JP3732340B2 - Thin film forming apparatus and thin film forming method - Google Patents

Description

【００１５】
ただし、下記の数式２、数式３で示した条件で、ｎ〜１とした場合である。
【００１６】
【数２】

【００１７】
【数３】

【００１８】
ここで、一般にＳ_e＞Ｓ_cなのでＶ_s＜０となる。基板面積が大きくなるほどカソード面積も大きくなるため、接地された部分である反応容器内壁・基板の面積とカソードの面積の比が１に近くなり、そのためにセルフバイアスの絶対値が小さくなるので、プラズマポテンシャルが大きくなる。その結果、プラズマポテンシャルと基板のポテンシャルとの間の電位差によって、プラズマ中の陽イオンが加速されて基板上の膜成長表面に衝突し、膜にダメージを与えて太陽電池の効率を低下させる。
【００１９】
また、前記基板面上に均一に前記半導体膜を形成するためには、前記原料ガスの導入を面内均一に行う手法（シャワー方式）を用いる必要がある。図４に示すようにガス供給をカソードの側面から行う場合、ガス導入ノズルと排気口を結ぶ方向に沿って原料ガスがプラズマ中を分解されながら進む結果、この方向に沿って膜特性や膜厚の不均一が生じるが、これはカソードの面積が大きいほど深刻になる。
【００２０】
他方の前記シャワー方式では、原料ガス供給の均一性の制御はカソード面積が大きくなるほど難しくなる。また、このシャワー方式ではカソードプレートを通してガス供給を行うため、カソードの構造が複雑になる。特に高周波プラズマを用いる場合、ガス供給配管を通じてアースに高周波電流が流れないよう、例えば、配管の一部に絶縁性の材質の管を用いるなどの工夫をしなければならない。また、このような工夫がなされたとしても、高周波電力の一部はこの配管において消費される分、余分の電力が必要となる。すなわち、これらの問題点はいずれも薄膜太陽電池の高コスト化の原因となる。
【００２１】
本発明は上記の問題点に鑑みてなされたものであり、その目的とするところは、比較的小さな装置を用いて、大面積の基板上に従来と同程度に高品質の薄膜を比較的安価に形成する装置及び方法を提供することにある。
【００２２】
【課題を解決するための手段】
本発明によれば、気密性を有する反応容器と、この反応容器に原料ガスを導入する原料ガス導入部と、導入された原料ガスを分解する原料ガス分解部と、前記原料ガス分解部を介して前記原料ガスを排出ガスとして排出する排気部と、前記反応容器に内設された基板保持部とを備え、基板保持部が、回転可能な円筒で、この円筒の外周面に薄膜半導体を形成すべき基板を保持し得ることを特徴とする薄膜形成装置が提供される。
【００２３】
すなわち、本発明の薄膜形成装置では、原料ガス分解部によって形成される原料ガスの分解空間を、基板が円筒の外周面に保持されて回転移動するので、この原料ガス分解空間に導入された原料ガスが分解されて基板上に薄膜を形成することができる。したがって、原料ガスの分解空間を円筒の外周部に静止した状態でかつ基板の一部分のみを臨む小容積の空間に限定して形成することができる。
【００２４】
つまり、長尺の可撓性基板上に薄膜を形成するに際して、従来のように基板を平面上で１方向に直線状に移動させなくてもすみ、基板の移動機構を構成が比較的簡単である回転機構とすることにより、小さな反応容器で薄膜の形成が可能となる。上記の利点は、本発明の薄膜形成装置の特徴である大面積の基板上に薄膜を形成する際に特に有効となる。
基板の移動機構を回転機構とすることにより、基板の移動速度及び回転位置の検出及びこれらに基づく基板移動の制御が容易になる。
【００２５】
本発明の薄膜形成装置は、多層からなる薄膜形成に有効である。すなわち、１つの基板上に異なる種類の層、例えば、半導体層を積層する場合、基板を１回または所定回数あるいは所定角度だけ回転させ第１層、第２層と順次積層することができる。原料ガス及び基板の回転速度を調整することにより、薄膜の種類、性質、膜厚あるいは積層数等を任意に変えることができる。
【００２６】
上記した本発明の薄膜形成装置を従来の方式と比較すると、さらに本発明の特徴が明らかになる。
すなわち、上記したロール・トゥ・ロール法のように複数のガス分解空間を基板送り方向に沿って並べた横長の大型装置を用いなくてもよい。また、多層からなる薄膜を形成する際、基板の回転速度を各層の最適条件に合わせて独立して設定できる。さらに、バッチ式で用いられる複数の反応容器間を移動させる搬送路を省略することができる。つまり、小型でかつ１つの反応容器で薄膜形成装置を構成することができる。
【００２７】
本発明は、別の観点によれば、気密性を有する反応容器の内部で回転可能な円筒の周りに原料ガス分解部と、原料ガス導入口と、この原料ガス導入口に対向する排気口とを少なくとも３組配置し、前記円筒の外周面に基板を保持させ、この基板を回転させながら第１の原料ガス導入口から第１の原料ガスを第１の原料ガス分解部に導入しつつ同時に前記第１の原料ガス分解部を介して排出された排出ガスを前記第１の原料ガス導入口に対向する第１の排気口から排気して前記基板上に第１の薄膜を形成し、この第１工程の終了後、前記基板を回転させながら第２の原料ガス導入口から第２の原料ガスを第２の原料ガス分解部に導入しつつ同時に第２の原料ガス分解部を介して排出された排出ガスを前記第２の原料ガス導入口に対向する第２の排気口から排気して前記基板上に第２の薄膜を形成し、この第２工程の終了後、前記基板を回転させながら第３の原料ガス導入口から第３の原料ガスを第３の原料ガス分解部に導入しつつ同時に第３の原料ガス分解部を介して排出された排出ガスを前記第３の原料ガス導入口に対向する第３の排気口から排気して前記基板上に第３の薄膜を形成することを特徴とする薄膜形成方法が提供される。
【００２８】
すなわち、本発明の薄膜形成方法を、薄膜太陽電池の半導体層の形成方法を例として説明すると、回転可能な円筒の周りに原料ガス導入口、排気口及び原料ガス分解部からなる薄膜形成手段を少なくとも３組配置する。例えばアノードとしての円筒と、この円筒に対向して配置された第１、第２、第３の各カソードとの間で少なくとも３か所の原料ガス分解部を形成する。第１の原料ガス分解部では、ｐ型ドーピングガスを含む原料ガスを導入・排気し、第１の原料ガス分解部に高周波電力を印加して高周波プラズマ空間を形成してｐ層を形成する。第２の原料ガス分解部では、ドーピングガスを加えずに原料ガスを導入・排気し、第２の原料ガス分解部に高周波電力を印加して高周波プラズマ空間を形成してｉ層を形成する。第３の原料ガス分解部では、ｎ型ドーピングガスを含む原料ガスを導入・排気し、第３の原料ガス分解部に高周波電力を印加して高周波プラズマ空間を形成してｎ層を形成する。このようにして、基板上にｐ，ｉ，ｎ各層を順次形成することができる。もちろん、ガスの種類を変えれば、導電型を変えることができ、基板上にｐ，ｉ，ｎ各層を順次形成することができる。
【００２９】
例えば、第１の原料ガス分解部に高周波電力を印加して形成される第１の高周波プラズマ空間を用いて第１の原料ガスによりｐ層（またはｎ層）を形成しているとき、ｎ層（またはｐ層）及びｉ層を形成するために用いる第２及び第３の原料ガス分解部に前記第１の原料ガスが流れ込むが、その際、第２及び第３の原料ガス分解部には高周波電力は印加されないので、形成される高周波プラズマ空間は第１の高周波プラズマ空間のみに限定され、第２及び第３の原料ガス分解部に流れ込んだ前記第１の原料ガスはプラズマにより分解されることがない。
【００３０】
したがって、第２及び第３の原料ガス分解部に面した壁面、カソード表面等に第１の原料ガスが反応生成物となって付着することがない。１つの原料ガス分解部から他の原料ガス分解部に流れ込んだ上記の原料ガスは、物理吸着により壁面等に付着するが、付着した原料ガスは、原料ガス分解部に高周波電力を印加して高周波プラズマ空間を形成する前に原料ガス導入口から排気口までそれぞれの原料ガスを流すことにより、反応容器の外部に排気することができる。
【００３１】
上記のように、第１の原料ガス分解部から流出したｐ型（またはｎ型）のドーピングガスを含む原料ガスが、第１の原料ガス分解部以外にも流れ込んで反応容器内の壁面等に付着するが、これは容易に離脱し、反応容器外へ排出される。また、第１の原料ガス分解部以外の部位ではプラズマが存在しないため、原料ガスの分解生成物の発生や反応容器内面への付着は起こらない。したがって、第２，第３の半導体層の形成において、それぞれの原料ガス分解部の近傍の反応容器内面上に付着した原料ガス分解生成物のプラズマによる再分解とその基板への堆積によるオートドーピングを生じない。
【００３２】
【発明の実施の形態】
本発明の薄膜形成装置において、薄膜とは10μｍ〜10^-3μｍ位の膜厚を有する半導体層が挙げられる。
「気密性を有する反応容器」とは、内部に基板保持部を収納できる容積を有し、大気圧の状態または、真空ポンプにより基板の周囲を減圧された状態に保持できるものが好ましい。
【００３３】
本発明の薄膜形成装置を適用できる基板とは、実質的に剛体からなる小さな平板であってもよいが、円筒の外周面に容易に保持できる点から可撓性を有する平板が好ましい。このような基板は、外力で容易に変形するシート状あるいはフィルム状のものが好ましい。
円筒は、その中心軸を回転軸と一致させて回転可能なものが好ましい。さらに円筒は、金属で形成されかつアースを施すことによりアノードとして使用できるものが好ましい。金属で形成することにより、さらにヒータを装着して円筒表面に保持された基板を加熱することができる。
基板保持部は、基板をその表面に保持する保持手段を回転可能な円筒の外周面に備えることが好ましい。保持手段としては、基板の取り付け、取り外しが容易にでき基板が弛まないものであればよく、例えば、機械式の保持チャック、静電チャック、真空チャックが挙げられる。
【００３４】
原料ガス導入部は、原料ガスの流量、流速、吐出圧力を制御できるものが好ましい。また、排気部は、反応容器内に拡散した原料ガスを吸引し、反応容器外へ排気できる吸引力を有する吸引源を備えたものが好ましい。
【００３５】
基板保持部の円筒が、円筒の回転速度を制御する回転速度制御手段を具備しておれば、形成すべき薄膜に応じてその膜厚等に関する最適条件を調整できる。回転速度制御手段としては、円筒駆動用モータの電力制御、ギアあるいはプーリーによる減加速装置が挙げられる。
【００３６】
基板保持部の円筒が、円筒の回転位置を検出する回転位置検出手段を具備しておれば、基板保持部に保持された基板の両端部、あるいは基板上の特定部位を検出して円筒の回転駆動及び停止をおこなうことができ、さらに、原料ガス分解部、原料ガス導入部及び排気部をそれぞれ単独に、あるいは円筒の回転駆動及び停止と連動して制御することができる。具体的には、円筒の回転位置を検出するフォトセンサと円筒駆動用のステッピングモータとの組み合わせが挙げられる。
