JP3813932B2 - Dry etching method for compound semiconductor multilayer film - Google Patents

Description

【００２８】
ｎ_c’およびｎ_env.は、それぞれ再成長層および共振器部の外側の屈折率である。上式から得られるｎ_GaAs’およびｎ_AlAs’を用いると、実効反射率Ｒ’は、下記式（５）で表わされる。
【００２９】
【数２】

【００３０】
ここで、Ｒ_top およびＲ_bottomは、それぞれ上部ＤＢＲおよび下部ＤＢＲの反射率であり、それぞれ下記式（６）および（７）で表わされる。
【００３１】
【数３】

【００３２】
【数４】

実効反射率Ｒ’より、閾利得ｇ_thは下記式（８）で求められる。
【００３３】
【数５】

したがって、レーザーが発振するための条件は、下記式（９）で表わされる。
【００３４】
【数６】

【００３５】
ここで、ｇ_s は活性層の飽和利得、α_i は共振器の内部損失、ｔ_a は活性層の厚さである。理想的な場合、すなわち共振器の内部損失α_i が０のとき、式（９）の右辺は第２項のみとなり、上述の式（１）が導かれる。
【００３６】
図３には、円柱の直径ｄが１μｍ，２μｍおよび５μｍの場合のそれぞれについて、再成長膜厚ｔと閾利得との関係を示している。なお、直径５μｍは、通常の面発光レーザーと呼ばれるサイズであり、図３から、直径が５μｍの場合には、閾利得は、再成長膜厚ｔにほとんど影響されないことがわかる。
【００３７】
一方、微小共振器の効果が顕著となる１μｍ径においては、再成長膜厚が増加するにつれて、閾利得が急激に増大している。ここで、飽和利得ｇ_s ＝５０００ｃｍ^-1とすると、再成長膜厚が０．２１μｍ以上で発振が不可能となるほどに閾利得が増大している。
【００３８】
このように、微小共振器レーザーの共振器サイズの領域では、共振器を包囲する埋め込み層の膜厚を薄くすることが、低閾値発振を実現する上で本質的に重要であることがわかる。
【００３９】
なお、参考例の半導体微小共振器発光素子においては、再成長膜の膜厚は、具体的には、もっとも厚い部分の厚さが、活性層のバンドギャップ相当光の再成長薄中での波長の３倍以下であることが好ましく、より好ましくは２倍以下である。
【００４０】
図１に示した半導体微小共振器発光素子１０の場合には、図３に示したグラフにおいて、飽和利得ｇ_s＝４０００ｃｍ^-1とすると、再成長膜の膜厚の最大値ｔ_maxは０．４７μｍとなり、この値は、再成長膜１中での波長の約１．６倍に相当する。
【００４１】
上述した半導体微小共振器発光素子は、例えば、以下のようにして製造することができる。
【００４２】
図５に、参考例の発光素子の製造工程を表わす断面図を示す。
【００４３】
まず、図５（ａ）に示すように、基板７上に、第１のＤＢＲ３、スペーサー層５ｂ、活性層４、スペーサー層５ａ、および第２のＤＢＲ２を成長させる。各層の成長に当たっては、例えば、分子ビームエピタキシャル成長法（ＭＢＥ）、および有機金属気相成長法（ＭＯＣＶＤ）等を用いることができる。さらに、第２のＤＢＲ２の表面には、化学気相成長法（ＣＶＤ）により二酸化珪素膜（ＳｉＯ₂ ）９を堆積する。
【００４４】
次に、紫外線露光プロセスまたは電子線露光プロセスを用いて、ＳｉＯ₂ 膜９上に円形のレジストパターンを形成し、四フッ化炭素／水素混合ガスを用いた反応性イオンエッチング（ＲＩＥ）により、ＳｉＯ₂ 膜９にレジストパターンを転写する。
【００４５】
酸素プラズマによりレジストを除去した後、図５（ｂ）に示すように、エピタキシャル成長させた半導体層を基板７までエッチングして円柱構造を作製する。ここでは、例えば、塩素ガスを用いた反応性イオンビームエッチング（ＲＩＢＥ）を用いることができる。
【００４６】
続いて、アンモニア系のエッチング液を用いて活性層側壁を１０ｎｍ程度エッチングして、ＲＩＢＥの際に側面に生じたエッチングダメージ層を除去する。さらに、五硫化二燐（Ｐ₂ Ｓ₅ ）を溶解した硫化アンモニウム溶液（(ＮＨ₄)₂Ｓ）に一分間浸すことによって、側面にパッシベーション膜を形成した後、純水で洗浄する。その後、ＭＯＣＶＤ法を用いて、ＳｉＯ₂ 膜９上には成長しない選択成長のモードで、円柱側壁にＡｌ_0.5 Ｇａ_0.5 Ａｓ薄膜１を成長させる。
【００４７】
次に、Ａｌ_0.5 Ｇａ_0.5 Ａｓ薄膜１が側面に形成された円柱をポリイミド８で埋め込み、酸素プラズマにより円柱頂部を露出させ、紫外線露光プロセスとリフトオフによりｐ電極６を形成する。
【００４８】
最後に裏面を研磨し、光出力部を避けてｎ電極（図示せず）を形成し、光出力部には減反射膜（図示せず）を堆積することによって、参考例の半導体微小共振器発光素子１０が得られる。
【００４９】
なお、参考例の半導体微小共振器発光素子は、以下のような変更が可能である。
【００５０】
第１の変更例を図６に示す。図６に示す発光素子の基本構造は、円柱共振器が絶縁体に完全に埋め込まれていない以外は、図１に示したものと同様である。すなわち、共振器の周囲を包囲する半導体薄膜１は、さらに、ＳｉＯ₂，ＳｉＮ_x，Ａｌ₂Ｏ₃などの絶縁体１２からなる薄膜によって柱状に囲まれている。
【００５１】
この半導体微小共振器発光素子１１においては、絶縁体１２の外側には、電極兼反射膜１３が設けられている。このように、活性層４の周囲に絶縁体を介して反射膜１３を配置することによって、活性層４から横方向に放射された自然放出光が、再び活性層に吸収されるという効果が得られるので、閾値をさらに低減することが可能となる。
【００５２】
以上の例においては、共振器の側面を包囲する再成長膜１として、単層のＡｌＧａＡｓを用いたが、屈折率の異なる複数の再成長膜により構成した多層膜を用いることもできる。さらに、厚さ方向の屈折率を連続的に変化させた再成長膜を用いてもよい。
【００５３】
かかる構造の一例を、図７に例を示す。図７（ｃ）に示すように、円柱の中心軸から半径方向の距離をｒとし、再成長膜の外周面までの距離をａ、再成長膜の膜厚をｔとした。
【００５４】
図７（ａ）には、屈折率の異なる２種類の半導体薄膜により再成長膜１を構成した場合の円柱の中心からの距離ｒと屈折率ｎとの関係を示す。この場合には、ＤＢＲおよび半導体薄膜の屈折率は、次の関係にある。
【００５５】
ｎ_in＜ｎ_out ＜ｎ_DBR
ｎ_DBR ，ｎ_inおよびｎ_out は、それぞれ、ＤＢＲの平均屈折率、内側薄膜の屈折率、および外側薄膜の屈折率を表わす。
【００５６】
図７（ｂ）には、再成長膜１の屈折率は、外側に向けて徐々に小さくなっている場合の例を示す。前述の図７（ａ）および（ｂ）のいずれの構造も、単層の再成長膜の場合に比べて、モードのＤＢＲ部分への閉じ込め係数がより大きくなるので、単層で再成長膜を形成する場合よりも、膜厚ｔを大きくすることができる。
【００５７】
さらに、共振器の断面（図８（ａ）中のＡ−Ａ’面）の形状は、何等限定されるものではなく、図８（ｂ）に示すような種々の形状とすることができる。また、再成長膜１を形成した後の断面も、任意形状とすることができる。すなわち、共振器部の側面に形成される再成長膜１の膜厚は、上述した範囲内であれば、その形状は限定されない。
【００５８】
以上の例では、基板としてｎ型の半導体を用いた例について示したが、もちろんｐ型基板を用いることもでき、この場合には、基板上に積層される各半導体層の導電型を反転すればよい。また、活性層の構造は、量子井戸構造に制限されるものではなく、量子細線、および量子箱構造を用いる事も可能である。さらに、材料系はここで述べたＡｌＧａＡｓ，ＩｎＧａＡｓ系のみならず、ＩｎＧａＡｓＰ，ＩｎＧａＡｌＰ系の材料も適用することができる。
【００５９】
また、分布反射膜は全て半導体のみで構成されている必要はなく、金属あるいは誘電体と半導体とを組み合わせて構成することもできる。その他、種々変形して実施することができる。
【００６０】
（実施例）
以下、本発明によるドライエッチング方法について、実施例を示して具体的に説明する。
【００６１】
まず、塩素ガスを用いた反応性イオンビームエッチング（ＲＩＢＥ）によるエッチングを例に挙げて説明する。使用する装置の概略図を図９に示す。図９に示すように、エッチング装置３０は、ＥＣＲ（Ｅｌｅｃｔｒｏｎ Ｃｙｃｌｏｔｒｏｎ Ｒｅｓｏｎａｎｃｅ）イオン源３２とエッチング室３５との二室から構成されており、二室の間にはイオン引き出し電極３３が設置されている。エッチング室３５には、ヒータ３９と熱電対４０が埋め込まれた基板ホルダー３８が設置されており、基板加熱が可能である。また、エッチング中の基板温度は、エッチング室３５の上方に配置されたパイロメータ３６により測定し、その測定値に基づいて加熱用ヒータ３９のパワーを制御する。
【００６２】
エッチングに当たっては、まず、基板３７をホルダー３８に取り付けてエッチング室３５内に設置し、所定の圧力までエッチング室を排気する。試料基板の温度が安定し、エッチング室の圧力が所定の値以下に達したところで、ＥＣＲイオン源３２に接続されたガス導入管３１より塩素ガスを導入し、プラズマを発生させる。このプラズマ中からイオン引き出し電極３３によってイオンを引き出す。この間、エッチング室内の圧力は、塩素ガスの流量をマスフローコントローラで制御することにより調節する。
【００６３】
次いで、ＥＣＲイオン源とエッチング室の間に設置されたシャッター３４を開けることにより、イオンを基板に照射してエッチングを開始する。なお、イオンの照射の開始にともないエッチング基板温度が上昇するため、パイロメータ３６で測定した基板温度をヒータ電源にフィードバックして、ヒータのパワーを調節し基板温度を一定に保持する。
【００６４】
プラズマの発生とともに生成された活性な中性分子は、拡散により基板表面へ到達する。エッチング底面においては、イオン照射による反応性スパッタに起因したエッチングが主に生じるが、エッチング側壁にはイオン照射はほとんどなく、中性分子と基板との反応によりエッチングが進行する。
【００６５】
前述の装置を用いて、半導体レーザ、ＬＥＤ、半導体光増幅器、および半導体光変調器などのブラッグ反射膜として用いられる多層膜のエッチングを行なった。この実験結果を、以下の詳細に説明する。
【００６６】
基板としては、ＧａＡｓ基板上に分子線エピタキシー法により成長したＡｌＡｓとＧａＡｓ１５対の多層膜の基板を用いた。この基板上に、ＣＶＤ法を用いてＳｉＯ₂ 膜を成膜した後、通常のフォトリソグラフィ法と反応性イオンエッチング（ＲＩＥ）により加工して、エッチングマスクとしての断面円形のＳｉＯ₂ 膜２１を、図１０に示すように形成した。
【００６７】
先に説明した装置のエッチング室３５内を５×１０^-6ｔｏｒｒまで真空にした後、上述の手順で基板のエッチングを行なった。なお、本実験例では、エッチング条件は、マイクロ波パワー２００Ｗ、イオン加速電圧４００Ｖ、塩素ガス圧０．２５〜１．２ｍｔｏｒｒとし、基板温度を３５〜２３０℃の範囲内で変化させて、エッチング温度と得られるエッチング側壁の形状との関係を調べた。
【００６８】
その結果、エッチング温度が１００℃以下の場合には、サイドエッチングはほとんど生ぜず、エッチングマスク直下にオーバーハングは認められなかった。なお、エッチング側壁には、図１５に示したようなＡｌＡｓ層２３が凸、ＧａＡｓ層２２が凹となる凹凸が生じていた。
【００６９】
この凹凸に関して、本発明者らは次のように考察した。すなわち、この温度範囲では、揮発性のエッチング反応生成物であるＧａＣｌ₃ の蒸気圧がＡｌＣｌ₃ の蒸気圧より約一桁以上高い。エッチング側壁ではイオン照射効果が少ないものの、イオンの散乱やエッチング底面からの反跳によるアシスト効果、および局所的温度上昇などによりＧａＡｓ層のエッチングが優先的に進行した。一方、ＡｌＡｓ層は極めて酸化されやすいため、イオン照射効果の小さいエッチング側壁ではエッチングが進行しない。これらの結果としてＡｌＡｓ層が凸、ＧａＡｓ層が凹の凹凸が側壁に形成された。
【００７０】
エッチング温度を１００℃〜１３０℃とした場合では、サイドエッチングが発生するもののほぼ垂直なエッチング側壁が得られた。しかし、側壁には依然としてＡｌＡｓ層が凸、ＧａＡｓ層が凹となる凹凸が認められた。
【００７１】
さらにエッチング温度を高くし１３０℃〜１７０℃の温度範囲では、サイドエッチング量がよりいっそう大きくなり、図１１に示したように、マスク２１の直下にはオーバーハング２４が生じた。この場合、エッチング側壁は、ほとんどの領域にわたり垂直であり、かつＡｌＡｓ層２３とＧａＡｓ層２２との凹凸がない平滑なエッチング側壁が得られた。これは、このエッチング温度の範囲では、ＡｌＡｓ層２３のサイドエッチング速度とＧａＡｓ層２２のサイドエッチング速度がほぼ等しくなったものと考えられる。
【００７２】
より高温の１７０℃〜１９０℃ではさらにサイドエッチング量が大きくなり、ＡｌＡｓ層のサイドエッチング速度がＧａＡｓ層のサイドエッチング速度を上回って、ＡｌＡｓ層２３が凹、ＧａＡｓ層２２が凸状の凹凸が生じはじめる。しかし、エッチング側壁はまだほぼ垂直であり、デバイスへの適用には支障のない程度であった。
【００７３】
１９０℃以上では側壁は大きくくびれ、垂直なエッチング側壁は得られなかった。以上の実験結果から、本発明者らは、エッチング側壁のサイドエッチングを利用して、垂直な側壁を得ることを見出だした。なお、エッチング温度とサイドエッチング量（Ｒ₁ ／Ｒ₂ ）との間に、図１２に示すような関係が得られた。ここで、Ｒ₁はサイドエッチングの深さであり、Ｒ₂は垂直方向のエッチング深さである。すなわち、サイドエッチング速度は、エッチング温度を上昇させるにしたがって増加するという温度依存性がある。サイドエッチング量は、参考例で述べたマイクロキャビティ側壁への再成長膜の膜厚ｔと同程度であることが好ましい。すなわち、（Ｒ₁／Ｒ₂）が、ｔ／ｈにほぼ等しく、かつ
（ｔ／ｈ）＜（ｔ_max ／ｈ）
の関係を満たせばよい。ここで、ｈは、マイクロキャビティの高さである。例えば、参考例に述べた直径２μｍのマイクロキャビティでは、Ｒ₁ ／Ｒ₂ は、約０．０８以下であることが好ましい。
【００７４】
