JP3589878B2

JP3589878B2 - Back-illuminated light-receiving device and method of manufacturing the same

Info

Publication number: JP3589878B2
Application number: JP32191398A
Authority: JP
Inventors: 弘伊藤; 忠夫石橋; 知史古田; 聡児玉
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 1998-11-12
Filing date: 1998-11-12
Publication date: 2004-11-17
Anticipated expiration: 2018-11-12
Also published as: JP2000150923A

Description

【０００１】
【発明の属する技術分野】
本発明は、基板の裏面から信号光を入射する構造の受光装置に係り、特に高速で、高効率の裏面入射型受光装置およびその作製方法に関する。
【０００２】
【従来の技術】
フォトダイオードに代表される半導体受光素子は、光信号を電気信号に変換する素子であり、光通信や光測定などの分野で幅広く使われいる。従来のｐｎ接合を用いたフォトダイオードは、例えば、図２０に示すように基板の端面から信号光を入射する「導波路型」受光素子と、図２１に示すように基板に垂直に信号光を入射する「面型」受光素子とに分類される。ここで、半導体層２５は、半導体基板２６上に形成されたｎ型電極層、光吸収層２７、ｐ型電極層からなる半導体積層構造を基本としており、必要とする層をさらに多層構造にすることにより、光導波路構造を兼ねる構造にすることもできる。受光素子２８は、メサ加工により絶縁分離されることが多く、素子の上部および下部電極層に接して、それぞれ上部電極３０および下部電極３１が形成される。導波路型受光素子の特徴は、入射光２９の進行方向と光励起キャリアの走行方向が互いに異なるため、受光素子の帯域と効率を独立に設計できることである。しかし、その反面、入射光との結合効率が低いことや、光入射端面に劈開面３３を用いるため、通常の半導体製造プロセス等との整合性が悪く、量産性の観点で問題があり、他の素子とのモノリシック集積化の自由度も低いなどの欠点がある。さらに、劈開しないと素子特性の測定ができないために、ウエハ状態での素子評価ができないのも欠点の一つである。これに対し、基板に垂直方向から信号光を入力する面型受光素子は、幾つかの点において利点を有している。すなわち、入射光との結合効率低下の問題が少なく、光吸収層の厚さを厚くすることにより効率を高くできるという特徴や、通常の半導体製造プロセスとの整合性が良いので量産性に富み、構造の類似性からも他の素子とのモノリシック集積化の自由度も高いなどの特徴がある。また、ウエハ状態での素子評価も容易である。
面型フォトダイオードのうち、信号光を基板表面側から入射する面型受光素子、すなわち表面入射型素子（図２１）は、入射光を導入するファイバー等とのモジュール化が簡便に行えることから、低速応用の領域では多く用いられている。しかし、表面入射型の場合は、上部電極３０領域内に光入射用の窓３２が必要であり、したがって窓領域（電極の無い部分）の広がり抵抗が大きくなり、素子特性の向上がはかれないという欠点があった。また、光吸収層２７の厚さと素子効率はトレードオフの関係にあり、吸収層を薄層化して高速化をはかると効率が低下するという問題があった。これに対し、高速応用や高効率が必要な領域では、面型受光素子である裏面入射型（図２２）が主流となっている。その理由は、上部電極３０領域全面に電極を形成することで素子抵抗の低減をはかることができると共に、上部電極３０で信号光を反射させ、反射光３４を再度、光吸収層へ導くことにより、素子の高効率化がはかれるからである。
【０００３】
【発明が解決しようとする課題】
しかしながら、従来の裏面入射面型フォトダイオードにおいて、さらなる高速化をはかるためには、キャリア走行距離を極端に短くすること、すなわち光吸収層の厚さを極端に薄くせざるを得ないために、高速性を保持したままで十分な素子効率を得ることができないという問題があった。
【０００４】
本発明の目的は、上記従来技術における問題点を解消し、受光素子の光吸収層部に信号光を斜めに入射すると共に、入射した信号光をさらに反射させ、より小さい素子面積で光吸収層に効率的に信号光を導入することが可能で、素子の高速化と高効率化をはかることができる裏面入射型受光装置およびその作製方法を提供することにある。
【０００５】
【課題を解決するための手段】
上記本発明の目的を達成するために、特許請求の範囲に記載のような構成とするものである。すなわち、
本発明は請求項１に記載のように、
裏面入射型受光装置であって、半導体基板上に形成された半導体受光素子と、上記基板表面に、上記受光素子の側部に独立して形成された単数もしくは複数の凹状の斜面反射部とを少なくとも備え、上記基板の裏面から入射した信号光が上記斜面反射部で反射し、基板面に対し斜め方向から上記受光素子に入射する構造に、上記受光素子および上記斜面反射部を配設し、かつ受光素子の全側面部、もしくは少なくとも斜面反射部とは反対側の側面部において、上記受光素子に対し斜め方向に入射する信号光を素子側へ反射させる構造となし、上記斜面反射部とは反対側の素子側面が、上記斜面反射部側の素子側面よりも深く掘り込まれた非対称なメサ状の断面形状を有する裏面入射型受光装置とするものである。
また、本発明は請求項２に記載のように、
請求項１において、半導体基板の厚さをＴ、斜面反射部の深さをＤ、上記斜面反射部で反射され受光素子へ入射する信号光の入射方向と基板面とのなす角度をθ′とした場合に、上記受光素子を構成する光吸収層の上記斜面反射部側の端部と、上記斜面反射部の最深部との水平距離ｚが、ｚ＜Ｔ／（１０・ｔａｎθ′）となる範囲に設定し、かつ受光素子の全側面部、もしくは少なくとも斜面反射部とは反対側の側面部において、上記受光素子に対し斜め方向に入射する信号光を素子側へ反射させる構造となし、上記斜面反射部とは反対側の素子側面が、上記斜面反射部側の素子側面よりも深く掘り込まれた非対称なメサ状の断面形状を有する裏面入射型受光装置とするものである。
また、本発明は請求項３に記載のように、
請求項１または請求項２において、斜面反射部を受光素子に対して対称な位置に、偶数個、配設し、かつ受光素子の全側面部、もしくは少なくとも斜面反射部とは反対側の側面部において、上記受光素子に対し斜め方向に入射する信号光を素子側へ反射させる構造となし、かつ上記斜面反射部とは反対側の素子側面が、上記斜面反射部側の素子側面よりも深く掘り込まれた非対称なメサ状の断面形状を有する裏面入射型受光装置とするものである。
また、請求項４に記載のように、
請求項１ないし請求項３のいずれか１項において、受光素子の少なくとも斜面反射部とは反対側の側面を、逆メサ状の断面形状に構成してなる裏面入射型受光装置とするものである。
また、請求項５に記載のように、
請求項１ないし請求項４のいずれか１項に記載の裏面入射型受光装置の作製方法であって、半導体基板上に凹状の斜面反射部を形成する際に、半導体基板に対し選択エッチング特性を有する半導体薄膜をマスクとして用い、化学的エッチング法により、基板結晶の（１１１）Ａ面、もしくはこれと等価な面が露出されるようにエッチングする工程を含む裏面入射型受光装置の作製方法とするものである。
本発明は、請求項１ないし請求項４のいずれか１項に記載のように、受光素子側部に独立した斜面反射部を設け、裏面からの入射光をすべて反射させ、光吸収層に斜め方向から信号光を入射させるようにしたものである。このような構成とすることにより、入射光にとって実効的な吸収長が増大するため、受光素子の効率を顕著に増大させることができる効果がある。そして、基板裏面から垂直に入射した入射信号光を、受光素子とは独立して基板内に形成された斜面反射部ですべて反射させることにより、光吸収層に対して低角度で入射させる点が従来技術とは異なるところである。
また、本発明は請求項１ないし請求項４に記載のように、受光素子の全側面部、もしくは少なくとも上記斜面反射部とは反対側に位置する素子側面部も反射部となるように構成するものであって、このような構成とすることにより、素子部に、斜めに入射した信号光をさらに反射させ、より小さい素子面積で光吸収層に効率的に信号光を導くことができる効果がある。そして、素子側面を反射部とし、短い素子寸法であっても低角度の入射信号光を効率良く光吸収層へ導くように構成した点が従来技術とは異なるところである。
さらに本発明は、請求項５に記載のように、半導体基板に対し選択エッチング特性を有する半導体薄膜をマスクとして用い、化学的エッチング法によって、基板結晶の（１１１）Ａ面、もしくはこれと等価な面が露出されるようにエッチングして斜面反射部を形成する方法であるので、容易に、再現性良く、高歩留まりで斜面反射部を有する半導体基板を作製できる効果がある。
【０００６】
【発明の実施の形態】
以下に、本発明の実施の形態について図面を引用しながら詳細に説明する。以下に述べる実施の形態では、受光素子の一例として、高速性、高出力性に優れるＵＴＣ−ＰＤ（Ｕｎｉ−Ｔｒａｖｅｌｉｎｇ−ＣａｒｒｉｅｒＰｈｏｔｏｄｉｏｄｅ）と呼ばれるフォトダイオード（特開平９−２７５２２４号公報）を用いた。
〈実施の形態１〉
図１は本発明の第１の実施の形態であって、１は半絶縁性のＩｎＰからなる半導体基板、２はｎ型ＩｎＰ電極層、３はアンドープＩｎＰキャリア走行層、４は信号光を吸収するｐ型ＩｎＧａＡｓ吸収層（膜厚２５００Å）、５はｐ型ＩｎＧａＡｓＰ電極層（組成：波長１．３μｍ相当）、６は下部電極、７は上部電極、８は素子側部に形成されたＶ溝からなる斜面反射部、９は入射光、１０は反射光である。また、基板裏面には反射防止膜１１が設けられている。なお、本発明の構成には直接関係しないため、ここではＵＴＣ−ＰＤの層構成は簡略化して記述している。また、有限の径を有する光ビームの半導体層界面での屈折の様子は、図３を除いて省略して記述している。