【００３７】
本発明における原料ガス分解部とは、原料ガスを基板上に薄膜として蒸着するガス分解空間を形成する手段であり、高周波プラズマ、マイクロ波プラズマ、光プラズマ等のプラズマ発生手段あるいは高温触媒表面における熱分解等の手段が挙げられる。
原料ガス分解部が、基板保持部に保持された基板との間にガス分解空間を形成するよう構成され、原料ガス導入部が反応容器内に開口した原料ガス導入口を有し、排気部が反応容器内に開口したガス排気口を有し、これら原料ガス導入口とガス排気口とが、前記ガス分解空間を挟んで互いに対向する位置に配設されることにより、原料ガス導入部から排気部まで均一で乱れの少ない原料ガスの流れを形成できる。
【００３８】
基板保持部が、原料ガス導入口からガス排気口までの原料ガスの流れの向きと同じ向きに円筒を回転させることにより、原料ガス分解部と基板との間のガス分解空間における原料ガスの流れの乱れを抑えることができる。
なお、原料ガス導入口及びガス排気口は、両者のうちの少なくともガス排気口が基板の幅（円筒の回転軸方向における長さ）よりも大きい開口幅を有することにより、基板の略全面にわたって原料ガスの均一な流れを容易に形成できる。
【００３９】
この発明の薄膜形成装置は、円筒の外周面に対向する位置に配置されたカソード電極を具備し、それによって原料ガス分解部が、基板保持部に保持された基板との間にプラズマ空間を形成する構成とすれば、プラズマＣＶＤ法により基板上に半導体層を積層することができる。
【００４０】
カソード電極が円筒の周りに複数配置され、それぞれのカソード電極が、原料ガス導入部と排気部とを１対ずつ備えることにより、１つの反応容器の内部で基板上に複数の半導体層を積層することができる。すなわち、第１、第２、第３の３つの原料ガスを用いる場合、これらの原料ガスが異なるガスであれば、１つの反応容器の内部で、各原料ガスの供給時間帯をずらすことにより異なる半導体層を形成することができる。上記３つのの原料ガスが同じガスであれば、１つの反応容器の内部で、同時に原料ガスを供給して同じ半導体層を多数形成することができる。
【００４１】
アノード電極が円筒自体を使用することにより、基板保持部とアノードを兼用でき、かつアノード電極はカソード電極より面積の大きいものを形成できる。すなわち、カソード電極の面積がアノード電極のそれより小さく形成されておれば、高周波プラズマＣＶＤ法による薄膜形成の場合、上述したように、発生したプラズマ空間におけるプラズマポテンシャルの増大を抑えることによってプラズマ中の陽イオンの基板上への衝突を減少させ形成される薄膜のダメージを最小限に抑えることができる。
【００４２】
原料ガス導入部及び排気部は、それら各部の管路が、カソードと対向する円筒の両側面を取り囲む１対の遮蔽壁体を構成するのが好ましい。このような遮蔽壁体は、発生したプラズマにより原料ガスを分解する空間を局所に限定し、上述したように原料ガスのオートドーピングを抑える。
【００４３】
本発明の薄膜形成方法を用いて、薄膜太陽電池を形成する場合の具体例を以下に示す。まず、円筒の外周面に基板を保持させ、反応容器内を減圧し、この基板を回転させながら第１原料ガス導入口から第１原料ガスを第１原料ガス分解部に導入しつつ同時に第１原料ガス導入口に対向する第１排気口から排出ガスを排気する。
次に、高周波電力を第１のカソード電極に印加して前記基板上に第１の導電型半導体層の薄膜を形成する。高周波電力の印加及び第１原料ガスの導入を停止し高周波プラズマを消失させた後、前記基板を回転させながら第２原料ガス導入口から第２原料ガスを第２原料ガス分解部に導入しつつ同時に第２原料ガス導入口に対向する第２排気口から排出ガスを排気する。
【００４４】
次に、高周波電力を第２のカソード電極に印加して前記基板上に実質的に真性な半導体層の薄膜を形成する。高周波電力の印加及び第２原料ガスの導入を停止し高周波プラズマを消失させた後、前記基板を回転させながら第３原料ガス導入口から第３原料ガスを第３原料ガス分解部に導入しつつ同時に第３原料ガス導入口に対向する第３排気口から排出ガスを排気する。
次に、高周波電力を第３のカソード電極に印加して前記基板上に第１の導電型半導体層と反対の導電性を有する半導体層を形成する。高周波電力の印加及び第３原料ガスの導入を停止し高周波プラズマを消失させた後、基板を室温近くまで冷却してから反応容器内を大気圧に戻し、前記基板を取り出す。
【００４５】
各層の形成時のガス圧力・流量・組成・高周波電力・周波数・アノード回転数（円筒回転数）などは個々に独立して設定できる。各々のカソードの面積は同じである必要はない。また、薄膜太陽電池において光電変換層であるｉ層の膜厚はｎ・ｐ各層に比べて厚くなければならないため、その形成には複数対のカソード、原料ガス導入部及び排気部を同時に用いて、ｉ層の作製に要する時間を短縮することも可能である。また、ｐ／ｉ界面やｎ／ｉ界面に界面特性改善のためのバッファー層を設けてもよいが、従来のバッチ式の形成装置と同様、ｐ／ｉ界面（またはｎ／ｉ界面）バッファー層形成のためのカソードはｐ層（またはｎ層）の形成に用いたものを使用できる。
【００４６】
さらに、前記したタンデム型薄膜太陽電池を形成する場合、複数のｐ・ｎ層形成のためのカソードをそれぞれ一つずつ用意すればよいので、前記ロール・トゥ・ロール法の形成装置に比べて小型化でき、しかも複数の反応容器を連結した前記バッチ式の形成装置のような基板搬送機構も不要となる。
【００４７】
また、ｐ層（またはｎ層）を形成しているとき、ｎ層（またはｐ層）およびｉ層を形成する原料ガス分解部にもｐ型（またはｎ型）のドーピングガスを含む原料ガスが流れ込むが、これら原料ガス分解部にはプラズマが存在しないので、この空間に面した内壁やカソードの表面にｐ型（またはｎ型）の反応生成物が付着することはない。したがって、壁面などに付着した反応生成物とプラズマとの反応に由来するオートドーピングが生じることはなく、各半導体層の高品質化が達成できる。
【００４８】
物理吸着したガスは、次にｉ層およびｎ層（またはｐ層）を形成するときに、プラズマの発生の前に原料ガスを流すことで容易に反応容器の外に排気することができる。したがって、前述したようなオートドーピングの心配はなく、高品質な太陽電池の形成が可能である。この事情は、複数の基板を連続的に処理する場合においても同様である。
【００４９】
また、ｐ，ｉ，ｎ各層を形成するそれぞれのプラズマを同時に発生させなければならないロール・トゥ・ロール法と違って、本発明で提案される装置においては、各層のプラズマは同時に発生していないので、各成膜空間の間での原料ガスの混合を防ぐためのエアカーテンなどの手段を講じる必要はない。したがって、ロール・トゥ・ロール法による形成装置に比べて簡単な構造で、しかも装置全体を小型化できる。
【００５０】
以下に、本発明を実施の形態により更に詳細に説明する。
図１及び図２は、薄膜形成装置としての半導体膜形成装置の構成を示す。図１はその平面図、図２は図１の部分断面正面図（II−II線断面図）である。
この半導体膜形成装置１０は、高周波プラズマＣＶＤ法を用いた薄膜太陽電池の半導体層の形成装置である。
【００５１】
半導体膜形成装置１０は、気密性を有する反応容器１と、反応容器１に内設された基板保持部としての円筒２と、反応容器１内に原料ガスを導入する原料ガス導入部３１、３２、３３と、円筒２に保持された基板Ｓに近接して配置され、各原料ガス導入部３１、３２、３３から導入された原料ガスを分解し基板Ｓ上に薄膜を形成させるカソード１１、１２、１３と、反応容器１から原料ガスを排気する排気部４１、４２、４３とを備える。
【００５２】
反応容器１は、脚６を有するベース７の上部に保持され、図示しない真空ポンプにより内部が減圧可能となっている。
円筒２は、ベース７に保持されたモータ８の駆動軸に着脱可能に接続される有蓋の円筒自体を電極としてなるアノード２６と、可撓性を有する基板Ｓを円筒２の外周面部２７に保持する真空チャック（図示せず）とを備える。円筒２は、ステンレス鋼を材料として形成され、アースを施されることにより電極として使用できるよう構成されている。
【００５３】
基板Ｓは、ステンレスやポリイミドなどの耐熱性があり可撓性のフィルム状のものが用いられ、基板保持機構である円筒２の曲面に沿って密着させる。また、外周面部２７に複数の基板を保持してもよい。基板Ｓの温度は、円筒２に備えられたヒータ（図示せず）により所定の温度に保持される。
【００５４】
カソード１１、１２、１３はアノード２６の外周面部２７に対向して配設され、それぞれのカソード１１、１２、１３の側方には各カソードを取り囲む１対のアースシールド２１、２２、２３が各カソード１１、１２、１３と電気的に絶縁された状態で配設されている。
【００５５】
これらのカソード１１、１２、１３は、プラズマの発生・維持のための機構として、望ましくは従来の平行平板型プラズマＣＶＤ装置と同様の、高周波電源から電気的エネルギーをプラズマに与えることができる。各カソード１１、１２、１３は、その表面が半径線と直角方向でかつ外周面部２７と平行に形成され、外周面部２７に円筒状に保持された基板Ｓの軸線方向における幅（高さ）を覆って基板Ｓの幅よりも大きく設定され、円筒２の軸線に平行な方向に均一なプラズマを発生できるような形状を有している。
【００５６】
さらに、アースシールド２１、２２、２３の各対の側方には、原料ガス導入部３１、３２、３３及び排気部４１、４２、４３が配置されている。それぞれの原料ガス導入部及び排気部は、それぞれが対をなしてカソード１１、１２、１３の表面に平行する線上で互いに向き合って反応容器１内に開口する導入口３１ａ、３２ａ、３３ａ及び排気口４１ａ、４２ａ、４３ａを有する。これらの開口は、各原料ガス導入部から対向する各排気部に向かう原料ガスの流れが、アースシールド２１、２２、２３、カソード１１、１２、１３及び基板保持部２に保持された基板Ｓによって遮られない位置に構成される。
【００５７】
原料ガス導入部３１、３２、３３及び排気部４１、４２、４３のそれぞれは、管路３１ｂ、３２ｂ、３３ｂ及び４１ｂ、４２ｂ、４３ｂによって反応容器１内を半径方向外方に延出し反応容器１の外部の吸・排気系に接続されている。
【００５８】
このような構成では、各カソード１１、１２、１３と各カソード１１、１２、１３に対向する基板面との間に形成されるプラズマ空間は、原料ガス導入部３１、３２、３３及び排気部４１、４２、４３により取り囲まれるので、発生したプラズマにより原料ガスを分解する空間が局所に限定される。例えば、図１に示した原料ガス分解空間５１で膜堆積を行う場合、原料ガスが反応容器１内の前記局所５１以外の空間へ流出しても、そこではプラズマが形成されていないので、原料ガス分解反応生成物は生じない。原料ガス自体は反応容器内壁表面に物理吸着するのみなので、原料ガス分解空間５２（または５３）に高周波プラズマを形成する前に、容易に反応容器外部に排出できる。上記の事情は、原料ガス分解空間５２・５３における膜堆積においても同様である。