しかしながら、実験結果で示されたように、エッチング温度が１７０℃を越えると、ＡｌＡｓ層が凹となる凹凸が生じるので、本発明の目的を達成できず、一方、１３０℃未満では、ＡｌＡｓ層が凸となる凹凸が生じてしまう。
【００７５】
なお、エッチング生成物であるＧａＣｌ₃ とＡｌＣｌ₃ との蒸発速度の比が１であれば、平滑なエッチング側壁が得られる。本発明者らは、上述のサイドエッチング量に及ぼす温度の影響、および垂直で平滑なエッチング側壁が得られるという理由に基づいて、基板の温度範囲を、反応生成物の蒸気圧の比（Ｐ（ＧａＣｌ₃ ）／Ｐ（ＡｌＣｌ₃ ））が０．２以上１．５以下となる範囲に限定した。
【００７６】
特に、蒸気圧の比（Ｐ（ＧａＣｌ₃ ）／Ｐ（ＡｌＣｌ₃ ）の範囲が０．２以上１以下となる基板温度においてエッチングを行なうことにより、サイドエッチングを発生させエッチングマスク直下にオーバーハング部を形成することができ、同時に、垂直でかつ平滑なエッチング側壁を得ることができる。
【００７７】
また、蒸気圧の比（Ｐ（ＧａＣｌ₃ ）／Ｐ（ＡｌＣｌ₃ ））の範囲が１以上１．５以下となる基板温度においてエッチングを行なった場合には、サイドエッチングが大きくなるため、オーバーハング部の形成というよりも、マスクサイズよりも微細なパターンサイズの多層膜を加工することを目的に使用することができ、しかも平滑なエッチング側壁が同時に得られる。
【００７８】
以上の実験例をもとに、本発明のエッチング方法をマイクロキャビティレーザの作製プロセスに適用した。図１３を参照して、このプロセスを説明する。
【００７９】
ここで用いたウエハー４３は、図１３（ａ）に示すようにＧａＡｓ基板４４上に、１８．５対の下部ＡｌＡｓ／ＧａＡｓＤＢＲ４５、ＧａＡｓスペーサ層４６、および１５対の上部ＡｌＡｓ／ＧａＡｓＤＢＲ４８をＭＢＥにより成長させたものであり、スペーサ層４６の中央には、３層のＩｎＧａＡｓからなる量子井戸活性層４７が形成されている。
【００８０】
さらに、上部ＤＢＲ４８の上には、エッチングマスクとしての断面円形のＳｉＯ₂ 膜４９が形成されている。このエッチングマスクの直径は、１〜１０μｍとした。
【００８１】
本実施例においては、エッチング条件はマイクロ波パワー２００Ｗ、イオン加速電圧４００Ｖ、エッチングガスとしては塩素ガスを用い、塩素ガス圧０．４５ｍｔｏｒｒとして、前述の装置を用いてエッチングを行なった。なお、エッチング温度は、上述の実験例に示した結果に基づいて１５０℃とした。
【００８２】
その結果、図１３（ｂ）に示すように、エッチングマスクの直下には、サイドエッチングによりオーバーハング部５０が形成されており、このオーバーハング部より下のほとんどの領域は垂直であった。しかも、凹凸がなく、表面が極めて平滑なエッチング側壁が形成された。したがって、本発明の方法を用いたエッチングにより得られた円柱構造マイクロキャビティの光学的散乱損失は、従来のエッチング法により作製したものよりも格段に低減することができた。
【００８３】
前述のようにサイドエッチングを利用しているので、本発明のドライエッチング方法を適用して得られた円柱構造マイクロキャビティの直径は、マスクサイズより小さくすることができる。例えば、直径１μｍのエッチングマスクを用いた場合には、垂直な側壁を有する部分の直径が０．８μｍという微細な円柱構造のマイクロキャビティを作製することができた。なお、エッチング温度を１７０〜１９０℃の間に設定して、サイドエッチング量を増やすことによって、さらに微細な円柱構造を得ることができる。
【００８４】
通常のパターニング法とドライエッチング法とを用いた場合では、半導体多層膜を１μｍ以下の直径に加工するのは極めて困難である。本発明のエッチング法では、サイドエッチングを積極的に利用し、この部分のエッチング量をエッチング温度で制御することによって、１μｍ以下の直径の制御性、再現性を著しく向上させることができる。
【００８５】
さらに、活性層表面でのキャリアの再結合速度を低減するために、図１３（ｃ）に示すように、円柱構造マイクロキャビティの側壁へＡｌＧａＡｓ薄膜５１を再成長させた。なお薄膜の形成に当たっては、ＭＯＣＶＤによる選択成長法を用いた。先に図１３（ｂ）で説明したように、本発明のエッチング方法を用いたことによって、ＳｉＯ₂ マスク直下にオーバーハング部が形成されている。エッチングマスクの直下にオーバーハング部が形成されていない場合には、図１６に示すように、周囲にＡｌＧａＡｓ成長させる際にＳｉＯ₂ マスク端に異常成長部８１が観察されることがある。本発明のエッチング方法では、オーバーハング部５０を形成しているので、このような異常成長を抑制し、良好な再成長を行なうことができた。
【００８６】
本発明のエッチング法では、積極的にサイドエッチングを引き起こしているので、ドライエッチングにより導入されるダメージ層を除去するという効果も有する。また、円柱構造マイクロキャビティの側壁に半導体薄膜を再成長させた場合には、従来のエッチング法により製造したマイクロキャビティと比較して、活性層表面のキャリアの再結合速度が格段に減少した。
【００８７】
以上実施例で示したように、本発明は半導体多層膜のドライエッチングにおいて、サイドエッチングを積極的に利用しているので、多層膜の各層のサイドエッチング速度が等しくなるよう、エッチング速度を設定し、垂直で平滑なエッチング側壁を得ることができる。
【００８８】
なお、本実施例では、マイクロキャビティレーザの作製を例に挙げて、本発明のドライエッチング方法を説明したが、本発明のエッチング法は他のＬＥＤ、光増幅器、光変調器など、半導体多層膜を用いた全てのデバイスの製造プロセスにも適用することが可能である。
【００８９】
また、エッチング方法としてとしては、反応性イオンエッチング（ＲＩＥ）、ケミカルアシステッドイオンビームエッチング（ＣＡＩＢＥ）などの他のドライエッチング方法にも適用できる。
【００９０】
さらに、本発明のドライエッチング方法は、Ａｌを含む化合物半導体膜と、Ｇａを含む半導体膜とを交互に積層した任意の構成の多層膜に適用することができ、その構成は何等限定されるものではない。例えば、Ａｌ_x Ｇａ_1-x Ｖ／Ａｌ_y Ｇａ_1-y Ｖ（０≦ｘ＜ｙ≦１）（Ｖ＝Ｎ，Ａｓ，Ｐ，Ｓｂ）、Ａｌ_x Ｇａ_1-x Ｖ’_z Ｖ”_1-z ／Ａｌ_y Ｇａ_1-y Ｖ’_z Ｖ”_1-z （０≦ｘ＜ｙ≦１、０≦ｚ≦１）（Ｖ’，Ｖ”＝Ｎ，Ａｓ，Ｐ，Ｓｂ）を満たす組み合わせの多層膜であれば、本発明を適用して上述と同様の効果を得ることができる。
【００９１】
【発明の効果】
以上詳述したように、本発明によれば、装置のコストおよび工程を増加させることなく、垂直で凹凸のない平滑なエッチング側壁を得ることができる。したがって、デバイスの特性を大幅に向上させることが可能であり、かかるドライエッチング方法は、半導体レーザー、ＬＥＤ、半導体光増幅器、半導体光変調器等、化合物半導体多層膜を有する全てのデバイスの製造に有効である。
【図面の簡単な説明】
【図１】 参考例の半導体微小共振器発発光素子の一例の概略構成を示す図。
【図２】 共振器モードの半導体埋め込み領域へのモードの滲み出しを説明する図。
【図３】 再成長膜厚と閾利得との関係を示すグラフ図。
【図４】 多層反射膜側壁からの光散乱損失を説明する図。
【図５】 参考例の半導体微小共振器発光素子の製造工程の一例を示す断面図。
【図６】 参考例の半導体微小共振器発光素子の他の例を示す断面図。
【図７】 参考例の半導体微小共振器発光素子の他の例を示す図。
【図８】 参考例の半導体微小共振器発光素子の断面構造の変形例を示す図。
【図９】 本発明に使用され得るエッチング装置の概略を示す図。
【図１０】 実験例で用いた基板の断面図。
【図１１】 本発明によるエッチングを行った後の半導体多層膜の断面図。
【図１２】 サイドエッチング量と温度依存性との関係を示すグラフ図。
【図１３】 本発明によるエッチング法を用いたマイクロキャビティレーザの作製工程を示断面図。
【図１４】 従来の面発光レーザーの断面図。
【図１５】 従来のエッチング法によりエッチングを行った半導体多層膜の断面図。
【図１６】 従来のエッチング法により製造されたマイクロキャビティレーザの断面図。
【符号の説明】
１…Ａｌ_0.5 Ｇａ_0.5 Ａｓ薄膜，２…ｐ−ＧａＡｓ／ＡｌＡｓ−ＤＢＲ，３…ｎ−ＧａＡｓ／ＡｌＡｓ分布ブラッグ反射膜（ＤＢＲ），４…ｉ−Ｉｎ_0.2 Ｇａ_0.8 Ａｓ歪み量子井戸活性層，５ａ…ｐ−Ａｌ_0.2 Ｇａ_0.8 Ａｓスペーサ層，５ｂ…ｎ−Ａｌ_0.2 Ｇａ_0.8 Ａｓスペーサ層，６…Ｃｒ／Ａｕ−ｐ電極，７…ｎ−ＧａＡｓ基板，８…絶縁体，９…絶縁体，１０…半導体微小共振器発光素子，１１…半導体微小共振器発光素子，１５…多重反射膜，１６…活性層からの発光，１７…散乱光，２１…エッチングマスク，２２…ＧａＡｓ層，２３…ＡｌａＡｓ層，２４…オーバーハング部，３１…反応性ガス導入管，３２…ＥＣＲイオン源，３３…イオン引出電極．３４…シャッター，３５…エッチング室，３６…パイロメータ，３７…半導体基板，３８…基板ホルダー，３９…ヒータ，４０…熱電対，４１…ヒータ電源，４３…ウエハー，４４…ＧａＡｓ基板，４５…ＧａＡｓ／ＡｌＡｓ下部ＤＢＲ，４６…ＧａＡｓスペーサ層，４７…ＩｎＧａＡｓ活性層，５８…ＧａＡｓ／ＡｌＡｓ上部ＤＢＲ，４９…ＳｉＯ₂ マスク，５０…オーバーハング部，５１…ＡｌＧａＡｓ再成長膜，６０…面発光レーザ，６１…基板，６２…第１の多層反射鏡，６３…スペーサー層，６４…活性層，６５…発光部，６６…第２の多層反射鏡，６７…電極，６８…面発光レーザー，６９…絶縁膜，７０…電極，７１…面発光レーザ，７２…埋め込み層，７３…電極，７４…基板，７５…下部ＤＢＲ，７６…スペーサー層，７７…活性層，７８…上部ＤＢＲ，７９…ＳｉＯ₂ マスク，８０…再成長層，８１…異常成長部。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device used for optical communication, optical interconnection, optical information processing, and the like, and particularly to a low power consumption LED or semiconductor laser suitable for high-density integration, a light receiving device, and a modulator.
[0002]
The present invention also relates to a dry etching method for producing a compound semiconductor device, particularly a semiconductor laser, LED, semiconductor optical amplifier, semiconductor optical modulator, etc. using a compound semiconductor multilayer film with good reproducibility.
[0003]
[Prior art]
In recent years, in the fields of optical communication, optical interconnection, optical information processing, and the like, demands for large capacity and high speed by parallel processing are increasing. In order to realize parallel processing, it is expected that LEDs, semiconductor laser arrays, light receiving elements, and modulator arrays integrated one-dimensionally or two-dimensionally are essential. As a light source capable of meeting such demand, a so-called vertical cavity surface emitting laser capable of extracting output light in a direction perpendicular to a substrate has been vigorously developed.
[0004]
As shown in FIG. 14, this type of laser typically has a lower multilayer reflector 62, a light emitting layer 65 including an active layer and a spacer layer (cladding layer), and an upper multilayer reflector 66 on a substrate 61. Have a structure of being sequentially stacked. In such a surface-emitting laser, in order to control the transverse mode, a part of the upper reflective film 66 or the part up to the lower reflective film 62 is processed into a columnar shape, and the refractive index is changed in the lateral direction, so that the lowest order is achieved. It aims at stable oscillation in mode. Currently, the typical lateral size of these devices is (30 μm)²To (5μm)²The oscillation threshold is about several mW to several hundred μW.