図１に示すように、本実施の形態では、メサ形状の受光素子１３が形成され、その側部にＶ溝からなる斜面反射部８が独立して形成されている。後述するように、この斜面が基板表面とのなす角度（鋭角側）は、約５４．７°（度）とした。まず、基板裏面から基板面に垂直に入射した入射光９は、斜面反射部（Ｖ溝）８ですべて反射され、メサ型素子部の光吸収層４に斜め方向から入射する。ここで、図２に示すように、基板表面１２に対する斜面反射部８の角度（鋭角側）をθとすると、光吸収層４への入射方向と基板面の角度θ′は２θ−９０°となる。この入射光９は、キャリア走行層３と、ＩｎＧａＡｓ層よりなる光吸収層４、および光吸収層４とＩｎＧａＡｓＰよりなる電極層５との界面で、図３に示すように、スネルの法則にしたがい屈折する。したがって、光吸収層４の厚ををｄ、アンドープＩｎＰキャリア走行層３の屈折率をｎ_１、光吸収層４の屈折率をｎ_２とすると、光吸収層４に入射した信号光は、光吸収層４内を、次の（数１）式で示される距離ｄ_ｅｆｆだけ伝搬し、光吸収層４の上部に出射する。
ｄ_ｅｆｆ＝ｄ／〔１−４（ｎ_１／ｎ_２）^２ｓｉｎ^２θｃｏｓ^２θ〕^１／２………（数１）
出射した信号光は、上部電極７で反射され、再度、光吸収層４へ入射し、光吸収層４内を再度、距離ｄ_ｅｆｆだけ伝搬する。したがって、実効的な吸収長は、
２ｄ_ｅｆｆとなる。これは、信号光が光吸収層４に対して垂直に入射した場合（吸収長２ｄ）に比べ、（数１）式に示す係数だけ長くなり、結果として素子の効率増大に寄与することになる。本実施の形態における構成では、吸収長の増大係数は約１．８倍となる。
信号光の実効的な吸収長（実効吸収長と言う）は、斜面反射部８の入射光９の角度θに依存するが、光吸収層４への入射方向と基板面の角度（θ′）が９０度に近づくと、斜め入射による実効吸収長の増大効果は少なくなり、一方、入射角を小さくし過ぎると、後述するが、素子面積、すなわち素子容量が増大して高速特性は低下することになる。また、実際には、入射信号光は有限のビーム径を有するので、受光素子１３の素子面積（実際には入射方向の素子長）をθ′と連動して増大させないと、図４に示すような信号光９の「遮り」が生じ、信号光９の一部しか光吸収層４に到達しないため素子効率は低下する。また、自明ではあるが、基板表面と斜面反射部とのなす角度（θ）が４５度よりも小さい場合には、反射光１０は素子に到達できないし、９０度以上の場合は入射光９がいったん基板から出射することなしには反射部として機能しない。したがって、θとして考えられる範囲は９０°＞θ＞４５°となる。すなわち、「斜面反射部８」は、基板表面と斜面反射部とのなす角（鋭角側）θは、上記の範囲内にあるものと定義される。
以上のように、素子の高速性と効率は共にθの関数であり、トレードオフの関係にある。図５に示すように、θが６７度を越えると、実効吸収長の増分（差分）％は３０％程度以下となってしまうので、実効吸収長の増分の効果は十分ではない。したがって、θは６７度以下が好ましい。他方、θが低くなり過ぎると、必要とする素子面積が増大し、素子容量の増大による素子高速特性の著しい低下を招くため、やはり実用的ではない。ところで、素子の高速性は、容量を制限する要素と、キャリア走行時間を制限する要素とがある。前者は素子面積と関連しており、後者は光吸収層の厚さと関連している。一般に、光吸収層の厚さは、所定の効率を確保するために有限の値に設定する（本実施の形態では約２５００Åに設定した）。したがって、ある程度の素子面積までは、その面積増大が素子の高速性に対し顕著な影響を及ぼさない。その影響が顕著になるのは、垂直入射の場合に比べて、素子面積がおおむね５倍以上に増大した場合であり、θとしては５０度以上が好適である。したがって、現実的な装置を考えた場合に、上記θの好適な範囲としては、６７°≧θ≧５０°となる。
本発明の裏面入射型受光装置の構成では、斜面反射部（Ｖ溝）８で、入射光９の全反射が起こるようにθを決定しなければならない。また、Ｖ溝表面に表面保護などの目的で膜を堆積する場含には、その屈折率についても考慮する必要がある。本実施の形態で用いたＩｎＰ基板の屈折率は３．１７（波長１．５５μｍの場合）であるため、表面が空気あるいは封入ガスなど（いずれも屈折率はほぼ１）の場合、θが４５度以上ではすべて全反射条件を満たしている。一方、保護膜を堆積する場合は、全反射条件を保つためには屈折率の制約が生じる。例えば、θを５４．７度と設定した場合にＶ溝表面に堆積する材料は、屈折率２．５９以下でなければならない。本実施の形態では用いていないが、一般には表面保護膜を用いる構成が好まれ、通常の半導体プロセスとの整合性の観点から、堆積膜としては、ポリイミド（屈折率１．５程度）、有機膜の一種であるＢＣＢ（ビスベンゾシクロブテン：屈折率１．５程度）、シリコン酸化膜（屈折率１．５程度）、シリコン窒化膜（屈折率１．８〜２．２程度）等が好適に用いられる。これらはいずれも、θが４５度以上で全反射条件を満たしている。堆積膜の種類は、上記以外の膜であっても良く、また屈折率の制約は、θによって適宜規定されるものであり、θを大きくすればこの制約は緩和される。
図１において、入射光９は上部電極７で反射すると述べたが、電極層５に用いる材料と光吸収層４に用いる材料との屈折率差を適宜選択することにより、図６に示すように、入射光９を吸収層４および電極層５′の界面で全反射させることができる。実際の素子では、半導体／金属界面で光の吸収が生じる場合があるため、この構造は素子効率の向上に有効である。例えば、本実施の形態のように、θ′＝１９．５°となるように、基板表面と斜面反射部とのなす角度θを選んで斜面反射部８を形成すると、光吸収層４および電極層５′の界面で全反射条件となるのに必要な屈折率比は約０．８３となる。したがって光吸収層４をＩｎＧａＡｓ（屈折率３．５９）で構成した場合、電極層５′は例えばＡ１ＰやＡ１Ａｓ（いずれも１．５５μｍにおける屈折率２．８以下）で構成すれば良いことになる。
【０００７】
〈実施の形態２〉
図７（ａ）、（ｂ）および図８は、半導体基板１に斜面反射部（Ｖ溝）８の作製方法を示す模式図である。Ｖ溝の作製方法としては、半導体の化学エッチング特性を利用するものである。すなわち、化合物半導体基板として一般的に用いられる（００１）面、あるいはこれと等価な面を表面とする基板を、メサエッチングした場合に、その（−１１０）断面および（１１０）断面（あるいは、これらと等価な面の組み合わせであっても良い。）からみたエッチング断面形状は、それぞれ、図７（ａ）および図７（ｂ）に示すように、前者がいわゆる「逆メサ」形状となり、後者が「順メサ」形状となる。より具体的には、通常市販されている基板のＯＦ（オリエンテーションフラット）に対して平行なストライブ状メサの側面（ＯＦから見て前後）は順メサとなり、９０度異なる方向の側面（ＯＦから見て左右）は逆メサとなる。これらのメサ斜面の基板表面に対する角度（順メサの場合は鋭角側、逆メサの場合は鈍角側）は、エッチング液によっても異なるが、適当なエッチング液、エッチング条件を選択すれば、メサ側面を表面として（１１１）面、あるいはこれと等価な面を露出させることができる。
これは、例えばＩＩＩ−Ｖ族化合物半導体の場合、金属原子面（通常、Ａ面と称する）でエッチングが停止しやすい性質を有しているからである。基板がＩｎＰの場合、この面はＩｎ面となり、典型的には、ブロムとメタノールの混合液、臭酸、塩酸と燐酸の混合液、硫酸と過酸化水素の混合液などを用いて露出させることができる（例えば、Ｓ．Ａｄａｃｈｉ他、Ｊ．Ｅ１ｅｃｔｒｏｃｈｅｍ．Ｓｏｃ．Ｖｏ１，１２８，Ｎｏ．６．１９８１，ｐｐ．１３４２〜１３４９）。（１１１）Ａ面、あるいはこれと等価な面でエッチングを止めた場合、（−１１０）断面、あるいはこれと等価な面から見た基板表面と斜面とのなす角度は約５４．７度となる。
図８は本実施の形態で作製した斜面反射部（Ｖ溝）を示す図であって、まず、ＩｎＰ基板上に形成されたＩｎＧａＡｓ層上に、フォトレジストで開口部を有するマスクを作製し、続いてクエン酸系のエッチング液でＩｎＧａＡｓ層を選択的にエッチングする。その後、塩酸と燐酸の混合液を用いてＩｎＰのみを選択的にエッチングし、（１１１）Ａ面でエッチングを自動的に停止させることによりＶ溝を形成する。図８に示すように、Ｖ溝の深さはエッチングマスクの開口部長さに依存する（開口部長さに対し深さは１／√２となる）ので、マスク設計によりＶ溝の深さを決定することができる。必要なＶ溝の深さは、受光素子とＶ溝との位置関係により決まる。ここでは、開口部長を約２８μｍ、Ｖ溝深さを約２０ μｍとした。また、光吸収層の端部からＶ溝最深部（中心）までの水平距離を２７μｍとした。これらの値は、素子からの引き出し電極の形状、配置、素子の放熱（基板の熱抵抗）の観点、基板の機械的強度の観点、膜堆積などによるＶ溝の埋め直しの必要性の有無、入射信号光の位置合わせ余裕度などにより、適宜設定することができる。
Ｖ溝位置を素子から離すほど、Ｖ溝深さを深くする必要がある。また、ある程度Ｖ溝深さを深くしておけば、斜面反射部を十分な長さにすることができ、入射光９の位置合わせ余裕度が増大する。しかし、Ｖ溝の位置は、その深さに応じて、基板裏面からの入射光が反射されて受光素子に到達できるように受光素子の近傍に配置しなければならない。また、基板の機械的強度を考慮すると、斜面反射部の深さは、おおむね基板厚さの１／１０程度に留めるのが望ましい。例えば、市販のＩｎＰ基板では、２インチ基板の厚さは４５０μｍ程度、３インチ基板の厚さは６００μｍ程度であるので、それぞれ斜面反射部の最適深さは約４５μｍ以下、および約６０μｍ以下となる。これらを勘案すると、光吸収層の斜面反射部側の端部と斜面反射部の最深部との水平距離ｚ（図９）に関する制約は以下のように記述される。すなわち、入射光ビーム径を無限小と仮定した場合、斜面反射部の最深部の光吸収層端部で反射した入射光が少なくとも光吸収層に到達するためには、基板厚さをＴ、斜面反射部深さをＤとすると、ｚ≦Ｔ／（１０・ｔａｎθ′）とする必要がある。実際にはビーム径は有限であるため上記の式は等号を除くべきである。したがって、必要な条件はｚ＜Ｔ／（１０・ｔａｎθ′）となる。上述の２インチおよび３インチ基板では、この距離ｚは、それぞれ約１２７μｍおよび約１６９μｍとなる。ただし、チップサイズを無意昧に大きくしないためには、必要十分な距離を置いて両者をできるだけ近接させるのが好適である。一方、逆に斜面反射部を光吸収層の端部に接して形成するのは好ましくない。それは、斜面の作製が困難になるだけではなく、電極の配置が制限されたり、放熱が阻害されるなどの問題が生じるからである。したがって、斜面反射部を光吸収層端部に極端に近接させて形成するのは、上記の問題点を上回るメリットがある場含に限られる。