各原料ガス導入部から対向する各排気部に向かう原料ガスは、その流れが遮られないので、プラズマ空間を通過して速やかにかつ均一に各排気口に向かうことができる。
【００５９】
上記のカソード１１、１２、１３の大きさは、基板Ｓの幅（図２中、上下方向の長さ寸法）より大きくなければならないが、基板Ｓの長さとは無関係に選ぶことができ、円筒２の直径よりも小さいことが望ましい。したがってカソード１１、１２、１３の面積を小さくすることにより、半導体膜形成装置１０の小型化および投入高周波電力の低減を図ることができるだけでなく、各カソードの面積を接地された部分である反応容器１内壁・基板Ｓの表面積に比べて非常に小さくすることができるため、プラズマポテンシャルを小さくすることが可能となり、膜形成中における陽イオンの膜表面への衝突ダメージによる太陽電池の特性の低下を抑えられる。
【００６０】
原料ガス導入部３１、３２、３３には、ガス導入ノズルを用いることが望ましい。カソード１１、１２、１３の主面に無数に開けられた小孔を通してのシャワー方式を採ることも技術的には可能だが、その場合は前述したように、配管における高周波電力の無駄な消費が避けられない。前記した導入口３１ａ、３２ａ、３３ａ及び排気口４１ａ、４２ａ、４３ａも、基板保持部を構成する円筒２の軸線に平行な方向の基板幅より大きくして、その軸線方向について均一なガス流を発生できるような形状を有している。
【００６１】
また、円筒２の回転方向は、その回転により基板Ｓの移動する向きが原料ガスの流れる向きと同じになるようにする。なぜならば、ガス流と基板Ｓの移動方向を逆向きにすると、基板Ｓ表面近傍におけるガスの流れが乱され、プラズマ内部における成膜前駆体の空間分布を乱し、その結果、半導体膜の品質や面内均一性を損なうからである。
【００６２】
半導体層形成装置１０は、カソード１１、１２、１３の面積が基板Ｓの面積よりずっと小さいが、アノード２６の回転により基板Ｓの全領域に成膜することができる。アノード２６にはその回転速度の制御機構があるので、半導体層の成膜速度はプラズマ条件に加えてその回転速度によっても制御できる。
さらに、円筒２には、その回転位置の検出と制御を可能ならしめる機構があり、成膜の開始と終了位置にその検出位置をあわせることにより、基板Ｓの移動方向に均一に成膜できる。したがって、基板Ｓの全面に均一に半導体層を形成することが可能である。成膜の開始と終了は、基板Ｓの端であることが望ましい。
【００６３】
半導体膜形成装置１０による薄膜太陽電池の製造方法の一例を以下に記載する。
まず、ステンレスやポリイミドなどの耐熱性があり可撓性のフィルム状の基板を用いて、その片面にＡｌ・Ａｇなどの反射率の高い金属薄膜電極をスパッタ法・蒸着法などの方法で全面に成膜する。さらに高い反射率を得るためには、その上にＺｎＯ・ＳｎＯ₂・ＩＴＯなどの透明導電膜をＭＯＣＶＤ法・スパッタ法・蒸着法などの方法で成膜する。望ましくは、基板Ｓの裏面電極表面が凹凸になるような条件を選んで、金属膜・透明導電膜を作製する。
【００６４】
次に、裏面電極が外側になるように、基板Ｓを基板保持部を構成する円筒２の外周面上に密着するように保持する。このときに基板全体に均一な張力がかかるようにする。円筒２の材質には、たとえばステンレスなどの導電性・熱伝導性・耐熱性・プラズマ耐性があるものが望ましい。
【００６５】
基板温度を制御するための機構として、この円筒２の内部には、基板温度を所定の温度に保つためのヒーターや熱電対が備え付けられていて、これらの導線は回転軸と同心の中空のパイプ内を通して反応容器１の外部に電気的に接続可能である。あるいは、円筒２の内部をその回転軸と同心になるように円筒形に中空にしておき、反応容器１内に、ヒーター・熱電対を内蔵した円筒形パイプを具備し、円筒２を反応容器１内に組み込んだときに、円筒２内部の中空部分に上記パイプが位置するようにしてもよい。
【００６６】
次に、前記基板Ｓを保持した円筒２を、反応容器１内に取り付ける。このとき、円筒２の回転軸は、モータ８の駆動軸に接続される。その後、反応容器１内を高真空で排気するか、もしくはＡｒなどの高純度な不活性ガスを大量に流すことにより、反応容器１内部に残るガス状不純物を除去する。同時に基板Ｓを所定の温度まで昇温する。
【００６７】
この後、前記基板Ｓを所定の回転速度で回転させながら、水素化アモルファスシリコンｎ，ｉ，ｐ層を順次形成する。このとき用いる原料ガスとしては、例えばＳｉＨ₄，Ｓｉ₂Ｈ₆，ＳｉＨ_xＦ_4-x（ｘ＝０〜３），ＳｉＨ_xＣｌ_4-x（ｘ＝０〜３），ＣＨ₄，ＧｅＨ₄があり、キャリアガスとしては、例えばＨ₂・Ａｒ・Ｈｅがある。原料ガスとしては、上記のほかに、ｐ型の導電型をもつ半導体層を形成するときにはＢ₂Ｈ₆などを、ｎ型の導電型の半導体層を形成するときにはＰＨ₃などを用いることができるが、これらに限られるものではない。
【００６８】
具体的には、まず、基板保持部２を回転させながら原料ガス導入部３１を用いて原料ガスを反応容器１内に流し、同時に排気部４１を用いて反応容器１内に残る原料ガスを排気する。ガスの流れが落ち着き次第、カソード１１に高周波電力を印加してｎ層を形成する。次に原料ガス導入部３１及び排気部４１の各駆動源の駆動を停止し、高周波電力の印加を停止する。
【００６９】
反応容器１内が所定の真空度に達した後、前記ｎ層を形成したときには用いなかった別の１組のカソード１２・原料ガス導入部３２・排気部４２を用いて、ｉ層を形成する。次に、前記ｎ層と前記ｉ層とのどちらを形成したときにも用いなかった１組のカソード１３・原料ガス導入部３３・排気部４３を用いて、ｐ層を形成する。基板の端がそれぞれのカソードの真下にあるような回転位置において各々の層を形成するための成膜の開始と終了を行う。
【００７０】
各々の形成条件を表１に示す。
【００７１】
【表１】

【００７２】
この形成条件は一例であって、各層は他の条件で形成されてもかまわない。また、この実施の形態では半導体薄膜として水素化アモルファスシリコンを用いたが、水素化微結晶シリコンや多結晶シリコン、または炭素やゲルマニウムなどとシリコンとの合金を用いてもよい。
【００７３】
次に、半導体層が形成された基板Ｓを反応容器１の外部に取り出し、その表面に窓側電極としてＺｎＯ・ＳｎＯ₂・ＩＴＯなどの導電率の高い透明導電膜をＭＯＣＶＤ法・スパッタ法・蒸着法などの方法で成膜する。さらにその上に、櫛形などのパターニングした集電極を、Ａｌなどの金属材質を用いて、スパッタ法・蒸着法などの方法で作製する。
【００７４】
このようにして形成された太陽電池の特性は、短絡電流密度Ｊ_sc＝１７．５ｍＡｃｍ²、開放電圧Ｖ_OC＝０．８６Ｖ、曲線因子ＦＦ＝０．７０、光電変換効率η＝１０．５％であった。これは従来の半導体層形成装置を用いて作製した薄膜太陽電池と比較して遜色ない値になっている。
【００７５】
なお、上記した実施の形態では、円筒２の回転軸を鉛直方向に配する構成を示したが、前記回転軸の方向は特に限定されるものではない。したがって、半導体膜形成装置の支持用部材、例えばベース、脚等は任意に設定できる。
【００７６】
【発明の効果】
本発明の薄膜形成装置では、原料ガス分解部によって形成される原料ガスの分解空間を、基板が円筒の外周面に保持されて回転移動するので、この原料ガス分解空間に導入された原料ガスが分解されて基板上に薄膜を形成することができる。したがって、原料ガスの分解空間を円筒の外周部に静止した状態でかつ基板の一部分のみを臨む小容積の空間に限定して所望の薄膜を形成することができる。
【００７７】
長尺の可撓性基板上に薄膜を形成するに際して、従来のように基板を平面上で１方向に直線状に移動させなくてもすみ、基板の移動機構を構成が比較的簡単である回転機構とすることにより、小さな反応容器で薄膜の形成が可能となる。上記の利点は、本発明の薄膜形成装置の特徴である大面積の基板上に薄膜を形成する際に特に有効となる。
基板の移動機構を回転機構とすることにより、基板の移動速度及び回転位置の検出及びこれらに基づく基板移動の制御が容易になる。
【００７８】
本発明を薄膜太陽電池の製造に用いた場合は、回転可能な基板保持部をアノードとして、１つの気密性の反応容器内に少なくとも３組のカソード・原料ガス導入部・排気部をもち、ｐ・ｉ・ｎ各層を可撓性基板上に形成できるので、従来に比べて比較的小さな形成装置を用いて、大面積の基板上に比較的安価でかつ従来と同程度の高品質な薄膜太陽電池を形成することにより、薄膜太陽電池の生産の低コスト化を実現する。
【図面の簡単な説明】
【図１】本発明に係る薄膜形成装置の平面図である。
【図２】図１のII−II線断面による正面図である。
【図３】高周波プラズマにおけるプラズマポテンシャルの概念を示す図である。
【図４】従来の平行平板型プラズマＣＶＤ法による薄膜太陽電池の半導体層の形成装置を示す図である。
【符号の説明】
１ 反応容器
２ 円筒（基板保持部）
１０ 半導体膜形成装置（薄膜形成装置）
１１，１２，１３ カソード（原料ガス分解部）
３１，３２，３３ 原料ガス導入部
４１，４２，４３ 排気部
２６ アノード
２７ 外周面部
Ｓ 基板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film forming apparatus and a thin film forming method, and more particularly to a thin film forming apparatus and a thin film forming method used for forming a semiconductor layer such as a thin film solar cell.
[0002]
[Prior art]