[0005]
When a large number of elements are integrated and used, the restriction on power consumption per element becomes stricter than in the case of a single element. For example, taking the surface emitting laser described above as an example, the power consumption is about 1 mW, so the power is 10 W for a 100 × 100 array and 1000 W for a 1000 × 1000 array. Thus, when the power consumption is large, not only good consistency with an electronic device with advanced power saving is obtained, but also high density integration itself becomes impossible due to a thermal problem.
[0006]
Recently, a laser based on a new principle called a microcavity laser has been proposed (Tetsuro Kobayashi et al .; Applied Physics Society Lectures 29a-B-6 (1982), H. Yokoyama and SD Bronson; J. Appl.Phys.66, p4801 (1989)). This laser is characterized by a very small resonator structure. Specifically, the size in the lateral direction is (oscillation wavelength)^ThreeSeveral times to several tens of times. It has been theoretically shown that by using this small resonator, spontaneously emitted light from the active layer is efficiently coupled to a resonator mode that contributes to oscillation, so that a laser with a very low threshold can be realized. Specifically, when applied to a semiconductor laser, it is said that the oscillation threshold can be reduced by two orders of magnitude or more compared to the conventional case.
[0007]
Such a microresonator laser can be basically manufactured by reducing the cross-sectional area of the surface-emitting laser described above. However, simply reducing the cross-sectional area of a conventional surface emitting laser increases the optical loss from the resonator and the loss of injected carriers due to surface and interface recombination on the active layer side wall. I will let you. For example, in the structure of FIG. 14A, the side wall of the active layer 64 is exposed, so that the loss of injected carriers due to surface recombination on the side wall of the active layer increases. Further, in the structure of FIG. 14B, since there is no mode confinement structure in the lower reflective film 62, a connection portion between the upper reflective film 66 and the lower reflective film 62 (indicated by B in FIG. 14B). In the region near the spacer layer 63, light loss due to diffraction increases. Further, in the structure of FIG. 14C, since the semiconductor exists on the side surface of the active layer 64, interface recombination is not a problem, but the refractive index difference between the resonator unit 62 and the embedded unit 72 is small. The proportion of the resonator mode that oozes into the buried layer increases. For this reason, the effective reflectance of the reflective film is reduced. In the structure normally used in these surface emitting lasers, the threshold value increases due to an increase in loss due to the miniaturization of the resonator when the resonator size is 5 μm or less.
[0008]
In the devices such as the above-described surface emitting lasers, LEDs, optical amplifiers, and optical modulators, a compound semiconductor multilayer film is used as a reflection mirror (DBR) for confining light. In order to increase the integration density of these devices, the compound semiconductor multilayer film must be processed to have a diameter of 0.5 μm to several tens of μm and a depth of several μm, and in order to prevent deterioration of characteristics. Requires a smooth and non-scattering DBR sidewall. In particular, in a surface emitting laser, in order to reduce the oscillation threshold current, it is essential to reduce the optical scattering loss in the DBR.
[0009]
In order to process the semiconductor multilayer film into the above-described fine dimensions, for example, dry processes such as reactive ion etching (RIE), reactive ion beam etching (RIBE), and chemical assisted ion beam etching (CAIBE) are used. An etching method is used. In addition, the etching rate of each compound semiconductor layer has a property that it varies depending on constituent elements and compositions. For example, when performing dry etching of a multilayer film of AlAs and GaAs using chlorine gas, the etching is usually performed at 100 ° C. or lower in order to suppress side etching and achieve vertical anisotropic etching. However, in this temperature range, as shown in FIG. 15, the AlAs layer is convex and the GaAs layer is concave on the etching sidewall of the multilayer film. The unevenness formed on the DBR side wall of the device causes an increase in light scattering loss, resulting in a problem that the device characteristics are remarkably deteriorated. Although it is conceivable to control the unevenness of the side wall by taking measures such as improving the apparatus or increasing the number of steps, it is not preferable from the viewpoint of cost.
[0010]
[Problems to be solved by the invention]
As described above, when the compound semiconductor multilayer film is processed by the dry etching method in order to form the DBR film, the formation of irregularities on the side walls is unavoidable, and the optical scattering loss increases and the device characteristics deteriorate. cause.
[0011]
An object of the present invention is to provide a dry etching method capable of forming a smooth etching sidewall without unevenness without causing an increase in the number of steps and the apparatus cost when etching a compound semiconductor multilayer film. And
[0012]
[Means for Solving the Problems]
  In order to solve the above problems, the present invention provides:
Alternatingly, a first compound semiconductor layer containing Al and a second compound semiconductor layer containing Ga and having a refractive index and an Al content different from those of the first compound semiconductor layerOn the boardIn the method of selectively dry-etching the laminated semiconductor multilayer film using a mask,Chlorine gas as etching gasUseThe substrate is heated to a temperature of 130 ° C. or more and 170 ° C. or less, and a reaction product is generated from each of the first compound semiconductor layer and the second compound semiconductor layer by etching, andFirstCompoundOriginated from the semiconductor layerSaidVapor pressure P of reaction product₁When,SaidSecondCompoundOriginated from the semiconductor layerSaidVapor pressure P of reaction product₂WithBetween,The relationship shown in the following formula (2)obtainA dry etching method is provided.