図８に示すように、半導体基板（ＩｎＰ基板）１の表面に、例えば５０Å程度以上のＩｎＧａＡｓ層やＩｎＧａＡｓＰ層などのＩｎＰ基板に対してエッチングの選択性のある層を残し、これをエッチングマスク２０として用いれば、ＩｎＰ基板のサイドエッチを防ぐことができるので、Ｖ溝の開口部長および深さを、より正確に制御することができる。また、本手法により（１１１）Ａ面、あるいはこれと等価な面を露出させた場合、上述のようにθは約５４．７度、したがって光吸収層への入射方向と基板面の角度θ′は約１９．５度となり、屈折を考慮すると実効吸収長は、垂直入射の場合に比べて約１．８倍となる。したがって、この手法は、本発明の受光装置を作製する上で好適である。その理由は、１．８倍の実効吸収長の増大は、高効率化の観点から十分な効果が得られる一方、それに対応する素子面積の増大（３倍）は、上述したように素子の高速性にそれほど悪影響を及ぼさないので、この本発明の手法により得られる素子構造は、高速性を犠性にせずに素子の高効率化がはかれるメリットがある。加えて、化学的な性質によりθが自律的に決定されるため、装置作製における再現性、均一性に対し非常に優れた手法となる。なお、本実施の形態ではＶ溝を形成する場合について述べたが、エッチングを途中で停止することにより、図７（ｂ）に示すように、溝の底部に平坦な箇所を残しても良い。また、ＩｎＰ以外の基板、例えばＧａＡｓやＧａＰ基板でも適宜エッチング液を選択することにより、上記と同様の工程が適用可能であり、同様の効果が得られる。
図１０は、θを５４．７度とした場合の、本発明の裏面入射型受光装置と従来の裏面入射型受光素子との効率の膜厚依存性（計算値）を比較して示したグラフである。ここでは、基板裏面での反射が無く、入射光の吸収は光吸収層のみで生じると仮定している。例えば、実施の形態１における光吸収層の厚さ２５００Åの場合、従来の受光素子では受光感度は０．４Ａ／Ｗ程度にしかならないが、本発明の受光装置では、受光感度を０．７Ａ／Ｗ程度にまで増大させることができる。また、θをさらに小さくすれば、受光感度をさらに増大させることもできる。
【０００８】
〈実施の形態３〉
図１１は、本発明の実施の形態３で例示する受光装置の上面図である。ここで１５は上部電極、１６は下部電極、１７は斜面反射部であるＶ溝である。ここで、光吸収層を含む素子部分は上部電極１５の下部に存在する。素子部の層構造は、実施の形態１と同様である。信号光を斜面反射部（Ｖ溝）１７の受光素子１３側の斜面で反射させ、素子領域（上部電極）１５に斜めに入射させるため、素子の形状は図４に示したような「遮り」が生じないように、信号光入射方向に対し長辺を有する長方形となっている。一方、短辺の長さは、信号光のビーム径と同程度以上であれば良い。本実施の形態では、ビーム径が３μｍであるのに対し、入射光ビームの合わせ余裕を考慮して素子寸法を４μｍ×１１μｍとした。この程度の面積では、例えばキャリア走行層の厚さが０．２μｍの場合、素子の帯域として１００ＧＨｚ以上を有している。また、本実施の形態のように、斜面反射部１７を受光素子に対して対称に２箇所設けるのが好適である。このようにすることにより、片方の反射部が劣化したり、製作プロセスの途中でダメージを受けたりした場合でも、もう一方の反射部で代用することが可能であることに加え、各々の反射部を独立に利用して、異なる信号を同時、あるいは時分割的に素子に入力させるようにすることもできる。また、反射部の位置が受光素子に関して対称であるため、図１２に示すように、片方の斜面反射部（Ｖ溝）８から受光素子１３に入射した信号光（入射光）９の反射光１０を再び基板裏面側へ垂直に導くことができ、基板裏面側に上記受光素子１３とは独立したフォトダイオード等のモニタ素子１８を配することにより、信号光のモニタリングを行うこともできる。また、受光素子あるいは発光素子を搭載した基板を積層し、基板間で信号を授受する光インターコネクト構成とすることもできる。
図１３は、本実施の形態で例示する受光装置の模式図あって、斜め溝１９を除いて、図１と同様である。斜面反射部８を作製する方法としては、実施の形態２で示した半導体のウェットエッチング特性を利用するものの他に、斜めドライエッチングを用いることもできる。すなわち、図１４に示すように、基板上にエッチングマスク（例えばＩｎＧａＡｓ層）２０を形成した後、半導体基板（例えばＩｎＰ基板）１を斜めに保持し、エッチングガス、およびエッチング条件を選択することにより、異方性および側面の平坦性に優れた深い溝を掘ることができる。例えば、本実施の形態のＩｎＰ基板に対する優れた異方性エッチング手法としては、Ｂｒ_２−Ｎ_２系のガスを用いたものが報告されている（Ｓ．Ｏｋｕ他、ＣｏｎｆｅｒｅｎｃｅＰｒｏｃｅｅｄｉｎｇｓｏｆｔｈｅＩｎｔｅｒｎａｔｉｏｎａｌＣｏｎｆｅｒｅｎｃｅｏｎＩｎｄｉｕｍＰｈｏｓｐｈｉｄｅａｎｄＲｅ１ａｔｅｄＭａｔｅｒｉａｌｓ、（１９９７）、ｐｐ．５７４〜５７７）。もちろん、これ以外の公知のガスや手法を用いることもできるし、他の基板、例えばＧａＡｓやＧａＰ基板をエッチングする場合でも、それぞれ公知の最適なガス種、手法を用いることにより、平坦性に優れた斜め溝を作製することができる。そして、この斜め溝１９は、実施の形態１で述べたＶ溝と等価な役割を果たす。このように、斜めドライエッチングを用いる場合は、結晶の性質で決まる特定の面を用いなければならないという実施の形態２で述べたような制約が無くなり、実施の形態１で述べた最適配置条件の範囲内で、斜面の角度や溝の深さを任意に設計することができる。さらに、斜めドライエッチングを用いれば、素子に対して順メサになる配置以外にも斜面反射部１９を設けることができるので、素子形状を三角形、矩形、円形、十字型等に適宜変形させ、素子側部の任意の位置に、任意の数の反射部となる溝を設けることもできる。このようにすることにより、代用反射部の数を任意の数まで増大させることができると共に、任意の数の複数の異なる信号を同時、あるいは時分割的に受光素子へ入力させることも可能となる。例えば、図１５（ａ）、（ｂ）は、上述の斜めドライエッチングを用いて、円形の受光素子の周囲４箇所に、斜面反射部２１を設けたものである（４入力装置）。図１５（ｂ）に示すように、受光素子１３に対し、順メサ断面が生じる結晶方位だけでなく、逆メサ断面が生じる方位など任意の方位に、断面が順メサ形状の斜面反射部（斜め溝）２１を形成することができる。なお、図１５（ｂ）は、図１５（ａ）のＡ−Ａ断面を示す。
図１６は、実施の形態１の受光素子に、有限なビーム径ｗを有する信号光を入射させる場合に必要な素子長Ｌを説明するものである。実施の形態１でも示したが、現実的こは図１に示すように、受光素子として、光吸収層までを第１段のメサ型に形成し、キャリア走行層は、もう一段広いメサ形状とするのが好適である。その理由は、２段目のメサをθ′（光吸収層への入射方向と基板面の角度）に応じた距離だけ広くしておけば、図４に示した入射光の「遮り」を防ぐことができるからである。また、この２段メサ構造は、キャリア走行層の側面部への電界集中を防止する意昧からも効果的である。「遮り」に関しては、下部電極層メサに関しても同様であり、必要な距離だけ広げて、さらにもう一段広いメサ形状となっている。このようにすれば、必要な素子長Ｌを考える場合、問題を光吸収層より上部のメサ長に簡略化することができる。以下の実施の形態では、このような仮定の下に詳述する。もちろん、２段メサを採用しない場合は、「遮り」についても考慮する必要がある。また、上述したように、ここでは半導体層界面での入射光の屈折の影響は省略して記述しているが、実際の素子では、素子長に比べ光吸収層の厚さは２桁程度小さいので、これは良い近似値となる。
図１６に示されているように、入射光９の入射方向と基板面の角度がθ′の場合、入射光９の遮りが生じないための最小限必要な素子長（光吸収層４までのメサ長）は、光吸収層４、および上部電極層（ｐ型ＩｎＧａＡｓＰ電極層）５の厚さの合計をｔ、入射光９のビーム径をｗとすると、ｗ／ｓｉｎθ′＋ｔ／ｔａｎθ′となる。例えば、ｗ＝３μｍ、θ′＝１９．５度、ｔ＝０．５μｍとすると、Ｌ＝１０．４μｍとなる。実施の形態１のところで述べたように、基板表面と斜面反射部とのなす角θの値によっては、この素子長の増大は、素子の高速特性を著しく低下させる原因となる。
【０００９】
〈実施の形態４〉
図１７は、本発明の第４の実施の形態を示すものであって、図１６で示した受光装置に比べて、素子長が異なる点と、素子側面が反射面となるように加工されている点を除いて、他は同様である。ここで、素子側面を反射加工面（素子側壁）２２とすることにより、図１７に示すように、光吸収層４に入射させた入射光９を素子側壁で反射させることができ、すべて光吸収層４へ導くことができる。そして、上部電極７で反射した信号光を効率的に再度、光吸収層４へ導くこともできる。これにより、入射光９の遮りが生じないための最小限必要な素子長をｗ／ｓｉｎθ′まで短縮することができる。例えばｗ＝３μｍ、θ′＝１９．５度とすると、Ｌ＝９μｍとなる。反射加工面２２は、斜面反射部と反対側の素子側面に設けるだけでも効果があるが、両面とも反射加工面２２としておけば、さらに効率よく入射光を光吸収層４へ導くことができる。ここで、素子側面における全反射条件についても考慮する必要がある。素子側壁が基板に対し垂直であり、素子周囲部に膜が堆積されていない（周囲部の屈折率１）と仮定した場合、ＩｎＧａＡｓよりなる光吸収層（屈折率３．５９）では、臨界角は約１６．２度となる。ＩｎＰ層の場合（屈折率３．１７）は、臨界角は約１８．４度となる。斜面反射部として実施の形態２で示した（１１１）面あるいはこれと等価な面を用いる場合、θ′は約１９．５度であり、光吸収層内では半導体層界面での屈折の結果、入射角は約３３．７度となっており、したがって、受光素子をＩｎＰ、ＩｎＧａＡｓ、およびＩｎＧａＡｓＰのいずれの材料で構成しても、この全反射条件を満足している。素子側壁に膜を堆積する場合は、側壁で全反射が生じる条件にするため、その屈折率に関して考慮する必要があるが、図１７のように構成すれば、側面反射は光吸収層４より上部で生じるため、屈折率の条件は緩和される。そして、ＩｎＧａＡｓ光吸収層の側壁においては屈折率が１．９９以下の材料であれば良いので、堆積膜としてボリイミド、ＢＣＢ、シリコン酸化膜、シリコン窒化膜等を用いることができる。基板に対し垂直な側壁は、例えばＩｎＧａＡｓであればクエン酸系のウェットエッチング液を用いることにより形成することができる。もちろん、ドライエッチングでも形成することができる。