Currently, a thin film forming apparatus using a parallel plate type plasma CVD method is generally used in the field of film formation, particularly in the manufacture of thin film solar cells. Specifically, for example, a thin film forming apparatus 100 as shown in FIG. 4 is used. The thin film forming apparatus 100 includes an airtight reaction vessel 110, a substrate holding unit 120 installed in the reaction vessel 110, a source gas introduction unit 130 for introducing a source gas into the reaction vessel 110, and a substrate holding unit 120. The cathode 140 and anode (which also serves as the substrate holding unit 120) for decomposing the source gas introduced near the surface of the flat substrate S held by the substrate, and the exhaust unit 150 for exhausting the source gas introduced into the reaction vessel 110. With. An earth shield 160 is formed above the cathode 140.
[0003]
When forming a thin film, first, SiH, which is a source gas, is used._Four, Si₂H₆, SiH_xCl_4-x(X = 0-3), SiH_xF_4-x(X = 0-3), CH_Four, GeH_FourFor semiconductor gases such as H₂, Ar, He, and other gases are mixed, introduced into the reaction vessel 110 by the source gas introduction unit 130, and the source gas is a radical that is a film-forming precursor in the plasma generated between the cathode 140 and the anode 120. The silicon-based semiconductor thin film is formed by decomposing into a film, and depositing and bonding the film-forming precursor on the substrate S placed on the anode 120. For introducing the source gas, a gas introduction nozzle is used, or a shower method is used in which the raw material gas is blown onto the substrate through a large number of small holes formed in the main surface of the cathode plate from the back surface of the cathode.
[0004]
In order to form a p-i-n junction, it is necessary to sequentially form a film by mixing a source gas to be supplied with a dopant gas.₂H₆When the n layer is formed, etc._ThreeAre mixed with the source gas and supplied. In addition, a buffer layer for improving interface characteristics may be provided at the p / i interface or the n / i interface. As the substrate S used at this time, a substrate in which a metal thin film such as silver serving as a back electrode is previously formed on one surface of a heat-resistant resin such as polyimide or a flexible flat plate such as stainless steel is used.
[0005]
In order to further increase the light absorption rate in the i layer serving as the photoelectric conversion layer, the back surface reflectance of incident light is increased. In order to obtain a high back surface reflectance, ZnO, SnO is formed on the metal thin film.₂A transparent conductive film such as ITO is formed by a method such as MOCVD, sputtering, or vapor deposition.
[0006]
When only one such reaction vessel is used and each of the p-i-n (or n-i-p) layers is produced in order by changing the composition of the raw material gas, after decomposition in the plasma, A reaction product containing a p-type (or n-type) dopant adhering to the wall is desorbed again into the gas phase by being exposed to plasma at the time of forming the i layer, and taken into the i layer on the substrate. The quality of the i layer is degraded. This is a phenomenon called autodoping. When autodoping is significant, the level in the gap in the i layer increases and the recombination of photogenerated carriers through this level increases. As a result, the internal electric field formed inside the junction is weakened, and the characteristics of the thin film solar cell are deteriorated.
[0007]
When thin film solar cells are continuously mass-produced using a plurality of substrates, p, i, n (or n, i, p) layers are formed on one substrate, and then the substrate is removed from the reaction vessel. Next, when a p-layer (or n-layer) is produced on a substrate newly installed in the reaction vessel, an n-type (or p-type) dopant attached in the reaction vessel at the time of the previous film formation is removed. The decomposition reaction product that is included is re-desorbed by being exposed to the plasma during the production of the p layer (or n layer), and n-type (or p-type) dopant is mixed in the p layer (or n layer). Autodoping occurs. As a result, the quality of the p layer (or n layer) is lowered, and the characteristics of the thin film solar cell are deteriorated. Therefore, in order to avoid these problems and obtain a good pin junction, the space for forming each pin layer in the reaction vessel is divided so that the auto-doping does not occur. The device needs to be configured.
[0008]
The specific apparatus configuration differs depending on whether the thin film forming method is a batch type or a roll-to-roll method. In the case of a batch-type forming apparatus that processes substrates one by one, prepare the reaction containers one by one corresponding to each p-i-n layer, and connect them to each other via a gate valve, a substrate transfer chamber, etc. It is necessary to have a structure in which one substrate can travel between the reaction vessels.