[0013]
0.2 ≦ P₂ / P₁ ≦ 1.5 (2)
[0014]
[Action]
In the dry etching method of the present invention, since the etching temperature is limited, the side etching rate of each semiconductor layer constituting the semiconductor multilayer film can be controlled. Thereby, a vertical etching side wall can be obtained, and a smooth etching side wall without unevenness can be achieved by equalizing the side etching rate of each semiconductor layer. Therefore, by applying the dry etching method of the present invention to the manufacture of a device such as a surface emitting laser, it is possible to prevent deterioration of device characteristics such as optical scattering loss without causing an increase in process steps and an increase in apparatus cost. In addition, the yield can be improved.
[0015]
【Example】
(Reference example)
First, a semiconductor microresonator light emitting device of a reference example will be described with reference to the drawings.
[0016]
FIG. 1 shows a configuration of an example of a semiconductor microresonator light emitting device of a reference example. 1A is a cutaway view of a main part of an example of the semiconductor microresonator light emitting element, and FIG. 1B is a cross-sectional view including an active layer.
[0017]
As shown in FIG. 1A, a semiconductor microresonator light emitting device 10 includes a substrate 7 made of n-GaAs, an n-GaAs / AlAs distributed Bragg reflection film (DBR) 3, an active layer 4, a spacer layer 5, And p-GaAs / AlAs-DBR2 are sequentially formed. In the light emitting portion, the active layer 4 is made of i-In._0.2 Ga_0.8 It consists of an As strained quantum well active layer (8 nm thickness is 3 layers) and an i-GaAs barrier layer, and the spacer layers 5b and 5a are made of n-Al_0.2Ga_0.8As and p-Al_0.2Ga_0.8It is composed of As. Furthermore, on the side surface of this cylindrical semiconductor laminated structure, i-Al_0.5Ga_0.5A regrowth layer 1 made of As is formed.
[0018]
Note that the diameter of the cylindrical laminated structure is 2 μm, and the thickness of the regrowth layer 1 is 50 nm. The thickness of the light emitting part including the spacer layers 5a and 5b and the active layer 4 is 293 nm, and the oscillation wavelength λ is 980 nm.
[0019]
A Cr / Au-p electrode 6 is deposited on the surface of the p-GaAs / AlAs-DBR2, and an AuGe / Au-n electrode (not shown) is deposited on a part of the back surface of the substrate.
[0020]
In the semiconductor microresonator light emitting element 10, the output light is radiated from the substrate side in the direction of the arrow through the SiN antireflection film (not shown). Note that the structural diagram shown in FIG. 1A is simplified for the sake of explanation. Actually, as shown in FIG. 1B, a cylindrical shape having a regrowth film 1 formed on the side surface is shown. The entire resonator section is embedded with an insulator 8 such as polyimide or spin-on-glass (SOG).
[0021]
In the semiconductor microresonator light emitting device 10 of the reference example, the side surface of the semiconductor microresonator configured in a columnar shape is surrounded by a sufficiently thin semiconductor thin film 1 (referred to as a regrowth film), and this regrowth film Is limited so that the loss due to oozing into the regrowth film does not exceed the saturation gain of the active layer.
[0022]
This limitation is based on the following considerations. First, the oozing of the mode into the semiconductor buried region in the resonator mode will be described with reference to FIG. The amount of oozing in the mode increases as the cross-sectional area decreases, and the cross-sectional area (oscillation wavelength) increases as shown in FIG.²When a microresonator that is several tens of times less than is embedded in a semiconductor growth film made of the same kind of material, the difference in refractive index between the two is small, so that the semiconductor layer outside the resonator mode oozes out. growing. As a result, the effective reflectivity of the multilayer reflective film decreases and the threshold gain increases.
[0023]
On the other hand, as shown in FIG. 2B, the cross-sectional area of the semiconductor multilayer film is (oscillation wavelength).²In a normal surface emitting laser that is sufficiently larger than the above, since the oozing to the semiconductor layer outside the resonator mode is small, the increase in the threshold gain is almost negligible. This is because even when the difference in refractive index is small, the mode is almost confined in the multilayer reflective film portion.
[0024]
Even when the cross-sectional area is small, when the side surface of the semiconductor multilayer film is surrounded by a sufficiently thin semiconductor film as shown in FIG. 2C, the amount of mode oozing is small enough not to cause a problem.
[0025]
The results of calculating these situations with a specific model are shown in FIG. The structure assumed as a model here is composed of a lower GaAs / AlAs distributed Bragg reflector (DBR), a GaAs / InGaAs strained quantum well (10 nm thickness, number of wells 3), and an upper GaAs / AlAs distributed Bragg reflector (DBR). On the side of the cylindrical resonator_0.5Ga_0.5A semiconductor thin film made of As was grown. The calculation was performed as follows, assuming that the diameter of the cylinder was d and the film thickness of the regrown film was t.
[0026]
First, the effective refractive index n of GaAs constituting the DBR_GaAs′ Is the lateral confinement factor γ of the mode to the DBR region and the regrowth film region₁And γ₂Is expressed as in the following formula (3). Similarly, the effective refractive index n of AlAs_AlAs'Is represented by the following formula (4).
[0027]
[Expression 1]