【００１０】
〈実施の形態５〉
図１８は本発明の第５の実施の形態を示すものであって、図１７の受光装置と比べて異なるところは、斜面反射部と反対側の受光素子の側部を、さらに、深さｔ′だけ掘り込んだ非対称の断面形状としている。そして、入射光９がすべて直接もしくは素子側壁２３で反射して吸収層に導かれるように、その位置および深さを選定している。また、ここでは側壁はすべて基板に対し垂直としている。素子側部の掘り込み深さｔ′は、直接、光吸収層に入射しない入射光をすべて光吸収層側へ反射させるだけの深さ以上とする必要があり、一方、素子長は、上記反射した入射光がすべて光吸収層へ導かれる範囲に設定しなければならない。そのためには、素子長が、１／２・ｗ／ｓｉｎθ′以上であれば良く、その場合、ｔ′はＬ・ｔａｎθ′以上であれば良い。したがって、最大で、素子長を実施の形態４の半分にまで短縮することができる。例えばｗ＝３μｍ、θ′＝１９．５度とすると、Ｌ＝４．５μｍとなる。このような構成とすることにより、図１８に示すように、入射光の遮りを生じさせずに、入射光をすべて光吸収層へ導くことができる。ここでは、上記側面のうち、斜面反射部側を光吸収層の下部まで、斜面反射部の反対側を、キャリア走行層以下の層にまで掘り込んだ構成としたが、メサ深さはこれに制限されるものではなく、前者をキャリア走行層以下の層まで堀り込んだ場合でも、後者をより深く掘り込むことにより非対称な断面形状とし、入射光９の一部を斜面反射部の反対側の側壁で反射させて光吸収層へ導くことができる。
【００１１】
〈実施の形態６〉
図１９は、本発明の第６の実施の形態であって、図１８と比べて異なるところは、斜面反射部の反対側の素子側壁２４を、断面が逆メサ形状（すなわち、φ＜９０度）となるようにしている。そして、図１８の場合と同様に、側壁で反射した信号光がすべて光吸収層４に到達するようにしている。ここでの条件は、実施の形態５と同様に、入射光１０がすべて光吸収層４へ導かれるように、素子長Ｌ、掘り込み深さｔ′、および側面角度φを設定するところである。ここでの素子長Ｌは、素子容量を規定する光吸収層４の下面の長さを意昧している。このような構成とすることにより、実施の形態５と同様、図１９に示されるように、入射光１０をすべて光吸収層４へ導くことができる。そして、素子側部を逆メサ状にしているため、入射光１０の遮りが生じないための最小限必要な素子長を、実施の形態５の場合よりもいっそう短縮することができる。ここで、斜面反射部側の素子側面は基板に対して垂直としたが、もちろん角度を有していても良い。これを逆メサ断面にすることにより、上部電極７のコンタクト面積を増大させ、コンタクト抵抗の低減をはかることもできる。
上述の各実施の形態では、受光素子としてフォトダイオードの一種であるＵＴＣ−ＰＤを用いた。ＵＴＣ−ＰＤの層構成は、本実施の形態で示したもの以外にも、吸収層とキャリア走行層との間に複数の層を挿入し、伝導帯不連続を低減することによりキャリアブロック現象を防止したり、キャリア走行層の一部を光吸収層として用いるハイブリッド構造など、さまざまなバリエーションが可能である。また、フォトダイオードとしては、高不純物濃度ｐ型電極層、アンドープもしくは低不純物濃度光吸収層、高不純物濃度ｎ型電極層の積層構造を基本とする通常のｐｉｎフォトダイオード、超格子構造の吸収層を用いたフォトダイオード、アバランシェフォトダイオード等、その他のフォトダイオードを用いることもできる。また受光素子としては、フォトダイオードやフォトトランジスタ等の単体受光素子以外にも、受光素子と他の素子を縦積みに形成した複合素子や、受光素子と電子デバイスとの集積回路であっても良い。
受光装置を構成する材料としては、上記実施の形態ではＩｎＰ基板に格子整合するＩｎＰ／ＩｎＧａＡｓ（Ｐ）系を用いたが、ＩｎＡｌ（Ｇａ）Ａｓ／ＩｎＧａＡｓ、ＩｎＡ１Ａｓ／ＧａＡｓＳｂなどのＩｎＰに格子整合する他の材料系、Ａ１ＧａＡｓ／（Ａ１）ＧａＡｓ、ＩｎＧａＰ／ＧａＡｓなどのＧａＡｓに格子整合する材料系、Ａ１ＧａＮ／ＧａＮ／ＩｎＧａＮなどのＧａＮに格子整合する半導体材料の組み合わせなどや、格子不整合系材料など、通常の半導体材料の組み合わせを用いることもできる。
基板としては、半絶縁性のものを用いたが、導電性のものであっても良い。
なお、詳述しなかったが、裏面入射型受光素子の場合は、基板が入射光に対して透明であることが必要であり、電極層なども極力入射光に対して透明であることが望ましい。
光通信で通常用いられている１．３μｍ帯や、１．５μｍ帯の波長の光に対し、ＩｎＰは透明であることから、一般に用いられているＩｎＰ基板上に作製されたＩｎＰ、ＩｎＧａＡｓ、ＩｎＧａＡｓＰなどを用いた受光素子は好適な例である。この他にも、例えば１．３μｍ帯や１．５μｍ帯の波長の光に対しＧａＡｓ、０．８５μｍ帯の波畏の光に対しＡ１ＧａＡｓを基板として用いることも好適な例である。
【００１２】
【発明の効果】
以上説明したように、本発明の裏面入射型受光装置は、受光素子の側部に受光素子とは独立した斜面反射部を設け、基板裏面からの入射光をすべて反射させ、吸収層に斜めから信号光を入射させることにより、吸収層内での実効的な吸収長を増大させることができるので、吸収層の厚さの増大による高速性の低下を招くことなく、素子効率を向上できる効果がある。また、上記構成に加え、少なくとも斜面反射部の反対側に位置する素子の側端部も反射部となるように構成することにより、素子部に斜めに入射した信号光の一部をさらに反射させ、より小さい素子面積で光吸収層に効率的に信号光を導くことができるので、素子の高速性の低下を招くことなく、さらに素子効率を向上できる効果がある。さらに、複数の斜面反射部を設けることにより、容易に複数の入射光の入力が可能となり、受光装置の機能化がはかれると共に、斜面反射部を代替えできる効果もある。また、半導体薄膜をエッチングマスクとして基板結晶の（１１１）Ａ面あるいはこれと等価な面が露出するようなエッチング液を用いて斜面反射部を形成することにより、斜面反射部として好適な角度を有するＶ溝を、寸法および角度の制御性良く形成できる効果がある。
本発明の受光装置は、従来の受光素子と比べ、層構成やプロセス上の特別の配慮が不要であり、単純に受光素子から必要な距離範囲で、素子が存在しない領域に斜面反射部となる凹部を形成するだけで良いので、他の素子、例えば、電子回路との集積化（ＯＥＩＣ化）などが容易に行える効果もある。また、素子効率をそれほど必要としない場合は、さらなる光吸収層の薄層化により、素子のさらなる高速化をはかることができる効果がある。
【図面の簡単な説明】
【図１】本発明の実施の形態１で例示した裏面入射型受光装置の構成を示す模式図。
【図２】本発明の実施の形態１で例示した裏面入射型受光装置の斜面反射部角度と吸収層への入射角の関係を示す模式図。
【図３】本発明の実施の形態１で例示した裏面入射型受光装置の斜め入射による実効吸収長の増大効果を示す模式図。
【図４】本発明の実施の形態１で例示した入射光の遮りが生じる場合を示す模式図。
【図５】本発明の実施の形態１で例示した実効吸収長の増分と素子速度（相対値）の斜面反射部の角度（θ）依存性を示すグラフ。
【図６】本発明の実施の形態１で例示した裏面入射型受光装置の他の構成を示す模式図。
【図７】本発明の実施の形態２で例示した基板結晶の面方位と逆メサ形状（ａ）および順メサ形状（ｂ）を示す模式図。
【図８】本発明の実施の形態２で例示したＶ溝の作製方法を示す模式図。
【図９】本発明の実施の形態２で例示した斜面反射部の深さ（Ｄ）と光吸収層からの距離（ｚ）の制約に関する説明図。
【図１０】本発明の実施の形態２で例示した本発明の受光装置の素子効率の光吸収層の厚さ依存性を、従来の素子と比較して示したグラフ。
【図１１】本発明の実施の形態３で例示した裏面入射型受光素子と斜面反射部の配置を示す模式図。
【図１２】本発明の実施の形態３で例示した裏面入射型受光装置の構成を示す模式図。
【図１３】本発明の実施の形態３で例示した裏面入射型受光装置の他の構成を示す模式図。
【図１４】本発明の実施の形態３で例示した斜めエッチングによる斜面反射部の作製方法を示す模式図。
【図１５】本発明の実施の形態３で例示した４入力型裏面入射型受光装置の構成を示す模式図。
【図１６】本発明の実施の形態３で例示した裏面入射型受光素子の最小素子長を示す説明図。
【図１７】本発明の実施の形態４で例示した裏面入射型受光素子の最小素子長を示す説明図。
【図１８】本発明の実施の形態５で例示した裏面入射型受光素子の最小素子長を示す説明図。
【図１９】本発明の実施の形態６で例示した裏面入射型受光素子の最小素子長を示す説明図。
【図２０】従来の導波路型受光素子の構成を示す模式図。
【図２１】従来の表面入射型受光素子の構成を示す模式図。
【図２２】従来の裏面入射型受光素子の構成を示す模式図。
【符号の説明】
１…半導体基板（ＩｎＰ基板など）
２…ｎ型ＩｎＰよりなる電極層（下部電極層）
３…アンドープＩｎＰよりなるキャリア走行層
４…ｐ型ＩｎＧａＡｓよりなる光吸収層
５…ｐ型ＩｎＧａＡｓＰよりなる電極層（上部電極層）
５′…ｐ型半導体よりなる電極層（上部電極層）
６…下部電極
７…上部電極
８…斜面反射部（Ｖ溝）
９…入射光
１０…反射光
１１…反射防止膜
１２…基板表面
１３…受光素子
１４…基板裏面
１５…上部電極
１６…下部電極
１７…斜面反射部（Ｖ溝）
１８…モニタ素子
１９…斜め溝
２０…エッチングマスク（ＩｎＧａＡｓ層など）
２１…斜面反射部（斜め溝）
２２…反射加工面（素子側壁）
２３…素子側壁
２４…素子側壁
２５…半導体層
２６…半導体基板
２７…光吸収層
２８…受光素子
２９…入射光
３０…上部電極
３１…下部電極
３２…入射窓
３３…劈開面
３４…反射光[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a light-receiving device having a structure in which signal light is incident from the back surface of a substrate, and particularly to a high-speed, high-efficiency back-illuminated light-receiving device and a method for manufacturing the same.