[0009]
Using a roll-to-roll method, i.e., a long flexible substrate, a substrate is fed in this direction at a constant speed in parallel to the long direction, and a pin (or in this direction) (or n-i-p), when using a method in which each layer is continuously formed in plasma generated simultaneously at different locations, the source gas of each layer, radicals generated in the plasma, etc. A measure such as providing an air curtain between two adjacent plasmas is taken so as not to be mixed with the source gas in the adjacent space being formed.
[0010]
[Problems to be solved by the invention]
In the roll-to-roll method described above, since it is necessary to produce the p-i-n layers at the same substrate feed rate, the formation conditions for each layer cannot be selected independently, and thus the formation conditions with the highest solar cell efficiency cannot be selected. . In any system, the overall size of the reaction vessel becomes very large.
[0011]
In the case of the above batch type forming apparatus, in order to uniformly form a semiconductor layer on the main surface of the substrate, the cathode is required to have a size equal to or larger than the area of the substrate. The reaction vessel becomes larger as becomes larger. At the same time, in order to perform film formation at a film formation rate that is suitable for production on an industrial scale, it is necessary to increase the high-frequency power applied to the cathode because an appropriate power density is required.
[0012]
Further, in the case of a roll-to-roll method forming apparatus in a tandem-type thin film solar cell in which a plurality of pin junctions are formed in the substrate vertical direction, as described above, the p is sequentially applied along the substrate feed direction. -I-n Since the formation space of each layer is lined up, the enlargement of the apparatus becomes further remarkable. In addition, in a batch type forming apparatus, one reaction container can be used for each layer. However, since the substrate transport mechanism is complicated, the forming apparatus becomes expensive.
[0013]
On the other hand, in capacitively coupled parallel plate plasma CVD, the area of the cathode is S_c, S is the area of the grounded part such as the substrate or the inner wall of the reaction vessel_e, High frequency voltage V_RF, The self-bias on the cathode surface is V_s, Plasma potential V_pThen, the following relationship is established between them (see the potential conceptual diagram of FIG. 3).
[0014]
[Expression 1]

[0015]
However, it is a case where it is set to n-1 on the conditions shown by the following Numerical formula 2 and Numerical formula 3.
[0016]
[Expression 2]

[0017]
[Equation 3]

[0018]
Where generally S_e> S_cSo V_s<0. Since the cathode area increases as the substrate area increases, the ratio of the area of the reaction vessel inner wall / substrate and the area of the cathode to the area of the cathode, which is grounded, is close to 1, and the absolute value of the self-bias decreases accordingly. The potential increases. As a result, due to the potential difference between the plasma potential and the substrate potential, cations in the plasma are accelerated and collide with the film growth surface on the substrate, damaging the film and reducing the efficiency of the solar cell.
[0019]
Further, in order to uniformly form the semiconductor film on the substrate surface, it is necessary to use a method (shower method) in which the source gas is uniformly introduced into the surface. As shown in FIG. 4, when the gas is supplied from the side of the cathode, the source gas proceeds while being decomposed in the plasma along the direction connecting the gas introduction nozzle and the exhaust port. This becomes more serious as the cathode area increases.
[0020]
On the other hand, in the shower method, the control of the uniformity of the raw material gas supply becomes more difficult as the cathode area increases. Further, in this shower system, the gas is supplied through the cathode plate, so that the structure of the cathode is complicated. In particular, when high-frequency plasma is used, it is necessary to devise measures such as using a pipe made of an insulating material for a part of the pipe so that a high-frequency current does not flow to the ground through the gas supply pipe. Moreover, even if such a device is made, a part of the high-frequency power is consumed in the piping, so that extra power is required. That is, all of these problems cause high cost of the thin film solar cell.
[0021]
The present invention has been made in view of the above-described problems, and the object of the present invention is to use a relatively small apparatus to produce a high-quality thin film on a large-area substrate at a relatively low cost. It is an object of the present invention to provide an apparatus and a method for forming a substrate.
[0022]
[Means for Solving the Problems]
According to the present invention, an airtight reaction vessel, a raw material gas introduction part for introducing a raw material gas into the reaction container, a raw material gas decomposition part for decomposing the introduced raw material gas, and the raw material gas decomposition part The substrate holding unit is a rotatable cylinder, and a thin film semiconductor is formed on the outer peripheral surface of the cylinder. There is provided a thin film forming apparatus characterized in that a substrate to be held can be held.
[0023]
That is, in the thin film forming apparatus of the present invention, since the substrate is held on the outer peripheral surface of the cylinder and rotates in the decomposition space of the source gas formed by the source gas decomposition unit, the source material introduced into the source gas decomposition space The gas can be decomposed to form a thin film on the substrate. Therefore, the decomposition space for the source gas can be limited to a small volume space that is stationary on the outer periphery of the cylinder and faces only a part of the substrate.
[0024]
That is, when forming a thin film on a long flexible substrate, it is not necessary to move the substrate linearly in one direction on a flat surface as in the prior art, and the structure of the substrate moving mechanism is relatively simple. By using a certain rotation mechanism, a thin film can be formed in a small reaction vessel. The above advantages are particularly effective when a thin film is formed on a large-area substrate, which is a feature of the thin film forming apparatus of the present invention.
By using the rotation mechanism as the substrate moving mechanism, detection of the moving speed and rotation position of the substrate and control of the substrate movement based on these can be facilitated.
[0025]
The thin film forming apparatus of the present invention is effective for forming a thin film consisting of multiple layers. That is, when different types of layers such as a semiconductor layer are stacked on one substrate, the substrate can be sequentially stacked with the first layer and the second layer by rotating the substrate once, a predetermined number of times, or a predetermined angle. By adjusting the source gas and the rotation speed of the substrate, the type, properties, film thickness, number of layers, etc. of the thin film can be arbitrarily changed.
[0026]
When the above-described thin film forming apparatus of the present invention is compared with a conventional system, the characteristics of the present invention will be further clarified.
That is, it is not necessary to use a horizontally long large apparatus in which a plurality of gas decomposition spaces are arranged along the substrate feed direction as in the roll-to-roll method described above. Moreover, when forming the thin film which consists of a multilayer, the rotational speed of a board | substrate can be set independently according to the optimal condition of each layer. Furthermore, the conveyance path which moves between the several reaction containers used by a batch type can be abbreviate | omitted. That is, the thin film forming apparatus can be configured with a small size and one reaction vessel.
[0027]
According to another aspect of the present invention, a raw material gas decomposition section, a raw material gas introduction port, and an exhaust port opposed to the raw material gas introduction port around a cylinder rotatable inside an airtight reaction vessel. At least three sets, holding the substrate on the outer peripheral surface of the cylinder, and simultaneously introducing the first source gas from the first source gas introduction port to the first source gas decomposition part while rotating the substrate Exhaust gas discharged through the first source gas decomposition unit is exhausted from a first exhaust port facing the first source gas introduction port to form a first thin film on the substrate. After completion of the first step, the second raw material gas is introduced into the second raw material gas decomposition part from the second raw material gas introduction port while rotating the substrate, and simultaneously discharged through the second raw material gas decomposition part. A second exhaust gas that is opposed to the second source gas inlet A second thin film is formed on the substrate by exhausting from the air port. After the completion of the second step, the third source gas is supplied from the third source gas introduction port while rotating the substrate. At the same time, the exhaust gas discharged through the third source gas decomposition unit while being introduced into the gas decomposition unit is exhausted from the third exhaust port opposed to the third source gas introduction port, and is then discharged onto the substrate. A thin film forming method characterized by forming a thin film is provided.
[0028]
That is, the thin film forming method of the present invention will be described by taking a method for forming a semiconductor layer of a thin film solar cell as an example. A thin film forming means comprising a source gas inlet, an exhaust port, and a source gas decomposition part around a rotatable cylinder. At least three sets are arranged. For example, at least three source gas decomposition portions are formed between a cylinder as an anode and each of the first, second, and third cathodes arranged to face the cylinder. In the first source gas decomposition unit, a source gas containing a p-type doping gas is introduced and exhausted, and high frequency power is applied to the first source gas decomposition unit to form a high frequency plasma space to form a p layer. In the second source gas decomposition unit, the source gas is introduced and exhausted without adding doping gas, and high frequency power is applied to the second source gas decomposition unit to form a high frequency plasma space to form an i layer. In the third source gas decomposition unit, a source gas containing n-type doping gas is introduced and exhausted, and high frequency power is applied to the third source gas decomposition unit to form a high frequency plasma space to form an n layer. In this way, the p, i, and n layers can be sequentially formed on the substrate. Of course, if the type of gas is changed, the conductivity type can be changed, and the p, i, and n layers can be sequentially formed on the substrate.
[0029]
For example, when the p layer (or n layer) is formed by the first source gas using the first high frequency plasma space formed by applying high frequency power to the first source gas decomposition portion, the n layer The first source gas flows into the second and third source gas decomposition units used to form the (or p layer) and i layers. At that time, the second and third source gas decomposition units Since no high frequency power is applied, the formed high frequency plasma space is limited to only the first high frequency plasma space, and the first source gas flowing into the second and third source gas decomposition sections is decomposed by the plasma. There is nothing.