[0028]
n_c'And n_env.Are the refractive indices outside the regrowth layer and resonator section, respectively. N obtained from the above equation_GaAs'And n_AlAsWhen 'is used, the effective reflectance R' is expressed by the following formula (5).
[0029]
[Expression 2]

[0030]
Where R_topAnd R_bottomAre the reflectivities of the upper DBR and the lower DBR, respectively, and are represented by the following equations (6) and (7), respectively.
[0031]
[Equation 3]

[0032]
[Expression 4]

From the effective reflectance R ′, the threshold gain g_thIs obtained by the following equation (8).
[0033]
[Equation 5]

Therefore, the conditions for the laser to oscillate are expressed by the following formula (9).
[0034]
[Formula 6]

[0035]
Where g_sIs the saturation gain of the active layer, α_iIs the internal loss of the resonator, t_aIs the thickness of the active layer. In the ideal case, i.e. internal loss α of the resonator_iWhen is 0, the right side of equation (9) is only the second term, and the above equation (1) is derived.
[0036]
FIG. 3 shows the relationship between the regrowth film thickness t and the threshold gain when the diameter d of the cylinder is 1 μm, 2 μm, and 5 μm. Note that the diameter of 5 μm is a size called a normal surface emitting laser, and it can be seen from FIG. 3 that the threshold gain is hardly influenced by the regrowth film thickness t when the diameter is 5 μm.
[0037]
On the other hand, at a diameter of 1 μm where the effect of the microresonator becomes significant, the threshold gain increases rapidly as the regrowth film thickness increases. Where saturation gain g_s= 5000cm^-1Then, the threshold gain increases to the extent that oscillation is impossible when the regrowth film thickness is 0.21 μm or more.
[0038]
Thus, it can be seen that in the region of the resonator size of the microcavity laser, it is essential to reduce the thickness of the buried layer surrounding the resonator in order to realize low threshold oscillation.
[0039]
In the semiconductor microresonator light emitting device of the reference example, the film thickness of the regrowth film is specifically the thickness of the thickest part is the wavelength in the regrowth film of light corresponding to the band gap of the active layer. Is preferably 3 times or less, more preferably 2 times or less.
[0040]
In the case of the semiconductor microresonator light-emitting element 10 shown in FIG. 1, in the graph shown in FIG._s= 4000cm^-1Then, the maximum thickness t of the regrowth film_maxIs 0.47 μm, and this value corresponds to about 1.6 times the wavelength in the regrowth film 1.
[0041]
The semiconductor microresonator light emitting element described above can be manufactured, for example, as follows.
[0042]
FIG. 5 is a cross-sectional view showing a manufacturing process of the light emitting device of the reference example.
[0043]
First, as shown in FIG. 5A, the first DBR 3, the spacer layer 5b, the active layer 4, the spacer layer 5a, and the second DBR 2 are grown on the substrate 7. For the growth of each layer, for example, molecular beam epitaxial growth (MBE), metal organic chemical vapor deposition (MOCVD), or the like can be used. Further, a silicon dioxide film (SiO 2) is formed on the surface of the second DBR 2 by chemical vapor deposition (CVD).₂) 9 is deposited.
[0044]
Next, using an ultraviolet exposure process or an electron beam exposure process, SiO₂A circular resist pattern is formed on the film 9, and reactive ion etching (RIE) using a carbon tetrafluoride / hydrogen mixed gas is used to form SiO.₂The resist pattern is transferred to the film 9.
[0045]
After removing the resist by oxygen plasma, the epitaxially grown semiconductor layer is etched to the substrate 7 as shown in FIG. Here, for example, reactive ion beam etching (RIBE) using chlorine gas can be used.
[0046]
Subsequently, the sidewall of the active layer is etched by about 10 nm using an ammonia-based etchant to remove the etching damage layer generated on the side surface during the RIBE. Furthermore, diphosphorus pentasulfide (P₂S_Five) Dissolved in ammonium sulfide solution ((NH_Four)₂A passivation film is formed on the side surface by immersing in S) for one minute, and then washed with pure water. Then, using MOCVD method, SiO₂In a mode of selective growth that does not grow on the film 9, Al is formed on the cylindrical sidewall._0.5Ga_0.5As film 1 is grown.
[0047]
Next, Al_0.5Ga_0.5A cylinder with the As thin film 1 formed on its side surface is filled with polyimide 8, the top of the cylinder is exposed by oxygen plasma, and a p-electrode 6 is formed by an ultraviolet exposure process and lift-off.
[0048]
Finally, the back surface is polished, an n-electrode (not shown) is formed avoiding the light output portion, and an anti-reflection film (not shown) is deposited on the light output portion, so that the semiconductor microresonator of the reference example The light emitting element 10 is obtained.
[0049]
The semiconductor microresonator light emitting device of the reference example can be modified as follows.
[0050]
A first modification is shown in FIG. The basic structure of the light emitting element shown in FIG. 6 is the same as that shown in FIG. 1 except that the cylindrical resonator is not completely embedded in the insulator. That is, the semiconductor thin film 1 surrounding the resonator is further made of SiO.₂, SiN_x, Al₂O_ThreeIt is surrounded by a thin film made of an insulator 12 such as a columnar shape.
[0051]
In the semiconductor microresonator light emitting element 11, an electrode / reflection film 13 is provided outside the insulator 12. As described above, by arranging the reflective film 13 around the active layer 4 via the insulator, the spontaneous emission light emitted from the active layer 4 in the lateral direction is again absorbed by the active layer. Therefore, the threshold value can be further reduced.
[0052]
In the above example, a single layer of AlGaAs is used as the regrowth film 1 that surrounds the side surface of the resonator, but a multilayer film composed of a plurality of regrowth films having different refractive indexes can also be used. Furthermore, a regrowth film in which the refractive index in the thickness direction is continuously changed may be used.
[0053]
An example of such a structure is shown in FIG. As shown in FIG. 7C, the radial distance from the central axis of the cylinder is r, the distance to the outer peripheral surface of the regrowth film is a, and the film thickness of the regrowth film is t.
[0054]
FIG. 7A shows the relationship between the distance r from the center of the cylinder and the refractive index n when the regrowth film 1 is constituted by two types of semiconductor thin films having different refractive indexes. In this case, the refractive indexes of the DBR and the semiconductor thin film have the following relationship.
[0055]
n_in<N_out<N_DBR
n_DBR, N_inAnd n_outDenote the average refractive index of the DBR, the refractive index of the inner thin film, and the refractive index of the outer thin film, respectively.
[0056]
FIG. 7B shows an example in which the refractive index of the regrowth film 1 gradually decreases toward the outside. Both the structures shown in FIGS. 7A and 7B have a higher confinement factor in the DBR portion of the mode than in the case of a single-layer regrowth film. The film thickness t can be made larger than when it is formed.
[0057]
Further, the shape of the cross section of the resonator (A-A ′ plane in FIG. 8A) is not limited in any way, and may be various shapes as shown in FIG. 8B. Further, the cross section after the regrowth film 1 is formed can also have an arbitrary shape. That is, the thickness of the regrowth film 1 formed on the side surface of the resonator portion is not limited as long as it is within the above-described range.
[0058]
In the above example, an example in which an n-type semiconductor is used as a substrate has been shown. However, a p-type substrate can be used as a matter of course, and in this case, the conductivity type of each semiconductor layer stacked on the substrate is reversed. That's fine. The structure of the active layer is not limited to the quantum well structure, and a quantum wire and a quantum box structure can also be used. Furthermore, the material system is not limited to the AlGaAs and InGaAs systems described here, and InGaAsP and InGaAlP materials can be applied.
[0059]
In addition, the distributed reflection film does not have to be composed of only a semiconductor, but can be composed of a metal or a dielectric and a semiconductor. In addition, various modifications can be made.
[0060]
(Example)
Hereinafter, the dry etching method according to the present invention will be specifically described with reference to examples.