[0002]
[Prior art]
A semiconductor light receiving element represented by a photodiode is an element that converts an optical signal into an electric signal, and is widely used in fields such as optical communication and optical measurement. A photodiode using a conventional pn junction includes, for example, a “waveguide-type” light receiving element that receives signal light from an end face of a substrate as shown in FIG. 20 and a signal light perpendicular to the substrate as shown in FIG. It is classified into a “surface type” light-receiving element that enters. Here, the semiconductor layer 25 is based on a semiconductor laminated structure including an n-type electrode layer, a light absorbing layer 27, and a p-type electrode layer formed on a semiconductor substrate 26, and the required layers are further multi-layered. Thereby, a structure also serving as an optical waveguide structure can be provided. The light receiving element 28 is often insulated and separated by mesa processing, and an upper electrode 30 and a lower electrode 31 are formed in contact with the upper and lower electrode layers of the element, respectively. The feature of the waveguide type light receiving element is that the traveling direction of the incident light 29 and the traveling direction of the photoexcited carrier are different from each other, and therefore the band and the efficiency of the light receiving element can be independently designed. However, on the other hand, since the coupling efficiency with the incident light is low and the cleavage plane 33 is used at the light incident end face, the compatibility with a normal semiconductor manufacturing process or the like is poor, and there is a problem in terms of mass productivity. There is a drawback that the degree of freedom of monolithic integration with the element is low. Further, one of the drawbacks is that the device characteristics cannot be measured without cleavage, so that the device cannot be evaluated in a wafer state. On the other hand, a surface-type light receiving element that inputs signal light to a substrate from a vertical direction has advantages in several points. In other words, there is little problem of a decrease in coupling efficiency with incident light, and a feature that the efficiency can be increased by increasing the thickness of the light absorption layer, and since the compatibility with a normal semiconductor manufacturing process is good, the mass productivity is high, Due to the similarity in structure, there is such a feature that the degree of freedom of monolithic integration with other elements is high. Also, device evaluation in a wafer state is easy.
Among the surface-type photodiodes, the surface-type light-receiving element for inputting signal light from the substrate surface side, that is, the front-illumination-type element (FIG. 21) can be easily modularized with a fiber or the like for introducing incident light. It is often used in low-speed applications. However, in the case of the front-illuminated type, the window 32 for light incidence is required in the region of the upper electrode 30. Therefore, the spreading resistance of the window region (portion without an electrode) is increased, and the device characteristics cannot be improved. There was a disadvantage. Further, there is a trade-off between the thickness of the light absorbing layer 27 and the element efficiency, and there is a problem that the efficiency is reduced when the absorbing layer is made thinner to increase the speed. On the other hand, in a region where high speed application and high efficiency are required, a back illuminated type (FIG. 22) which is a surface type light receiving element is mainly used. The reason is that by forming an electrode on the entire surface of the upper electrode 30 region, it is possible to reduce the element resistance, and at the same time, reflect the signal light on the upper electrode 30 and guide the reflected light 34 to the light absorption layer again. This is because the efficiency of the device can be improved.
[0003]
[Problems to be solved by the invention]
However, in the conventional back-illuminated surface-type photodiode, in order to further increase the speed, in order to shorten the carrier traveling distance extremely, that is, in order to force the thickness of the light absorbing layer to be extremely thin, There has been a problem that sufficient element efficiency cannot be obtained while maintaining high speed.
[0004]
SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems in the prior art and to make a signal light obliquely incident on a light absorbing layer portion of a light receiving element, further reflect the incident signal light, and reduce the light absorbing layer with a smaller element area. It is an object of the present invention to provide a back-illuminated light-receiving device capable of efficiently introducing signal light into a light-emitting device, achieving high-speed and high-efficiency elements, and a method for manufacturing the same.
[0005]
[Means for Solving the Problems]
In order to achieve the above object of the present invention, the present invention is configured as described in the claims. That is,
The present invention as defined in claim 1
A back-illuminated light-receiving device, comprising: a semiconductor light-receiving element formed on a semiconductor substrate; and, on the surface of the substrate, one or more concave slope reflection portions formed independently on a side of the light-receiving element. At least the light receiving element and the inclined surface reflecting portion are arranged in a structure in which the signal light incident from the back surface of the substrate is reflected by the inclined surface reflecting portion and is incident on the light receiving element from an oblique direction with respect to the substrate surface. And, on the entire side surface portion of the light receiving element, or at least on the side surface portion opposite to the slope reflection portion, there is no structure for reflecting the signal light incident obliquely to the light receiving element toward the element side, and the slope reflection portion Has an asymmetrical mesa-shaped cross-sectional shape in which the opposite element side surface is dug deeper than the element side surface on the side of the inclined reflector. This is a back illuminated light receiving device.
Further, the present invention provides, as described in claim 2,
2. The semiconductor device according to claim 1, wherein the thickness of the semiconductor substrate is T, the depth of the inclined reflecting portion is D, and the angle between the incident direction of the signal light reflected by the inclined reflecting portion and incident on the light receiving element and the substrate surface is θ ′. In this case, the horizontal distance z between the end of the light absorbing layer constituting the light receiving element on the side of the inclined reflecting portion and the deepest portion of the inclined reflecting portion is z <T / (10 · tan θ ′). Set to range And, on the entire side surface portion of the light receiving element, or at least on the side surface portion opposite to the slope reflection portion, there is no structure for reflecting the signal light incident obliquely to the light receiving element toward the element side, and the slope reflection portion Has an asymmetrical mesa-shaped cross-sectional shape in which the opposite element side surface is dug deeper than the element side surface on the side of the inclined reflector. This is a back illuminated light receiving device.
Further, the present invention provides, as set forth in claim 3,
In Claim 1 or Claim 2, an even number of the slope reflection portions are disposed at positions symmetrical with respect to the light receiving element. And, on the entire side surface portion of the light receiving element, or at least the side surface portion opposite to the inclined surface reflection portion, the signal light obliquely incident on the light receiving element is reflected to the element side, and the inclined surface reflection portion The element side surface opposite to the side has an asymmetrical mesa-shaped cross-sectional shape dug deeper than the element side surface on the slope reflection portion side. This is a back illuminated light receiving device.
Also, as described in claim 4,
4. A light receiving element according to claim 1, wherein Little At least the side opposite the slope reflector Is formed into an inverted mesa-shaped cross-section This is a back illuminated light receiving device.
Ma Claims 5 As described in
Claims 1 to Claim 4 The method for manufacturing a back-illuminated light-receiving device according to any one of the preceding claims, wherein a semiconductor thin film having selective etching characteristics with respect to the semiconductor substrate is used as a mask when forming the concave slope reflection portion on the semiconductor substrate. This is a method of manufacturing a back illuminated light receiving device including a step of etching by chemical etching so that a (111) A plane of a substrate crystal or a plane equivalent thereto is exposed.
The present invention relates to claims 1 to 4 As described in any one of (1) to (4), an independent inclined reflecting portion is provided on the side of the light receiving element to reflect all the incident light from the back surface and to allow the signal light to enter the light absorbing layer from an oblique direction. It is. With such a configuration, the effective absorption length for the incident light increases, so that the efficiency of the light receiving element can be significantly increased. Then, by reflecting all the incident signal light perpendicularly incident from the back surface of the substrate on the slope reflection portion formed in the substrate independently of the light receiving element, the incident signal light is incident on the light absorption layer at a low angle. This is different from the prior art.
Also, the present invention Claims 1 to 4 As described in the above, the entire side surface portion of the light receiving element, or at least the element side surface portion located on the opposite side to the slope reflection portion is also configured to be a reflection portion, and such a configuration is adopted. Accordingly, there is an effect that the signal light obliquely incident on the element portion is further reflected, and the signal light can be efficiently guided to the light absorption layer with a smaller element area. This is different from the prior art in that the side surface of the element is used as a reflecting portion so that even if the element has a short dimension, the incident signal light at a low angle is efficiently guided to the light absorbing layer.
Further, the present invention provides 5 As described above, using a semiconductor thin film having selective etching characteristics for a semiconductor substrate as a mask, etching is performed by a chemical etching method so that the (111) A plane of the substrate crystal or a plane equivalent thereto is exposed. This is a method of forming a slope reflection portion, and thus has an effect that a semiconductor substrate having a slope reflection portion can be easily manufactured with good reproducibility and high yield.
[0006]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the embodiment described below, a photodiode called a UTC-PD (Uni-Traveling-Carrier Photodiode) having excellent high-speed performance and high output performance (Japanese Patent Application Laid-Open No. 9-275224) is used as an example of a light receiving element.
<Embodiment 1>
FIG. 1 shows a first embodiment of the present invention, in which 1 is a semiconductor substrate made of semi-insulating InP, 2 is an n-type InP electrode layer, 3 is an undoped InP carrier traveling layer, and 4 is a signal light absorbing layer. P-type InGaAs absorption layer (film thickness 2500 Å), 5 is a p-type InGaAsP electrode layer (composition: equivalent to a wavelength of 1.3 μm), 6 is a lower electrode, 7 is an upper electrode, and 8 is a V-groove formed on a side of the element. 9 is incident light, and 10 is reflected light. Further, an antireflection film 11 is provided on the back surface of the substrate. Note that the UTC-PD layer configuration is simplified and described here because it is not directly related to the configuration of the present invention. In addition, the state of refraction of a light beam having a finite diameter at the interface of the semiconductor layer is omitted and described except for FIG.
As shown in FIG. 1, in the present embodiment, a light receiving element 13 having a mesa shape is formed, and a slope reflecting portion 8 formed of a V groove is independently formed on a side portion thereof. As will be described later, the angle (the acute angle side) formed by this slope with the substrate surface was about 54.7 ° (degrees). First, all the incident light 9 which is perpendicularly incident on the substrate surface from the back surface of the substrate is reflected by the inclined reflecting portion (V-groove) 8 and is incident on the light absorption layer 4 of the mesa element portion from an oblique direction. Here, as shown in FIG. 2, assuming that the angle (the acute angle side) of the inclined reflecting portion 8 with respect to the substrate surface 12 is θ, the angle θ ′ between the incident direction to the light absorbing layer 4 and the substrate surface is 2θ−90 °. Become. The incident light 9 is at the interface between the carrier transit layer 3, the light absorbing layer 4 made of an InGaAs layer, and the electrode layer 5 made of the light absorbing layer 4 and InGaAsP, according to Snell's law, as shown in FIG. Bend. Therefore, the thickness of the light absorbing layer 4 is d, and the refractive index of the undoped InP carrier traveling layer 3 is n. ₁ , The refractive index of the light absorbing layer 4 is n ₂ Then, the signal light incident on the light absorbing layer 4 travels within the light absorbing layer 4 at a distance d represented by the following equation (1). _eff And the light is emitted to the upper part of the light absorption layer 4.
d _eff = D / [1-4 (n ₁ / N ₂ ) ² sin ² θcos ² θ) ^1/2 ...... (Equation 1)
The emitted signal light is reflected by the upper electrode 7, enters the light absorption layer 4 again, and travels again within the light absorption layer 4 by the distance d. _eff Only propagates. Therefore, the effective absorption length is
2d _eff It becomes. This is longer than the case where the signal light is perpendicularly incident on the light absorbing layer 4 (absorption length 2d) by the coefficient shown in the expression (1), thereby contributing to an increase in the efficiency of the device. . In the configuration of the present embodiment, the absorption length increase coefficient is about 1.8 times.
