processing is performed on the output of the sensor.   3. The focus servo system according to claim 1, wherein the displacement is estimated based on a drive signal given to a drive source for displacing the objective lens. A recording light having an exposure wavelength for recording on the optical disc master, the focusing of an auxiliary light condensed via the movable condenser lens to the optical disc master having a different wavelength from that of the exposure wavelength lens and through an objective lens having and chromatic aberration movable with respect to the optical disc master is incident on the illuminated surface of the optical disc master, an optical disk by driving Dogen exposed recorded on the optical disc master while displacing the objective lens master An exposure apparatus,
After positioning the condensing lens with respect to the optical disc master, the second focus error information obtained by the return light of the auxiliary light from the irradiated surface and the deviation of the relative positions of the condensing lens and the objective lens The displacement of the objective lens is controlled by the drive source so that the recording light is focused on the irradiated surface based on first focus error information corresponding to the focus error of the recording light generated from Thus, an optical disk master exposure apparatus comprising control means for correcting a shift in focusing condition between the recording light and the auxiliary light .   7. The optical disk master exposure apparatus according to claim 6, wherein the shift of the focusing condition between the recording light and the auxiliary light is an offset caused by the displacement of the objective lens. A sensor for detecting the displacement of the objective lens;
8. The optical disc master exposure apparatus according to claim 6, wherein a deviation in focusing conditions of the recording and auxiliary light is estimated based on an output of the sensor.   9. The optical disk master exposure apparatus according to claim 8, wherein the output of the sensor is subjected to a high pass filter process.   The optical disk master exposure apparatus according to claim 6 or 7, wherein the displacement of the objective lens is estimated based on a drive signal applied to the drive source.

Images