[0061]
First, an example of etching by reactive ion beam etching (RIBE) using chlorine gas will be described. A schematic diagram of the apparatus used is shown in FIG. As shown in FIG. 9, the etching apparatus 30 includes two chambers, an ECR (Electron Cyclotron Resonance) ion source 32 and an etching chamber 35, and an ion extraction electrode 33 is installed between the two chambers. . A substrate holder 38 in which a heater 39 and a thermocouple 40 are embedded is installed in the etching chamber 35, and the substrate can be heated. The substrate temperature during etching is measured by a pyrometer 36 disposed above the etching chamber 35, and the power of the heater 39 is controlled based on the measured value.
[0062]
In the etching, first, the substrate 37 is attached to the holder 38 and installed in the etching chamber 35, and the etching chamber is evacuated to a predetermined pressure. When the temperature of the sample substrate is stabilized and the pressure in the etching chamber reaches a predetermined value or less, chlorine gas is introduced from the gas introduction pipe 31 connected to the ECR ion source 32 to generate plasma. Ions are extracted from the plasma by the ion extraction electrode 33. During this time, the pressure in the etching chamber is adjusted by controlling the flow rate of chlorine gas with a mass flow controller.
[0063]
Next, by opening the shutter 34 installed between the ECR ion source and the etching chamber, the substrate is irradiated with ions to start etching. Since the etching substrate temperature rises with the start of ion irradiation, the substrate temperature measured by the pyrometer 36 is fed back to the heater power supply to adjust the heater power and keep the substrate temperature constant.
[0064]
Active neutral molecules generated with the generation of the plasma reach the substrate surface by diffusion. Etching caused by reactive sputtering due to ion irradiation mainly occurs on the bottom surface of etching, but there is almost no ion irradiation on the etching side wall, and etching proceeds by reaction between neutral molecules and the substrate.
[0065]
The multilayer film used as a Bragg reflection film for semiconductor lasers, LEDs, semiconductor optical amplifiers, semiconductor optical modulators, and the like was etched using the above-described apparatus. The results of this experiment will be described in detail below.
[0066]
As the substrate, a multilayer substrate of AlAs and GaAs 15 pairs grown on a GaAs substrate by molecular beam epitaxy was used. On this substrate, the CVD method is used to make SiO₂After the film is formed, it is processed by a normal photolithography method and reactive ion etching (RIE) to obtain a SiO 2 having a circular cross section as an etching mask.₂A film 21 was formed as shown in FIG.
[0067]
5 × 10 in the etching chamber 35 of the apparatus described above.^-6After evacuating to torr, the substrate was etched according to the procedure described above. In this experimental example, the etching conditions are a microwave power of 200 W, an ion acceleration voltage of 400 V, a chlorine gas pressure of 0.25 to 1.2 mtorr, and the substrate temperature is changed within a range of 35 to 230 ° C. And the shape of the etching sidewall obtained were investigated.
[0068]
As a result, when the etching temperature was 100 ° C. or lower, side etching hardly occurred, and no overhang was found immediately below the etching mask. Note that the etching sidewalls had irregularities such that the AlAs layer 23 was convex and the GaAs layer 22 was concave as shown in FIG.
[0069]
The present inventors considered this unevenness as follows. That is, in this temperature range, GaCl which is a volatile etching reaction product is used._ThreeThe vapor pressure of AlCl_ThreeAbout one order of magnitude higher than the vapor pressure. Although the ion irradiation effect is small on the etching side wall, the etching of the GaAs layer preferentially progressed due to the assist effect due to ion scattering and recoil from the etching bottom, and the local temperature rise. On the other hand, since the AlAs layer is very easily oxidized, the etching does not proceed on the etching sidewall having a small ion irradiation effect. As a result, the AlAs layer was convex and the GaAs layer was concave on the side wall.
[0070]
In the case where the etching temperature was set to 100 ° C. to 130 ° C., side etching occurred, but almost vertical etching sidewalls were obtained. However, the sidewalls still have irregularities in which the AlAs layer is convex and the GaAs layer is concave.
[0071]
Further, when the etching temperature was increased and the temperature range from 130 ° C. to 170 ° C., the amount of side etching was further increased, and an overhang 24 was generated immediately below the mask 21 as shown in FIG. In this case, the etching side wall was vertical over most of the region, and a smooth etching side wall having no unevenness between the AlAs layer 23 and the GaAs layer 22 was obtained. This is considered to be because the side etching rate of the AlAs layer 23 and the side etching rate of the GaAs layer 22 are substantially equal within this etching temperature range.
[0072]
At higher temperatures of 170 ° C. to 190 ° C., the amount of side etching increases further, the side etching rate of the AlAs layer exceeds the side etching rate of the GaAs layer, and the AlAs layer 23 is concave and the GaAs layer 22 is convex and concave. Start. However, the etching side walls are still almost vertical, so that they are not problematic for application to devices.
[0073]
Above 190 ° C., the side wall was greatly constricted, and a vertical etching side wall was not obtained. From the above experimental results, the present inventors have found that vertical sidewalls can be obtained by using side etching of etched sidewalls. Etching temperature and side etching amount (R₁/ R₂The relationship shown in FIG. 12 was obtained. Where R₁Is the depth of side etching, R₂Is the etching depth in the vertical direction. That is, the side etching rate has a temperature dependency that increases as the etching temperature increases. The side etching amount is preferably about the same as the film thickness t of the regrowth film on the side wall of the microcavity described in the reference example. That is, (R₁/ R₂) Is approximately equal to t / h, and
(T / h) <(t_max/ H)
Satisfy this relationship. Here, h is the height of the microcavity. For example, in the microcavity having a diameter of 2 μm described in the reference example, R₁/ R₂Is preferably about 0.08 or less.
[0074]
However, as shown in the experimental results, when the etching temperature exceeds 170 ° C., the unevenness of the AlAs layer becomes concave, so that the object of the present invention cannot be achieved. Concavities and convexities that become convex occur.
[0075]
Etching product GaCl_ThreeAnd AlCl_ThreeIf the ratio of the evaporation rate to 1 is 1, smooth etching sidewalls can be obtained. Based on the effect of temperature on the amount of side etching described above and the reason that a vertical and smooth etching sidewall is obtained, the inventors have determined the temperature range of the substrate as the ratio of the vapor pressure of the reaction product (P ( GaCl_Three) / P (AlCl_Three)) Is limited to a range of 0.2 to 1.5.
[0076]
In particular, the vapor pressure ratio (P (GaCl_Three) / P (AlCl_Three) Is performed at a substrate temperature in which the range of 0.2 to 1 is generated, side etching can be generated to form an overhang portion directly under the etching mask, and at the same time, a vertical and smooth etching sidewall can be formed. Obtainable.
[0077]
Further, the ratio of vapor pressure (P (GaCl_Three) / P (AlCl_ThreeWhen etching is performed at a substrate temperature in which the range of)) is 1 or more and 1.5 or less, side etching becomes large, so that a multilayer having a pattern size finer than the mask size is formed rather than forming an overhang portion. It can be used for the purpose of processing the film, and a smooth etching sidewall can be obtained at the same time.
[0078]
Based on the above experimental examples, the etching method of the present invention was applied to a process for producing a microcavity laser. This process will be described with reference to FIG.
[0079]
In the wafer 43 used here, as shown in FIG. 13A, 18.5 pairs of lower AlAs / GaAsDBR 45, GaAs spacer layer 46, and 15 pairs of upper AlAs / GaAsDBR 48 are grown by MBE on a GaAs substrate 44. In the center of the spacer layer 46, a quantum well active layer 47 made of three layers of InGaAs is formed.
[0080]