Although the effective absorption length of the signal light (referred to as the effective absorption length) depends on the angle θ of the incident light 9 of the inclined reflecting portion 8, the angle between the direction of incidence on the light absorbing layer 4 and the substrate surface (θ ′). When the angle approaches 90 degrees, the effect of increasing the effective absorption length due to oblique incidence decreases. On the other hand, when the incidence angle is too small, the element area, that is, the element capacitance increases, and the high-speed characteristics deteriorate, as described later. become. In fact, since the incident signal light has a finite beam diameter, the element area of the light receiving element 13 (actually, the element length in the incident direction) must be increased in conjunction with θ ′ as shown in FIG. The signal light 9 is "blocked", and only a part of the signal light 9 reaches the light absorbing layer 4, so that the element efficiency is reduced. Also, it is obvious that the reflected light 10 cannot reach the element when the angle (θ) between the substrate surface and the inclined reflecting portion is smaller than 45 degrees, and the incident light 9 is smaller than 90 degrees. It does not function as a reflection part without being emitted from the substrate once. Therefore, the range considered as θ is 90 °>θ> 45 °. That is, the “slope reflection portion 8” is defined such that the angle (the acute angle side) θ between the substrate surface and the slope reflection portion is within the above range.
As described above, the speed and efficiency of the element are both functions of θ and have a trade-off relationship. As shown in FIG. 5, if θ exceeds 67 degrees, the increase (difference)% of the effective absorption length becomes about 30% or less, and the effect of the increase of the effective absorption length is not sufficient. Therefore, θ is preferably 67 degrees or less. On the other hand, if θ becomes too low, the required element area increases, and the element high-speed characteristics are remarkably reduced due to an increase in element capacitance. By the way, the high-speed properties of the element include an element that limits the capacity and an element that limits the carrier transit time. The former is related to the element area, and the latter is related to the thickness of the light absorbing layer. Generally, the thickness of the light absorbing layer is set to a finite value in order to secure a predetermined efficiency (set to about 2500 ° in the present embodiment). Therefore, up to a certain element area, the increase in the area does not significantly affect the high speed of the element. The effect becomes remarkable when the element area is increased about 5 times or more as compared with the case of normal incidence, and θ is preferably 50 degrees or more. Therefore, when considering a realistic device, a preferable range of the above θ is 67 ° ≧ θ ≧ 50 °.
In the configuration of the back illuminated light receiving device according to the present invention, θ must be determined so that the incident light 9 is totally reflected at the inclined reflecting portion (V-groove) 8. In addition, when a film is deposited on the surface of the V-groove for the purpose of protecting the surface, it is necessary to consider the refractive index. Since the refractive index of the InP substrate used in the present embodiment is 3.17 (in the case of a wavelength of 1.55 μm), when the surface is air or a sealing gas (the refractive index is almost 1 in each case), θ is 45. Above degree all satisfy the total reflection condition. On the other hand, when depositing a protective film, there is a restriction on the refractive index in order to maintain the total reflection condition. For example, when θ is set to 54.7 degrees, the material deposited on the surface of the V-groove must have a refractive index of 2.59 or less. Although not used in this embodiment, a configuration using a surface protective film is generally preferred. From the viewpoint of compatibility with a normal semiconductor process, polyimide (about 1.5 in refractive index), organic BCB (bisbenzocyclobutene: a refractive index of about 1.5), a silicon oxide film (a refractive index of about 1.5), a silicon nitride film (a refractive index of about 1.8 to 2.2), etc., which are a kind of film, are preferable. Used for All of these satisfy the condition of total reflection when θ is 45 degrees or more. The type of the deposited film may be a film other than those described above, and the restriction on the refractive index is appropriately defined by θ. If θ is increased, this restriction is eased.
In FIG. 1, the incident light 9 is described as being reflected by the upper electrode 7, but by appropriately selecting the refractive index difference between the material used for the electrode layer 5 and the material used for the light absorbing layer 4, as shown in FIG. The incident light 9 can be totally reflected at the interface between the absorption layer 4 and the electrode layer 5 '. In an actual device, light absorption may occur at the semiconductor / metal interface, so this structure is effective for improving device efficiency. For example, as in the present embodiment, when the slope reflection portion 8 is formed by selecting the angle θ between the substrate surface and the slope reflection portion so that θ ′ = 19.5 °, the light absorption layer 4 and the electrode The refractive index ratio required to achieve the total reflection condition at the interface of the layer 5 'is about 0.83. Therefore, when the light absorption layer 4 is made of InGaAs (refractive index 3.59), the electrode layer 5 'may be made of, for example, A1P or A1As (both have a refractive index of 2.8 or less at 1.55 μm). .
[0007]
<Embodiment 2>
FIGS. 7A, 7B, and 8 are schematic views showing a method of forming the inclined reflection portion (V-groove) 8 on the semiconductor substrate 1. FIG. The method for forming the V-groove utilizes the chemical etching characteristics of the semiconductor. That is, when a substrate having a (001) plane generally used as a compound semiconductor substrate or a plane equivalent thereto is subjected to mesa etching, its (−110) cross section and (110) cross section (or these 7 (a) and FIG. 7 (b), the etched cross-sectional shape is a so-called "reverse mesa" shape, and the latter is a so-called "reverse mesa" shape. It has a "normal mesa" shape. More specifically, the side surfaces (front and rear as viewed from the OF) of the stripe-shaped mesas parallel to the OF (orientation flat) of a commercially available substrate are forward mesas, and the side surfaces (the directions from the OF) differ by 90 degrees. (Left and right when viewed) is an inverted mesa. The angle of these mesa slopes with respect to the substrate surface (the acute angle side in the case of a forward mesa, the obtuse angle side in the case of a reverse mesa) differs depending on the etching solution. However, if an appropriate etching solution and etching conditions are selected, the mesa side surface can be changed. A (111) plane or a plane equivalent thereto can be exposed as the surface.
This is because, for example, in the case of a group III-V compound semiconductor, the property is such that the etching is easily stopped at the metal atom plane (usually referred to as the A plane). When the substrate is InP, this surface is an In surface, and is typically exposed using a mixed solution of bromide and methanol, a mixed solution of bromic acid, hydrochloric acid and phosphoric acid, or a mixed solution of sulfuric acid and hydrogen peroxide. (Eg, S. Adachi et al., J. E1 electrochem. Soc. Vo1, 128, No. 6.1981, pp. 1342-1349). When the etching is stopped on the (111) A plane or a plane equivalent thereto, the angle between the substrate surface and the slope as viewed from the (-110) cross section or the plane equivalent thereto is about 54.7 degrees. .
FIG. 8 is a diagram showing a slope reflection portion (V groove) manufactured in the present embodiment. First, a mask having an opening with a photoresist is manufactured on an InGaAs layer formed on an InP substrate. Subsequently, the InGaAs layer is selectively etched with a citric acid-based etchant. After that, only InP is selectively etched using a mixed solution of hydrochloric acid and phosphoric acid, and the etching is automatically stopped on the (111) A plane to form a V groove. As shown in FIG. 8, since the depth of the V groove depends on the length of the opening of the etching mask (the depth is 1 / √2 with respect to the length of the opening), the depth of the V groove is determined by the mask design. can do. The required depth of the V groove is determined by the positional relationship between the light receiving element and the V groove. Here, the opening length was about 28 μm, and the V-groove depth was about 20 μm. The horizontal distance from the end of the light absorbing layer to the deepest part (center) of the V groove was set to 27 μm. These values are determined based on the shape and arrangement of the extraction electrodes from the element, the viewpoint of heat radiation of the element (thermal resistance of the substrate), the viewpoint of mechanical strength of the substrate, the necessity of refilling the V-groove by film deposition, etc. It can be set appropriately according to the positioning margin of the incident signal light.
It is necessary to increase the depth of the V groove as the position of the V groove is further away from the element. Also, if the V-groove depth is increased to some extent, the slope reflecting portion can be made sufficiently long, and the alignment margin of the incident light 9 increases. However, the position of the V-groove must be arranged in the vicinity of the light receiving element such that incident light from the back surface of the substrate can be reflected and reach the light receiving element according to the depth. Further, in consideration of the mechanical strength of the substrate, it is desirable that the depth of the inclined reflecting portion is limited to about 1/10 of the thickness of the substrate. For example, in the case of a commercially available InP substrate, the thickness of a 2-inch substrate is about 450 μm, and the thickness of a 3-inch substrate is about 600 μm. Therefore, the optimum depths of the slope reflection portions are about 45 μm or less and about 60 μm or less, respectively. . Considering these, the restriction on the horizontal distance z (FIG. 9) between the end of the light absorbing layer on the side of the slope reflection portion and the deepest portion of the slope reflection portion is described as follows. That is, assuming that the diameter of the incident light beam is infinitely small, in order for the incident light reflected at the end of the light absorbing layer at the deepest part of the slope reflecting portion to reach at least the light absorbing layer, the thickness of the substrate must be T, and Assuming that the depth of the reflecting portion is D, it is necessary to satisfy z ≦ T / (10 · tan θ ′). Since the beam diameter is actually finite, the above equation should exclude the equal sign. Therefore, the necessary condition is z <T / (10 · tan θ ′). For the above-mentioned 2 inch and 3 inch substrates, this distance z is about 127 μm and about 169 μm, respectively. However, in order not to unnecessarily increase the chip size, it is preferable to keep both as close as possible with a necessary and sufficient distance. On the other hand, it is not preferable to form the slope reflection portion in contact with the end of the light absorption layer. This is because not only is it difficult to fabricate the slope, but also there are problems such as restrictions on the arrangement of the electrodes and inhibition of heat radiation. Therefore, the formation of the slope reflecting portion extremely close to the end of the light absorbing layer is limited to the case where there is a merit exceeding the above problem.
As shown in FIG. 8, a layer having an etching selectivity with respect to an InP substrate such as an InGaAs layer or an InGaAsP layer of, for example, about 50 ° or more is left on the surface of the semiconductor substrate (InP substrate) 1, and this is left as an etching mask 20. If used, the side etching of the InP substrate can be prevented, so that the opening length and depth of the V-groove can be controlled more accurately. When the (111) A plane or a plane equivalent thereto is exposed by this method, θ is about 54.7 degrees as described above, and therefore, the angle θ ′ between the direction of incidence on the light absorbing layer and the substrate plane. Is about 19.5 degrees, and when refraction is taken into account, the effective absorption length is about 1.8 times that in the case of normal incidence. Therefore, this method is suitable for manufacturing the light receiving device of the present invention. The reason is that a 1.8-fold increase in the effective absorption length can provide a sufficient effect from the viewpoint of high efficiency, while a corresponding increase in the element area (3 times) increases the speed of the element as described above. The device structure obtained by the method of the present invention has a merit that the efficiency of the device can be increased without sacrificing high-speed performance since the performance is not so adversely affected. In addition, since θ is autonomously determined by chemical properties, it is a very excellent method for reproducibility and uniformity in device fabrication. Although the case where the V-groove is formed has been described in this embodiment, the etching may be stopped halfway to leave a flat portion at the bottom of the groove, as shown in FIG. 7B. Also, by appropriately selecting an etchant for a substrate other than InP, for example, a GaAs or GaP substrate, the same steps as above can be applied, and the same effects can be obtained.
FIG. 10 is a graph comparing the thickness dependency (calculated value) of the efficiency of the back illuminated light receiving device of the present invention and the conventional back illuminated light receiving element when θ is set to 54.7 degrees. It is. Here, it is assumed that there is no reflection on the back surface of the substrate, and that the incident light is absorbed only by the light absorbing layer. For example, when the thickness of the light absorbing layer is 2500 ° in Embodiment 1, the light receiving sensitivity of the conventional light receiving element is only about 0.4 A / W, but the light receiving sensitivity of the present invention is 0.7 A / W. It can be increased to about W. Further, if θ is further reduced, the light receiving sensitivity can be further increased.