[0030]
Therefore, the first source gas does not adhere as a reaction product to the wall surfaces facing the second and third source gas decomposition sections, the cathode surface, and the like. The source gas flowing from one source gas decomposition unit to the other source gas decomposition unit adheres to the wall surface or the like by physical adsorption, but the attached source gas applies high frequency power to the source gas decomposition unit to generate high-frequency power. By flowing each source gas from the source gas introduction port to the exhaust port before the plasma space is formed, the gas can be exhausted to the outside of the reaction vessel.
[0031]
As described above, the source gas containing the p-type (or n-type) doping gas that has flowed out of the first source gas decomposition part flows into other than the first source gas decomposition part, and enters the wall surface in the reaction vessel. Although it adheres, it is easily detached and discharged out of the reaction vessel. In addition, since no plasma exists in a portion other than the first source gas decomposition section, generation of decomposition products of source gas and adhesion to the inner surface of the reaction vessel do not occur. Therefore, in the formation of the second and third semiconductor layers, auto-doping by re-decomposition of the source gas decomposition products adhering to the inner surface of the reaction vessel in the vicinity of each source gas decomposition portion by plasma and deposition on the substrate is performed. Does not occur.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
In the thin film forming apparatus of the present invention, the thin film is 10 μm to 10 μm.^-3A semiconductor layer having a thickness of about μm can be given.
The “airtight reaction vessel” preferably has a volume capable of accommodating the substrate holding portion therein and can hold the substrate at an atmospheric pressure or in a state where the periphery of the substrate is decompressed by a vacuum pump.
[0033]
The substrate to which the thin film forming apparatus of the present invention can be applied may be a small flat plate made substantially of a rigid body, but is preferably a flexible flat plate because it can be easily held on the outer peripheral surface of a cylinder. Such a substrate is preferably in the form of a sheet or film that is easily deformed by an external force.
The cylinder is preferably rotatable with its central axis coinciding with the rotation axis. Further, the cylinder is preferably made of a metal and can be used as an anode by grounding. By forming with a metal, it is possible to heat the substrate held on the cylindrical surface by mounting a heater.
The substrate holding part preferably includes a holding means for holding the substrate on the surface thereof on the outer peripheral surface of the rotatable cylinder. The holding means may be any means as long as the substrate can be easily attached and detached and the substrate does not loosen. Examples thereof include a mechanical holding chuck, an electrostatic chuck, and a vacuum chuck.
[0034]
The source gas introduction part is preferably one that can control the flow rate, flow rate, and discharge pressure of the source gas. Further, the exhaust part is preferably provided with a suction source having a suction force capable of sucking the raw material gas diffused into the reaction vessel and exhausting it outside the reaction vessel.
[0035]
If the cylinder of the substrate holding part includes a rotation speed control means for controlling the rotation speed of the cylinder, the optimum conditions regarding the film thickness and the like can be adjusted according to the thin film to be formed. Examples of the rotational speed control means include power control of a cylinder driving motor and a reduction acceleration device using gears or pulleys.
[0036]
If the cylinder of the substrate holding part is equipped with a rotation position detecting means for detecting the rotation position of the cylinder, both ends of the substrate held by the substrate holding part or specific parts on the substrate are detected to rotate the cylinder. Driving and stopping can be performed, and further, the raw material gas decomposition unit, the raw material gas introduction unit, and the exhaust unit can be controlled individually or in conjunction with the rotational driving and stopping of the cylinder. Specifically, a combination of a photosensor that detects the rotational position of a cylinder and a stepping motor for driving the cylinder can be used.
[0037]
The source gas decomposition part in the present invention is a means for forming a gas decomposition space for depositing a source gas as a thin film on a substrate. Plasma generating means such as high-frequency plasma, microwave plasma, and optical plasma, or heat on the surface of a high-temperature catalyst. Means such as decomposition can be mentioned.
The source gas decomposition unit is configured to form a gas decomposition space between the source gas decomposition unit and the substrate held by the substrate holding unit, the source gas introduction unit has a source gas introduction port opened in the reaction vessel, and the exhaust unit has A gas exhaust port opened in the reaction vessel is provided, and the source gas introduction port and the gas exhaust port are disposed at positions facing each other across the gas decomposition space, thereby exhausting from the source gas introduction unit. It is possible to form a flow of the raw material gas that is uniform and less turbulent.
[0038]
The substrate holding unit rotates the cylinder in the same direction as the flow of the source gas from the source gas introduction port to the gas exhaust port, so that the source gas flows in the gas decomposition space between the source gas decomposition unit and the substrate. Disturbance can be suppressed.
Note that the source gas introduction port and the gas exhaust port are such that at least the gas exhaust port of both has a larger opening width than the width of the substrate (the length in the direction of the rotation axis of the cylinder), so A uniform gas flow can be easily formed.
[0039]
The thin film forming apparatus according to the present invention includes a cathode electrode disposed at a position facing the outer peripheral surface of a cylinder, whereby the source gas decomposition unit forms a plasma space between the substrate and the substrate held by the substrate holding unit. If it is set as the structure to perform, a semiconductor layer can be laminated | stacked on a board | substrate by plasma CVD method.
[0040]
A plurality of cathode electrodes are arranged around a cylinder, and each cathode electrode includes a pair of source gas introduction part and exhaust part, thereby stacking a plurality of semiconductor layers on a substrate inside one reaction vessel. be able to. That is, when the first, second, and third source gases are used, if these source gases are different gases, they differ by shifting the supply time zone of each source gas within one reaction vessel. A semiconductor layer can be formed. If the above three source gases are the same gas, a large number of the same semiconductor layers can be formed by simultaneously supplying the source gases inside one reaction vessel.
[0041]
By using the cylinder itself as the anode electrode, the substrate holding portion and the anode can be used together, and the anode electrode can have a larger area than the cathode electrode. That is, if the area of the cathode electrode is smaller than that of the anode electrode, in the case of thin film formation by the high-frequency plasma CVD method, as described above, the increase in the plasma potential in the generated plasma space is suppressed, thereby suppressing the increase in the plasma potential. The collision of cations on the substrate can be reduced, and damage to the formed thin film can be minimized.
[0042]
It is preferable that the source gas introduction part and the exhaust part constitute a pair of shielding wall bodies in which the pipe lines of these parts surround both side surfaces of the cylinder facing the cathode. Such a shielding wall body locally limits a space for decomposing the source gas by the generated plasma, and suppresses auto-doping of the source gas as described above.
[0043]
Specific examples in the case of forming a thin film solar cell using the thin film forming method of the present invention are shown below. First, the substrate is held on the outer peripheral surface of the cylinder, the inside of the reaction vessel is depressurized, and the first source gas is introduced into the first source gas decomposing portion from the first source gas introduction port while rotating the substrate. Exhaust gas is exhausted from the first exhaust port facing the source gas inlet.
Next, high frequency power is applied to the first cathode electrode to form a thin film of the first conductive type semiconductor layer on the substrate. After the application of the high frequency power and the introduction of the first source gas are stopped and the high frequency plasma is extinguished, the second source gas is introduced into the second source gas decomposition part from the second source gas introduction port while rotating the substrate. At the same time, exhaust gas is exhausted from the second exhaust port facing the second source gas introduction port.
[0044]
Next, high frequency power is applied to the second cathode electrode to form a substantially intrinsic semiconductor layer thin film on the substrate. After the application of the high frequency power and the introduction of the second source gas are stopped and the high frequency plasma is extinguished, the third source gas is introduced into the third source gas decomposition section from the third source gas introduction port while rotating the substrate. At the same time, exhaust gas is exhausted from the third exhaust port facing the third source gas introduction port.
Next, high frequency power is applied to the third cathode electrode to form a semiconductor layer having conductivity opposite to that of the first conductivity type semiconductor layer on the substrate. After the application of the high frequency power and the introduction of the third source gas are stopped and the high frequency plasma is extinguished, the substrate is cooled to near room temperature, the inside of the reaction vessel is returned to atmospheric pressure, and the substrate is taken out.
[0045]
The gas pressure, flow rate, composition, high frequency power, frequency, anode rotation speed (cylinder rotation speed), etc. at the time of forming each layer can be set independently. The area of each cathode need not be the same. In addition, since the film thickness of the i layer, which is a photoelectric conversion layer in a thin film solar cell, must be thicker than that of each of the n and p layers, a plurality of pairs of cathodes, a source gas introduction part and an exhaust part are used simultaneously for the formation. It is also possible to shorten the time required for producing the i layer. In addition, a buffer layer for improving the interface characteristics may be provided at the p / i interface or the n / i interface, but the p / i interface (or n / i interface) buffer layer as in the conventional batch type forming apparatus. As the cathode for formation, the cathode used for forming the p layer (or n layer) can be used.
[0046]
Furthermore, when forming the tandem-type thin film solar cell described above, it is only necessary to prepare one cathode for forming a plurality of pn layers, so that the size is smaller than that of the roll-to-roll method forming apparatus. In addition, a substrate transport mechanism such as the batch-type forming apparatus in which a plurality of reaction vessels are connected is also unnecessary.