Further, on the upper DBR 48, SiO 2 having a circular cross section as an etching mask.₂A film 49 is formed. The diameter of this etching mask was 1 to 10 μm.
[0081]
In this example, etching was performed using the above-described apparatus under the etching conditions of microwave power 200 W, ion acceleration voltage 400 V, chlorine gas as the etching gas, and chlorine gas pressure 0.45 mtorr. The etching temperature was set to 150 ° C. based on the results shown in the above experimental example.
[0082]
As a result, as shown in FIG. 13B, an overhang portion 50 was formed by side etching immediately below the etching mask, and most of the region below the overhang portion was vertical. In addition, etching sidewalls having no irregularities and a very smooth surface were formed. Therefore, the optical scattering loss of the cylindrical microcavity obtained by etching using the method of the present invention could be significantly reduced compared to that produced by the conventional etching method.
[0083]
Since side etching is used as described above, the diameter of the cylindrical microcavity obtained by applying the dry etching method of the present invention can be made smaller than the mask size. For example, when an etching mask having a diameter of 1 μm is used, a microcavity having a fine columnar structure in which a diameter of a portion having a vertical side wall is 0.8 μm can be manufactured. A finer columnar structure can be obtained by setting the etching temperature between 170 to 190 ° C. and increasing the amount of side etching.
[0084]
When the normal patterning method and dry etching method are used, it is extremely difficult to process the semiconductor multilayer film to a diameter of 1 μm or less. In the etching method of the present invention, side etching is actively used, and the controllability and reproducibility of a diameter of 1 μm or less can be remarkably improved by controlling the etching amount of this part by the etching temperature.
[0085]
Furthermore, in order to reduce the recombination rate of carriers on the surface of the active layer, as shown in FIG. 13C, the AlGaAs thin film 51 was regrown on the side wall of the cylindrical microcavity. In forming the thin film, a selective growth method by MOCVD was used. As described above with reference to FIG. 13B, by using the etching method of the present invention, SiO 2 is used.₂An overhang portion is formed directly under the mask. In the case where no overhang is formed immediately below the etching mask, as shown in FIG.₂An abnormally grown portion 81 may be observed at the mask edge. In the etching method of the present invention, since the overhang portion 50 is formed, such abnormal growth can be suppressed and good regrowth can be performed.
[0086]
In the etching method of the present invention, side etching is positively caused, so that it also has an effect of removing a damaged layer introduced by dry etching. In addition, when the semiconductor thin film was regrown on the side wall of the cylindrical microcavity, the carrier recombination rate on the surface of the active layer was remarkably reduced as compared with the microcavity manufactured by the conventional etching method.
[0087]
As shown in the above embodiments, the present invention actively uses side etching in dry etching of a semiconductor multilayer film, so the etching rate is set so that the side etching rate of each layer of the multilayer film is equal. A vertical and smooth etching sidewall can be obtained.
[0088]
In this embodiment, the dry etching method of the present invention has been described by taking the production of a microcavity laser as an example. However, the etching method of the present invention can be applied to semiconductor multilayer films such as other LEDs, optical amplifiers, and optical modulators. It is also possible to apply to the manufacturing process of all devices using.
[0089]
As an etching method, it can be applied to other dry etching methods such as reactive ion etching (RIE) and chemical assisted ion beam etching (CAIBE).
[0090]
Furthermore, the dry etching method of the present invention can be applied to a multilayer film having an arbitrary configuration in which a compound semiconductor film containing Al and a semiconductor film containing Ga are alternately stacked, and the configuration is not limited in any way. is not. For example, Al_xGa_1-xV / Al_yGa_1-yV (0 ≦ x <y ≦ 1) (V = N, As, P, Sb), Al_xGa_1-xV ’_zV "_1-z/ Al_yGa_1-yV ’_zV "_1-z(0.ltoreq.x <y.ltoreq.1, 0.ltoreq.z.ltoreq.1) If the multilayer film is a combination satisfying (V ', V "= N, As, P, Sb), the present invention is applied and the same as described above. An effect can be obtained.
[0091]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to obtain a smooth etching side wall which is vertical and has no unevenness without increasing the cost and the process of the apparatus. Therefore, it is possible to greatly improve the characteristics of the device, and such a dry etching method is effective for manufacturing all devices having a compound semiconductor multilayer film such as a semiconductor laser, an LED, a semiconductor optical amplifier, and a semiconductor optical modulator It is.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an example of a semiconductor microresonator light emitting device of a reference example.
FIG. 2 is a diagram for explaining mode bleeding into a semiconductor buried region in a resonator mode;
FIG. 3 is a graph showing the relationship between the regrowth film thickness and the threshold gain.
FIG. 4 is a view for explaining light scattering loss from the side wall of a multilayer reflective film.
FIG. 5 is a cross-sectional view showing an example of a manufacturing process of a semiconductor microresonator light emitting device of a reference example.
FIG. 6 is a cross-sectional view showing another example of the semiconductor microresonator light emitting device of the reference example.
FIG. 7 is a diagram showing another example of the semiconductor microresonator light emitting device of the reference example.
FIG. 8 is a view showing a modification of the cross-sectional structure of the semiconductor microresonator light emitting device of the reference example.
FIG. 9 is a schematic diagram of an etching apparatus that can be used in the present invention.
FIG. 10 is a cross-sectional view of a substrate used in an experimental example.
FIG. 11 is a cross-sectional view of a semiconductor multilayer film after etching according to the present invention.
FIG. 12 is a graph showing the relationship between side etching amount and temperature dependence.
13 is a cross-sectional view showing a manufacturing process of a microcavity laser using an etching method according to the present invention. FIG.
FIG. 14 is a sectional view of a conventional surface emitting laser.
FIG. 15 is a cross-sectional view of a semiconductor multilayer film etched by a conventional etching method.
FIG. 16 is a cross-sectional view of a microcavity laser manufactured by a conventional etching method.
[Explanation of symbols]
1 ... Al_0.5Ga_0.5As thin film, 2 ... p-GaAs / AlAs-DBR, 3 ... n-GaAs / AlAs distributed Bragg reflector (DBR), 4 ... i-In_0.2Ga_0.8As strained quantum well active layer, 5a ... p-Al_0.2Ga_0.8As spacer layer, 5b ... n-Al_0.2Ga_0.8As spacer layer, 6 ... Cr / Au-p electrode, 7 ... n-GaAs substrate, 8 ... insulator, 9 ... insulator, 10 ... semiconductor microresonator light emitting element, 11 ... semiconductor microresonator light emitting element, 15 ... Multiple reflection film, 16 ... Light emission from active layer, 17 ... Scattered light, 21 ... Etching mask, 22 ... GaAs layer, 23 ... AlaAs layer, 24 ... Overhang part, 31 ... Reactive gas introduction tube, 32 ... ECR ion Source 33 ... Ion extraction electrode. 34 ... Shutter, 35 ... Etching chamber, 36 ... Pyrometer, 37 ... Semiconductor substrate, 38 ... Substrate holder, 39 ... Heater, 40 ... Thermocouple, 41 ... Heater power supply, 43 ... Wafer, 44 ... GaAs substrate, 45 ... GaAs / AlAs lower DBR, 46 ... GaAs spacer layer, 47 ... InGaAs active layer, 58 ... GaAs / AlAs upper DBR, 49 ... SiO₂Mask: 50 ... Overhang portion, 51 ... AlGaAs regrowth film, 60 ... Surface emitting laser, 61 ... Substrate, 62 ... First multilayer reflector, 63 ... Spacer layer, 64 ... Active layer, 65 ... Light emitting portion, 66 2nd multilayer reflecting mirror, 67 ... electrode, 68 ... surface emitting laser, 69 ... insulating film, 70 ... electrode, 71 ... surface emitting laser, 72 ... buried layer, 73 ... electrode, 74 ... substrate, 75 ... lower DBR 76 ... Spacer layer, 77 ... Active layer, 78 ... Upper DBR, 79 ... SiO₂Mask, 80 ... regrowth layer, 81 ... abnormally grown portion.