[0008]
<Embodiment 3>
FIG. 11 is a top view of the light receiving device exemplified in the third embodiment of the present invention. Here, 15 is an upper electrode, 16 is a lower electrode, and 17 is a V-groove, which is a slope reflection portion. Here, the element portion including the light absorption layer exists below the upper electrode 15. The layer structure of the element portion is the same as in the first embodiment. Since the signal light is reflected by the slope on the light receiving element 13 side of the slope reflecting portion (V-groove) 17 and is obliquely incident on the element region (upper electrode) 15, the shape of the element is “blocked” as shown in FIG. This is a rectangle having a long side with respect to the signal light incident direction so as not to cause a problem. On the other hand, the length of the short side may be equal to or larger than the beam diameter of the signal light. In the present embodiment, while the beam diameter is 3 μm, the element size is set to 4 μm × 11 μm in consideration of the margin for matching the incident light beam. In such an area, for example, when the thickness of the carrier transit layer is 0.2 μm, the band of the element has 100 GHz or more. Further, as in the present embodiment, it is preferable to provide two inclined reflection portions 17 symmetrically with respect to the light receiving element. In this way, even if one of the reflecting portions is deteriorated or damaged during the manufacturing process, the other reflecting portion can be used instead. Can be used independently to input different signals to the elements simultaneously or in a time-division manner. In addition, since the positions of the reflecting portions are symmetrical with respect to the light receiving element, as shown in FIG. 12, the reflected light 10 of the signal light (incident light) 9 incident on the light receiving element 13 from one of the inclined reflecting portions (V-grooves) 8. Can be guided vertically again to the backside of the substrate, and signal light can be monitored by disposing a monitor element 18 such as a photodiode independent of the light receiving element 13 on the backside of the substrate. Alternatively, an optical interconnect configuration in which a substrate on which a light-receiving element or a light-emitting element is mounted is stacked and a signal is transmitted and received between the substrates may be employed.
FIG. 13 is a schematic diagram of a light receiving device exemplified in the present embodiment, and is similar to FIG. 1 except for an oblique groove 19. As a method for manufacturing the inclined reflecting portion 8, in addition to the method using the wet etching characteristics of the semiconductor described in Embodiment 2, oblique dry etching can be used. That is, as shown in FIG. 14, after forming an etching mask (for example, InGaAs layer) 20 on the substrate, holding the semiconductor substrate (for example, InP substrate) 1 at an angle, and selecting an etching gas and etching conditions. A deep groove having excellent anisotropy and flatness on side surfaces can be dug. For example, as an excellent anisotropic etching method for the InP substrate of the present embodiment, Br ₂ -N ₂ A report using a system gas has been reported (S. Oku et al., Conference Proceedings of the International Conference on Indium Phosphide and Released Materials, (1997), pp. 574-577). Of course, other known gases and techniques can be used, and even when etching other substrates, for example, GaAs or GaP substrates, excellent flatness can be obtained by using known optimal gas types and techniques. Inclined grooves can be produced. This oblique groove 19 plays a role equivalent to the V-groove described in the first embodiment. As described above, when oblique dry etching is used, the restriction described in Embodiment 2 that a specific surface determined by the properties of the crystal must be used is eliminated, and the optimal arrangement condition described in Embodiment 1 is not satisfied. Within the range, the angle of the slope and the depth of the groove can be arbitrarily designed. Further, if the oblique dry etching is used, the slope reflection portion 19 can be provided in addition to the arrangement in which the element becomes a normal mesa. Therefore, the element shape is appropriately changed to a triangle, a rectangle, a circle, a cross, or the like. At any position on the side portion, any number of grooves serving as reflection portions can be provided. By doing so, it is possible to increase the number of substitute reflecting portions to an arbitrary number, and to input an arbitrary number of different signals to the light receiving element simultaneously or in a time-division manner. . For example, FIGS. 15A and 15B show the case where the inclined reflecting portions 21 are provided at four places around the circular light receiving element by using the above-described oblique dry etching (four-input device). As shown in FIG. 15B, with respect to the light-receiving element 13, not only the crystal orientation in which a forward mesa section occurs but also an arbitrary direction such as an orientation in which an inverse mesa section occurs, the oblique reflection portion (oblique oblique section) having a forward mesa shape. (Groove) 21 can be formed. FIG. 15B shows a cross section taken along the line AA of FIG.
FIG. 16 illustrates an element length L required when a signal light having a finite beam diameter w is incident on the light receiving element of the first embodiment. As shown in Embodiment 1, as shown in FIG. 1, the reality is that as shown in FIG. 1, the light receiving element is formed in the first mesa shape up to the light absorption layer, and the carrier transit layer has another mesa shape which is one step wider. It is preferred to do so. The reason is that if the mesa of the second stage is widened by a distance corresponding to θ ′ (the angle between the incident direction to the light absorbing layer and the substrate surface), the “blocking” of the incident light shown in FIG. 4 is prevented. Because you can do it. The two-step mesa structure is also effective from the viewpoint of preventing electric field concentration on the side surface of the carrier transit layer. The same applies to the "blocking" of the lower electrode layer mesa, and the mesa shape is further widened by a necessary distance. In this way, when considering the necessary element length L, the problem can be simplified to the mesa length above the light absorbing layer. The following embodiment will be described in detail under such an assumption. Of course, when the two-step mesa is not used, it is necessary to consider “blocking”. Further, as described above, the influence of the refraction of incident light at the semiconductor layer interface is omitted here, but in an actual device, the thickness of the light absorbing layer is smaller by about two orders of magnitude than the device length. So this is a good approximation.
As shown in FIG. 16, when the angle between the incident direction of the incident light 9 and the substrate surface is θ ′, the minimum necessary element length (the distance from the light absorbing layer 4 to the light absorbing layer 4) so that the incident light 9 is not blocked. Assuming that the total thickness of the light absorbing layer 4 and the upper electrode layer (p-type InGaAsP electrode layer) 5 is t and the beam diameter of the incident light 9 is w, w / sin θ ′ + t / tan θ ′ Become. For example, if w = 3 μm, θ ′ = 19.5 degrees, and t = 0.5 μm, L = 10.4 μm. As described in the first embodiment, depending on the value of the angle θ between the substrate surface and the inclined surface reflection portion, this increase in the element length causes a significant decrease in the high-speed characteristics of the element.
[0009]
<Embodiment 4>
FIG. 17 shows a fourth embodiment of the present invention. Compared to the light receiving device shown in FIG. 16, the device has a different element length and is processed so that the side surface of the device becomes a reflective surface. Others are similar, except that Here, by forming the side surface of the element as a reflection processed surface (element side wall) 22, incident light 9 made incident on the light absorbing layer 4 can be reflected on the element side wall as shown in FIG. It can lead to layer 4. Then, the signal light reflected by the upper electrode 7 can be efficiently guided to the light absorbing layer 4 again. As a result, the minimum necessary element length for preventing the incident light 9 from being blocked can be reduced to w / sin θ ′. For example, if w = 3 μm and θ ′ = 19.5 degrees, L = 9 μm. It is effective to provide the reflection processing surface 22 only on the element side surface opposite to the inclined reflection portion. However, if both surfaces are formed as the reflection processing surface 22, incident light can be guided to the light absorbing layer 4 more efficiently. Here, it is necessary to consider the total reflection condition on the element side surface. Assuming that the element side wall is perpendicular to the substrate and that no film is deposited around the element (a refractive index of the peripheral part is 1), the critical angle of the light absorbing layer made of InGaAs (refractive index: 3.59) Is about 16.2 degrees. In the case of an InP layer (refractive index 3.17), the critical angle is about 18.4 degrees. When the (111) plane described in the second embodiment or a plane equivalent thereto is used as the inclined reflecting portion, θ ′ is about 19.5 degrees, and as a result of refraction at the semiconductor layer interface in the light absorbing layer, The incident angle is about 33.7 degrees. Therefore, the total reflection condition is satisfied regardless of whether the light receiving element is made of InP, InGaAs, or InGaAsP. In the case of depositing a film on the side wall of the element, it is necessary to consider the refractive index in order to satisfy the condition that the total reflection occurs on the side wall. , The condition of the refractive index is relaxed. Since a material having a refractive index of 1.99 or less may be used on the side wall of the InGaAs light absorbing layer, a polyimide, BCB, silicon oxide film, silicon nitride film, or the like can be used as the deposited film. The side wall perpendicular to the substrate can be formed by using a citric acid-based wet etching solution in the case of InGaAs, for example. Of course, it can also be formed by dry etching.
[0010]
<Embodiment 5>
FIG. 18 shows a fifth embodiment of the present invention. The difference from the light receiving device of FIG. 17 is that the side of the light receiving element opposite to the inclined reflecting portion is further provided with a depth t. ′, And has an asymmetrical cross-sectional shape. The position and the depth are selected so that all the incident light 9 is directly or reflected by the element side wall 23 and guided to the absorption layer. Here, all the side walls are perpendicular to the substrate. The digging depth t 'on the side of the device must be greater than the depth that allows all incident light not directly incident on the light absorbing layer to be reflected on the light absorbing layer side. It must be set in a range where all the incident light is guided to the light absorbing layer. For this purpose, the element length need only be equal to or more than ・ · w / sin θ ′. In this case, t ′ needs to be equal to or more than L · tan θ ′. Therefore, the element length can be reduced to half of the fourth embodiment at the maximum. For example, if w = 3 μm and θ ′ = 19.5 degrees, L = 4.5 μm. With such a configuration, as shown in FIG. 18, all the incident light can be guided to the light absorbing layer without blocking the incident light. Here, of the side surfaces, the slope reflection portion side was dug down to the lower part of the light absorption layer, and the opposite side of the slope reflection portion was dug into a layer below the carrier traveling layer, but the mesa depth was Without limitation, even if the former is dug down to a layer below the carrier traveling layer, the latter is dug deeper to form an asymmetrical cross-sectional shape, and a part of the incident light 9 is on the opposite side of the inclined reflection portion. And reflected to the light absorbing layer.
[0011]
<Embodiment 6>
FIG. 19 shows a sixth embodiment of the present invention. The difference from FIG. 18 is that the element side wall 24 on the opposite side of the inclined reflecting portion has an inverted mesa-shaped cross section (that is, φ <90 degrees). ). Then, as in the case of FIG. 18, all the signal light reflected on the side wall reaches the light absorbing layer 4. The conditions here are to set the element length L, the digging depth t ′, and the side surface angle φ so that all the incident light 10 is guided to the light absorbing layer 4 as in the fifth embodiment. The element length L here means the length of the lower surface of the light absorption layer 4 that defines the element capacitance. With this configuration, as in the fifth embodiment, as shown in FIG. 19, all the incident light 10 can be guided to the light absorption layer 4. Since the side of the element is formed in an inverted mesa shape, the minimum necessary element length for preventing the incident light 10 from being blocked can be further reduced as compared with the fifth embodiment. Here, the side surface of the element on the side of the inclined reflecting portion is perpendicular to the substrate, but may of course have an angle. By making this a reverse mesa cross section, the contact area of the upper electrode 7 can be increased and the contact resistance can be reduced.