[0047]
In addition, when the p layer (or n layer) is formed, a source gas containing a p-type (or n-type) doping gas is also present in the source gas decomposition part that forms the n layer (or p layer) and the i layer. However, since no plasma is present in these raw material gas decomposition parts, p-type (or n-type) reaction products do not adhere to the inner wall facing the space or the surface of the cathode. Therefore, autodoping resulting from the reaction between the reaction product adhering to the wall surface or the like and the plasma does not occur, and high quality of each semiconductor layer can be achieved.
[0048]
The physically adsorbed gas can be easily exhausted outside the reaction vessel by flowing the source gas before the generation of plasma when the i layer and the n layer (or p layer) are formed next time. Therefore, there is no worry of auto-doping as described above, and a high-quality solar cell can be formed. This situation is the same when processing a plurality of substrates continuously.
[0049]
In addition, unlike the roll-to-roll method in which the plasmas forming the p, i, and n layers must be generated at the same time, the plasma of each layer is not generated at the same time in the apparatus proposed in the present invention. Therefore, it is not necessary to take measures such as an air curtain for preventing the raw material gas from being mixed between the film formation spaces. Therefore, it is possible to reduce the size of the entire apparatus with a simple structure as compared with a forming apparatus using a roll-to-roll method.
[0050]
Hereinafter, the present invention will be described in more detail with reference to embodiments.
1 and 2 show a configuration of a semiconductor film forming apparatus as a thin film forming apparatus. 1 is a plan view thereof, and FIG. 2 is a partial sectional front view (II-II sectional view) of FIG.
This semiconductor film forming apparatus 10 is an apparatus for forming a semiconductor layer of a thin film solar cell using a high frequency plasma CVD method.
[0051]
The semiconductor film forming apparatus 10 includes an airtight reaction vessel 1, a cylinder 2 as a substrate holding unit provided in the reaction vessel 1, and source gas introduction units 31 and 32 that introduce source gas into the reaction vessel 1. , 33 and cathodes 11, 12 which are arranged in proximity to the substrate S held in the cylinder 2 and which decompose the source gas introduced from each source gas introduction part 31, 32, 33 to form a thin film on the substrate S. 13, and exhaust parts 41, 42, 43 for exhausting the source gas from the reaction vessel 1.
[0052]
The reaction vessel 1 is held on an upper portion of a base 7 having legs 6 and can be decompressed by a vacuum pump (not shown).
The cylinder 2 holds an anode 26 having a covered cylinder itself detachably connected to a drive shaft of a motor 8 held by a base 7 as an electrode, and a flexible substrate S on an outer peripheral surface portion 27 of the cylinder 2. A vacuum chuck (not shown). The cylinder 2 is made of stainless steel, and is configured to be used as an electrode by being grounded.
[0053]
The substrate S is made of a heat-resistant and flexible film such as stainless steel or polyimide, and is closely attached along the curved surface of the cylinder 2 which is a substrate holding mechanism. A plurality of substrates may be held on the outer peripheral surface portion 27. The temperature of the substrate S is maintained at a predetermined temperature by a heater (not shown) provided in the cylinder 2.
[0054]
The cathodes 11, 12, and 13 are disposed to face the outer peripheral surface portion 27 of the anode 26, and a pair of earth shields 21, 22, and 23 surrounding each cathode are provided on the sides of the cathodes 11, 12, and 13, respectively. The cathodes 11, 12, and 13 are disposed in a state of being electrically insulated.
[0055]
These cathodes 11, 12, and 13 can apply electric energy to the plasma from a high-frequency power source as a mechanism for generating and maintaining plasma, which is desirably the same as that of a conventional parallel plate type plasma CVD apparatus. Each cathode 11, 12, 13 has a surface in a direction perpendicular to the radial line and parallel to the outer peripheral surface portion 27, and has a width (height) in the axial direction of the substrate S held in the outer peripheral surface portion 27 in a cylindrical shape. The shape is set so as to be larger than the width of the substrate S so as to generate uniform plasma in a direction parallel to the axis of the cylinder 2.
[0056]
Further, source gas introduction parts 31, 32, 33 and exhaust parts 41, 42, 43 are arranged on the sides of each pair of the ground shields 21, 22, 23. Each of the raw material gas introduction part and the exhaust part is paired with an introduction port 31a, 32a, 33a and an exhaust port that open in the reaction vessel 1 facing each other on a line parallel to the surfaces of the cathodes 11, 12, and 13, respectively. 41a, 42a, 43a. These openings are formed by the substrate S held by the earth shields 21, 22, 23, the cathodes 11, 12, 13 and the substrate holding part 2 so that the flow of the raw material gas from each raw material gas introduction part to the respective exhaust parts facing each other. It is configured in an unobstructed position.
[0057]
Each of the raw material gas introduction sections 31, 32, 33 and the exhaust sections 41, 42, 43 extends radially outward in the reaction vessel 1 by pipes 31b, 32b, 33b and 41b, 42b, 43b. It is connected to the external intake / exhaust system.
[0058]
In such a configuration, the plasma spaces formed between the cathodes 11, 12, 13 and the substrate surface facing each of the cathodes 11, 12, 13 are the source gas introduction parts 31, 32, 33 and the exhaust part 41. , 42 and 43, the space in which the source gas is decomposed by the generated plasma is locally limited. For example, when film deposition is performed in the source gas decomposition space 51 shown in FIG. 1, even if the source gas flows out to a space other than the local 51 in the reaction vessel 1, no plasma is formed there. No gas decomposition reaction product is produced. Since the source gas itself is only physically adsorbed on the inner wall surface of the reaction vessel, it can be easily discharged outside the reaction vessel before the high frequency plasma is formed in the source gas decomposition space 52 (or 53). The above situation also applies to film deposition in the source gas decomposition spaces 52 and 53.
Since the flow of the raw material gas from each raw material gas introduction part to each opposing exhaust part is not blocked, it can pass through the plasma space and quickly and uniformly to each exhaust port.
[0059]
The size of the cathodes 11, 12, and 13 must be larger than the width of the substrate S (the vertical dimension in FIG. 2), but can be selected regardless of the length of the substrate S. It is desirable that the diameter is smaller than 2. Therefore, by reducing the area of the cathodes 11, 12, and 13, not only can the semiconductor film forming apparatus 10 be reduced in size and the input high-frequency power can be reduced, but also the reaction vessel in which the area of each cathode is a grounded part. 1 Since the inner wall and the surface area of the substrate S can be made very small, it is possible to reduce the plasma potential and to reduce the characteristics of the solar cell due to collision damage of the cation to the film surface during film formation. It can be suppressed.
[0060]
It is desirable to use gas introduction nozzles for the source gas introduction sections 31, 32 and 33. Although it is technically possible to adopt a shower system through countless small holes formed in the main surfaces of the cathodes 11, 12, and 13, in this case, as described above, wasteful consumption of high-frequency power in the piping is avoided. I can't. The introduction ports 31a, 32a, 33a and the exhaust ports 41a, 42a, 43a are also made larger than the substrate width in the direction parallel to the axis of the cylinder 2 constituting the substrate holding part, and a uniform gas flow is made in the axis direction. It has a shape that can be generated.
[0061]
Further, the rotation direction of the cylinder 2 is set so that the direction in which the substrate S moves by the rotation is the same as the direction in which the source gas flows. This is because if the gas flow and the moving direction of the substrate S are reversed, the gas flow in the vicinity of the surface of the substrate S is disturbed, disturbing the spatial distribution of the deposition precursor inside the plasma, and as a result, the quality of the semiconductor film This is because the in-plane uniformity is impaired.
[0062]
In the semiconductor layer forming apparatus 10, the areas of the cathodes 11, 12, and 13 are much smaller than the area of the substrate S, but the film can be formed on the entire region of the substrate S by the rotation of the anode 26. Since the anode 26 has a mechanism for controlling the rotation speed, the film formation speed of the semiconductor layer can be controlled by the rotation speed in addition to the plasma conditions.
Further, the cylinder 2 has a mechanism that makes it possible to detect and control the rotational position. By aligning the detection position with the start and end positions of film formation, the film can be formed uniformly in the moving direction of the substrate S. Therefore, it is possible to form a semiconductor layer uniformly on the entire surface of the substrate S. It is desirable that the start and end of film formation be at the end of the substrate S.
[0063]
An example of the manufacturing method of the thin film solar cell by the semiconductor film formation apparatus 10 is described below.
First, using a heat-resistant and flexible film-like substrate such as stainless steel or polyimide, a thin metal electrode with high reflectivity such as Al / Ag is applied to the entire surface by sputtering or vapor deposition on one side. Form a film. In order to obtain higher reflectance, ZnO.SnO₂A transparent conductive film such as ITO is formed by a method such as MOCVD, sputtering, or vapor deposition. Desirably, a metal film and a transparent conductive film are produced by selecting conditions such that the surface of the back electrode of the substrate S is uneven.
[0064]
Next, the substrate S is held so as to be in close contact with the outer peripheral surface of the cylinder 2 constituting the substrate holding portion so that the back electrode is on the outside. At this time, uniform tension is applied to the entire substrate. The material of the cylinder 2 is preferably a material having conductivity, thermal conductivity, heat resistance, and plasma resistance such as stainless steel.