Claims (4)

A semiconductor multilayer in which first compound semiconductor layers containing Al and second compound semiconductor layers containing Ga and having a refractive index and an Al content different from those of the first compound semiconductor layers are alternately stacked on a substrate. In a method of selectively dry-etching a film using a mask,
Chlorine gas is used as an etching gas , the substrate is heated to a temperature of 130 ° C. or higher and 170 ° C. or lower, and reaction products are generated from the first compound semiconductor layer and the second compound semiconductor layer by etching, respectively. is allowed, and the vapor pressure P ₁ of the reaction product resulting from the first compound semiconductor layer, between the vapor pressure P ₂ of the reaction product resulting from the second compound semiconductor layer, the following formula ( A dry etching method characterized by obtaining the relationship shown in 2).
0.2 ≦ P ₂ / P ₁ ≦ 1.5 (2) The first compound semiconductor layer is Al _y Ga _1-y V, and the second compound semiconductor layer is Al _x Ga _1-x V (0 ≦ x <y ≦ 1, V = N, As, P, Sb). The first compound semiconductor layer is made of Al _y Ga _1-y V ′ _w V ″ _1-w , and the second compound semiconductor layer is made of Al _x Ga _1-x V ′ _z V ″ _{1 -z} (0.ltoreq.x <y.ltoreq.1, 0.ltoreq.w.ltoreq.1, 0.ltoreq.z.ltoreq.1, V ', V "= N, As, P, Sb). The dry etching method as described.   3. The dry etching method according to claim 1, wherein the first compound semiconductor layer is made of AlAs and the second compound semiconductor layer is made of GaAs. The dry etching method according to claim 3 , wherein the ratio (P ₂ / P ₁ ) is 1 or less .

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