In each of the above embodiments, UTC-PD, which is a kind of photodiode, is used as the light receiving element. The layer configuration of the UTC-PD is different from that shown in the present embodiment in that a plurality of layers are inserted between the absorption layer and the carrier traveling layer to reduce the conduction band discontinuity, thereby reducing the carrier blocking phenomenon. Various variations are possible, such as a hybrid structure in which a part of the carrier traveling layer is used as a light absorbing layer, or a light absorbing layer. As the photodiode, a normal pin photodiode based on a laminated structure of a high impurity concentration p-type electrode layer, an undoped or low impurity concentration light absorbing layer, and a high impurity concentration n-type electrode layer, and an absorption layer having a superlattice structure are used. Other photodiodes such as a photodiode using an avalanche photodiode and the like can also be used. Further, as the light receiving element, in addition to a single light receiving element such as a photodiode or a phototransistor, a composite element formed by vertically stacking a light receiving element and another element, or an integrated circuit of a light receiving element and an electronic device may be used. .
In the above-described embodiment, an InP / InGaAs (P) system lattice-matched to the InP substrate was used as a material constituting the light-receiving device. However, lattice matching to InP such as InAl (Ga) As / InGaAs or InA1As / GaAsSb was used. Other material systems, such as A1GaAs / (A1) GaAs, a material system lattice-matched to GaAs such as InGaP / GaAs, a combination of semiconductor materials lattice-matched to GaN such as A1GaN / GaN / InGaN, and a lattice-mismatched material For example, a combination of ordinary semiconductor materials can be used.
Although a semi-insulating substrate is used as the substrate, a conductive substrate may be used.
Although not described in detail, in the case of a back-illuminated light-receiving element, the substrate needs to be transparent to incident light, and it is desirable that the electrode layer and the like be as transparent as possible to incident light. .
Since InP is transparent to light having a wavelength in the 1.3 μm band or 1.5 μm band generally used in optical communication, InP, InGaAs, InGaAsP manufactured on a commonly used InP substrate is used. A light receiving element using such a method is a preferable example. In addition, it is also preferable to use GaAs as a substrate for light having a wavelength of, for example, 1.3 μm or 1.5 μm, and use A1GaAs for light having a wavelength of 0.85 μm.
[0012]
【The invention's effect】
As described above, the back-illuminated light-receiving device of the present invention is provided with a slope reflection portion independent of the light-receiving element on the side of the light-receiving element, reflects all incident light from the back surface of the substrate, and obliquely reflects on the absorption layer. By injecting the signal light, the effective absorption length in the absorption layer can be increased, so that the efficiency of the device can be improved without reducing the high speed due to the increase in the thickness of the absorption layer. is there. In addition, in addition to the above configuration, at least a side end of the element located on the opposite side of the slope reflection part is also configured to be a reflection part, so that a part of the signal light obliquely incident on the element part is further reflected. Since the signal light can be efficiently guided to the light absorbing layer with a smaller device area, there is an effect that the device efficiency can be further improved without lowering the speed of the device. Further, by providing a plurality of slope reflection sections, it is possible to easily input a plurality of incident lights, and the function of the light receiving device can be achieved, and the slope reflection section can be replaced. In addition, by forming the inclined reflecting portion using an etching solution that exposes the (111) A surface of the substrate crystal or an equivalent surface thereof using the semiconductor thin film as an etching mask, the inclined reflecting portion has a suitable angle. There is an effect that the V-groove can be formed with good controllability of dimensions and angles.
The light-receiving device of the present invention does not require special consideration on the layer configuration and process as compared with the conventional light-receiving element, and simply becomes a slope reflection portion in a region where no element exists within a necessary distance range from the light-receiving element. Since it is only necessary to form a concave portion, there is an effect that integration with other elements, for example, an electronic circuit (OEIC) can be easily performed. Further, when the device efficiency is not so required, there is an effect that the speed of the device can be further increased by further reducing the thickness of the light absorption layer.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a configuration of a back illuminated light receiving device exemplified in Embodiment 1 of the present invention.
FIG. 2 is a schematic diagram showing the relationship between the angle of the inclined reflector and the angle of incidence on the absorption layer of the back-illuminated light-receiving device exemplified in the first embodiment of the present invention.
FIG. 3 is a schematic diagram showing the effect of increasing the effective absorption length by oblique incidence of the back-illuminated light-receiving device exemplified in the first embodiment of the present invention.
FIG. 4 is a schematic diagram showing a case where the incident light is blocked as exemplified in the first embodiment of the present invention.
FIG. 5 is a graph showing the dependence of the increment of the effective absorption length and the element speed (relative value) on the angle (θ) of the inclined surface reflection section as exemplified in the first embodiment of the present invention.
FIG. 6 is a schematic diagram showing another configuration of the back illuminated light receiving device exemplified in the first embodiment of the present invention.
FIG. 7 is a schematic diagram showing the plane orientation of a substrate crystal and an inverted mesa shape (a) and a forward mesa shape (b) exemplified in the second embodiment of the present invention.
FIG. 8 is a schematic view illustrating a method for manufacturing a V-groove illustrated in Embodiment 2 of the present invention.
FIG. 9 is an explanatory diagram regarding restrictions on a depth (D) of a slope reflection portion and a distance (z) from a light absorption layer, which are exemplified in the second embodiment of the present invention.
FIG. 10 is a graph showing the dependence of the element efficiency of the light receiving device of the present invention exemplified in the second embodiment of the present invention on the thickness of the light absorbing layer in comparison with a conventional element.
FIG. 11 is a schematic diagram showing the arrangement of a back-illuminated light-receiving element and a slope reflection section exemplified in Embodiment 3 of the present invention.
FIG. 12 is a schematic diagram illustrating a configuration of a back illuminated light receiving device exemplified in Embodiment 3 of the present invention.
FIG. 13 is a schematic diagram showing another configuration of the back illuminated light receiving device exemplified in the third embodiment of the present invention.
FIG. 14 is a schematic view illustrating a method for manufacturing a slope reflection portion by oblique etching illustrated in Embodiment 3 of the present invention.
FIG. 15 is a schematic diagram illustrating a configuration of a four-input back-illuminated light-receiving device exemplified in Embodiment 3 of the present invention.
FIG. 16 is an explanatory diagram showing a minimum element length of the back illuminated light receiving element exemplified in the third embodiment of the present invention.
FIG. 17 is an explanatory diagram showing a minimum element length of the back illuminated light receiving element exemplified in the fourth embodiment of the present invention.
FIG. 18 is an explanatory diagram showing a minimum element length of the back illuminated light receiving element exemplified in the fifth embodiment of the present invention.
FIG. 19 is an explanatory diagram showing a minimum element length of the back illuminated light receiving element exemplified in the sixth embodiment of the present invention.
FIG. 20 is a schematic view showing a configuration of a conventional waveguide type light receiving element.
FIG. 21 is a schematic view showing a configuration of a conventional front-illuminated light receiving element.
FIG. 22 is a schematic view showing a configuration of a conventional back illuminated light receiving element.
[Explanation of symbols]
1. Semiconductor substrate (InP substrate, etc.)
2 .... n-type InP electrode layer (lower electrode layer)
3. Carrier transit layer made of undoped InP
4: light absorption layer made of p-type InGaAs
5 ... P-type InGaAsP electrode layer (upper electrode layer)
5′—electrode layer made of p-type semiconductor (upper electrode layer)
6 Lower electrode
7 Upper electrode
8 ... Slope reflection part (V groove)
9… incident light
10: reflected light
11 ... Anti-reflection film
12 ... substrate surface
13 ... Light receiving element
14 ... Back side of substrate
15 ... Upper electrode
16 Lower electrode
17 ... Slope reflection part (V groove)
18 Monitor element
19 ... Slanted groove
20: Etching mask (InGaAs layer etc.)
21 ... Slope reflection part (diagonal groove)
22 ... Reflection processing surface (element side wall)
23 ... element side wall
24 ... Element side wall
25 ... Semiconductor layer
26 ... Semiconductor substrate
27 ... Light absorbing layer
28 ... Light receiving element
29 ... Incident light
30 ... upper electrode
31 ... Lower electrode
32 ... entrance window
33 ... cleavage plane
34 ... Reflected light

Claims

A back-illuminated light-receiving device, comprising: a semiconductor light-receiving element formed on a semiconductor substrate; and, on the surface of the substrate, one or more concave slope reflection portions formed independently on a side of the light-receiving element. At least, the signal light incident from the back surface of the substrate is reflected by the inclined reflection portion, and the light receiving element and the inclined reflection portion are arranged in a structure in which the signal light is incident on the light receiving element from an oblique direction with respect to the substrate surface . And, on the entire side surface of the light receiving element, or at least the side surface opposite to the inclined surface reflection portion, the signal light incident obliquely to the light receiving element is configured to be reflected to the element side, and the inclined surface reflection portion is A back-illuminated light-receiving device, wherein the opposite element side surface has an asymmetrical mesa-shaped cross-sectional shape dug deeper than the element side surface on the side of the inclined reflector .

2. The semiconductor device according to claim 1, wherein the thickness of the semiconductor substrate is T, the depth of the inclined reflecting portion is D, and the angle between the incident direction of the signal light reflected by the inclined reflecting portion and incident on the light receiving element and the substrate surface is θ ′. In this case, the horizontal distance z between the end of the light absorbing layer constituting the light receiving element on the side of the inclined reflecting portion and the deepest portion of the inclined reflecting portion is z <T / (10 · tan θ ′). Set to a range , and on all side surfaces of the light receiving element, or at least on the side surface opposite to the slope reflection portion, a structure for reflecting signal light incident obliquely to the light receiving element toward the element side, A back-illuminated light-receiving device, wherein an element side surface opposite to the slope reflection portion has an asymmetrical mesa-shaped cross-sectional shape dug deeper than the element side surface on the slope reflection portion side .

3. The light receiving element according to claim 1, wherein an even number of the inclined reflecting portions are disposed at positions symmetrical with respect to the light receiving element , and all the side surfaces of the light receiving element or at least a side surface portion opposite to the inclined reflecting portion. In the above, the structure is such that the signal light incident obliquely to the light receiving element is reflected toward the element side, and the element side surface opposite to the inclined surface reflection portion is dug deeper than the element side surface on the inclined surface reflection portion side. A back-illuminated light-receiving device having an asymmetrical mesa-shaped cross-section .

In any one of claims 1 to 3, the back-illuminated, characterized by comprising constitute the opposite side, the inverted mesa-shaped cross-sectional shape and small without even slope reflecting portion of the light-receiving element Light receiving device.

5. The method of manufacturing a back-illuminated light-receiving device according to claim 1 , wherein when forming a concave slope reflection portion on the semiconductor substrate, the semiconductor substrate has a selective etching characteristic. 6. Using a semiconductor thin film as a mask and etching by a chemical etching method so that the (111) A plane of the substrate crystal or a plane equivalent thereto is exposed. How to make.