[0065]
As a mechanism for controlling the substrate temperature, the cylinder 2 is provided with a heater and a thermocouple for keeping the substrate temperature at a predetermined temperature, and these lead wires are hollow pipes concentric with the rotating shaft. It can be electrically connected to the outside of the reaction vessel 1 through the inside. Alternatively, the inside of the cylinder 2 is hollowed in a cylindrical shape so as to be concentric with the rotation axis, and the reaction vessel 1 is provided with a cylindrical pipe containing a heater and a thermocouple. The pipe may be positioned in a hollow portion inside the cylinder 2 when incorporated in the cylinder 2.
[0066]
Next, the cylinder 2 holding the substrate S is mounted in the reaction vessel 1. At this time, the rotation shaft of the cylinder 2 is connected to the drive shaft of the motor 8. Thereafter, gaseous impurities remaining inside the reaction vessel 1 are removed by evacuating the reaction vessel 1 with a high vacuum or flowing a large amount of high-purity inert gas such as Ar. At the same time, the temperature of the substrate S is raised to a predetermined temperature.
[0067]
Thereafter, hydrogenated amorphous silicon n, i, and p layers are sequentially formed while rotating the substrate S at a predetermined rotational speed. As the source gas used at this time, for example, SiH_Four, Si₂H₆, SiH_xF_4-x(X = 0-3), SiH_xCl_4-x(X = 0-3), CH_Four, GeH_FourAs a carrier gas, for example, H₂・ There are Ar and He. As the source gas, in addition to the above, when forming a semiconductor layer having p-type conductivity, B₂H₆When forming an n-type conductive semiconductor layer, etc._ThreeHowever, it is not limited to these.
[0068]
Specifically, first, the source gas is introduced into the reaction vessel 1 using the source gas introduction unit 31 while rotating the substrate holding unit 2, and at the same time, the source gas remaining in the reaction vessel 1 is exhausted using the exhaust unit 41. To do. As soon as the gas flow settles, high frequency power is applied to the cathode 11 to form an n layer. Next, the drive of each drive source of the source gas introduction part 31 and the exhaust part 41 is stopped, and the application of high frequency power is stopped.
[0069]
After the inside of the reaction vessel 1 reaches a predetermined degree of vacuum, the i layer is formed by using another set of the cathode 12, the raw material gas introduction part 32, and the exhaust part 42 that were not used when the n layer was formed. . Next, a p-layer is formed using a set of cathode 13, source gas introduction part 33, and exhaust part 43 that was not used when either the n-layer or the i-layer was formed. The start and end of film formation for forming each layer is performed at a rotational position where the edge of the substrate is directly under each cathode.
[0070]
Each forming condition is shown in Table 1.
[0071]
[Table 1]

[0072]
This formation condition is an example, and each layer may be formed under other conditions. In this embodiment, hydrogenated amorphous silicon is used as the semiconductor thin film. However, hydrogenated microcrystalline silicon, polycrystalline silicon, or an alloy of silicon, such as carbon or germanium, may be used.
[0073]
Next, the substrate S on which the semiconductor layer is formed is taken out of the reaction vessel 1, and ZnO.SnO is formed on the surface thereof as a window side electrode.₂A high-conductivity transparent conductive film such as ITO is formed by a method such as MOCVD, sputtering, or vapor deposition. Further thereon, a patterned collector electrode such as a comb is formed by a method such as sputtering or vapor deposition using a metal material such as Al.
[0074]
The characteristics of the solar cell thus formed are as follows._sc= 17.5 mAcm², Open circuit voltage V_OC= 0.86 V, fill factor FF = 0.70, and photoelectric conversion efficiency η = 10.5%. This is comparable to a thin film solar cell manufactured using a conventional semiconductor layer forming apparatus.
[0075]
In the above-described embodiment, the configuration in which the rotation axis of the cylinder 2 is arranged in the vertical direction is shown, but the direction of the rotation axis is not particularly limited. Therefore, the supporting members of the semiconductor film forming apparatus, such as the base and the legs, can be arbitrarily set.
[0076]
【The invention's effect】
In the thin film forming apparatus of the present invention, since the substrate is held on the outer peripheral surface of the cylinder and rotates in the decomposition space of the source gas formed by the source gas decomposition unit, the source gas introduced into the source gas decomposition space is It can be decomposed to form a thin film on the substrate. Therefore, it is possible to form a desired thin film by limiting the decomposition space of the source gas to a small volume space that is stationary on the outer periphery of the cylinder and faces only a part of the substrate.
[0077]
When a thin film is formed on a long flexible substrate, it is not necessary to move the substrate linearly in one direction on a flat surface as in the prior art, and the substrate moving mechanism has a relatively simple structure. By using the mechanism, a thin film can be formed in a small reaction vessel. The above advantages are particularly effective when a thin film is formed on a large-area substrate, which is a feature of the thin film forming apparatus of the present invention.
By using the rotation mechanism as the substrate moving mechanism, detection of the moving speed and rotation position of the substrate and control of the substrate movement based on these can be facilitated.
[0078]
When the present invention is used for the production of a thin film solar cell, a rotatable substrate holding part is used as an anode, and at least three cathodes, a raw material gas introduction part and an exhaust part are provided in one airtight reaction vessel, and p・ Because i and n layers can be formed on a flexible substrate, a relatively thin and high quality thin-film solar on a large-area substrate is used on a large-area substrate by using a relatively small forming apparatus. By forming the battery, the production cost of the thin film solar cell can be reduced.
[Brief description of the drawings]
FIG. 1 is a plan view of a thin film forming apparatus according to the present invention.
FIG. 2 is a front view taken along the line II-II in FIG.
FIG. 3 is a diagram showing the concept of plasma potential in high-frequency plasma.
FIG. 4 is a view showing a conventional apparatus for forming a semiconductor layer of a thin film solar cell by a parallel plate type plasma CVD method.
[Explanation of symbols]
1 reaction vessel
2 Cylinder (substrate holder)
10 Semiconductor film forming equipment (thin film forming equipment)
11, 12, 13 Cathode (source gas decomposition part)
31, 32, 33 Source gas introduction part
41, 42, 43 Exhaust section
26 Anode
27 Outer peripheral surface
S substrate

Claims (8)

An airtight reaction vessel, a raw material gas introduction unit for introducing a raw material gas into the reaction vessel, a raw material gas decomposition unit for decomposing the introduced raw material gas, and discharging the raw material gas through the raw material gas decomposition unit comprising an exhaust portion for discharging a gas, and the reaction substrate holding portion that is provided inside the container, the substrate holder is a rotatable cylindrical, capable of holding the substrate to be formed thin film semiconductor on the outer circumferential surface of the cylinder And
The source gas decomposition unit is configured to form a gas decomposition space between the source gas decomposition unit and the substrate held by the substrate holding unit, the source gas introduction unit has a source gas introduction port opened in the reaction vessel, and the exhaust unit has It has a gas exhaust port opened in the reaction vessel, and the raw material gas introduction port and the gas exhaust port have the same flow direction of the raw material gas from the raw material gas introduction port to the gas exhaust port as the rotation direction of the cylinder, A thin film forming apparatus, wherein the thin film forming apparatus is disposed at positions facing each other across the gas decomposition space.  The thin film forming apparatus according to claim 1, wherein the cylinder of the substrate holding unit includes a rotation speed control unit that controls a rotation speed.  The thin film forming apparatus according to claim 1, wherein the cylinder of the substrate holding portion includes a rotational position detecting unit that detects a rotational position. Material gas decomposition unit, comprising a cathode electrode disposed in a position facing the outer circumferential surface of the cylinder, one of claims 1 to 3 for thereby forming a plasma space between the substrate held by the substrate holder The thin film formation apparatus as described in any one. 5. The thin film forming apparatus according to claim 4 , wherein a plurality of cathode electrodes are arranged around a cylinder, and each cathode electrode includes a pair of a source gas introduction part and an exhaust part.  At least three sets of a raw material gas decomposition part, a raw material gas introduction port, and an exhaust port opposite to the raw material gas introduction port are arranged around a cylinder that can be rotated inside an airtight reaction vessel. The substrate is held on the surface, and the first source gas is introduced into the first source gas decomposition unit from the first source gas introduction port while rotating the substrate, and at the same time through the first source gas decomposition unit. The discharged exhaust gas is exhausted from a first exhaust port facing the first source gas introduction port to form a first thin film on the substrate, and the substrate is rotated after the completion of the first step. The second source gas is introduced into the second source gas decomposition section from the second source gas introduction port, and the exhaust gas discharged through the second source gas decomposition section is simultaneously supplied to the second source gas decomposition section. Exhaust from the second exhaust port facing the introduction port and on the substrate A second thin film is formed, and after the completion of the second step, the third source gas is introduced into the third source gas decomposition part from the third source gas introduction port while rotating the substrate, and the third source gas is decomposed. The third thin film is formed on the substrate by exhausting the exhaust gas discharged through the raw material gas decomposition section from the third exhaust port facing the third raw material gas introduction port. Thin film forming method. The first conductive type semiconductor layer is formed by the first thin film forming step, the substantially intrinsic semiconductor layer is formed by the second thin film forming step, and the opposite of the first conductive type semiconductor layer is formed by the third thin film forming step. The thin film forming method according to claim 6 , wherein a second conductive type semiconductor layer having conductivity is formed. The thin film forming method according to claim 6 or 7 , wherein the thin film to be formed is a semiconductor layer of a thin film solar cell.

Images