JP3558479B2 - Guided light detector and method of manufacturing the same - Google Patents

Description

【００６６】
図３に示す形状のテーパ部２６では、図１に示すテーパ部６の斜度ψ（窒化ケイ素膜２と誘電体層３（バッファ層）との境界に対する傾き角）のように厳密に斜度φは定義できないものの、表１が示すように最大斜度は１０゜以下になっている。
【００６７】
次に、図４に基づき本実施形態３の導波光検出器の光結合特性について説明する。但し、図４の横軸ｘはテーパ終端Ｅと不純物拡散領域５の受光部９の起点（図３中に符号Ｏで示す）との距離を示し、符号＋は受光部９とテーパ部２６が重なる場合を示している。
【００６８】
図４において、まず、光結合効率の最大値に着目する。図４の光結合効率の実験値について、テーパ部２６のエッジＥとの距離ｘが＋１０μｍを越えると、光結合効率の変化は小さくなり、一定の値に近づく。また、距離ｘが＋２０μｍ以上に達すれば、光結合効率は９０％以上に達する。一方、光結合効率の理論値は、テーパ部２６のエッジＥとの距離ｘが１０μｍを越えるとほぼ一定となり、ｘが＋２０μｍ以上に達すれば、１００％になることを示している。即ち、理論値と実験値との双方の変化は、ｘが＋２０μｍ以上に達すれば光結合がほぼ１００％完了することを意味している。なお、実験値ではｘが＋３０μｍのときでも光結合効率は９５％にしかならないが、残りの５％の損失は光散乱損失によるものであり、損失の別の要因であるテーパ部２６での空気側への放射損失は十分抑制されている。つまり、図３の具体例に示した構造の導波光検出器で空気側への放射損失が除去されたことになる。テーパ斜度制限による空気側への放射損失の低減の効果は、窒化ケイ素膜がないテーパ導波路で、テーパ導波路の放射特性を調べた実験（１９９５年秋季応用物理学会学術講演会２８ａ−ＳＱ−２９；シャープ（株）南 他）で既に明らかであるが、図４の実験結果が示すように窒化ケイ素膜がある状態でも、窒化ケイ素膜の影響で放射条件が変わるにも拘わらず、テーパの最大斜度が１０゜以下の場合に空気側への放射低減効果が発揮され、高光結合効率が達成できる。
【００６９】
但し、実際に作製した素子では、各層境界での凹凸の影響で光散乱損失が発生する場合がよくある。また、この損失の影響は、光導波路層４３から光電変換素子の受光部９に光結合が完了するまでの距離が長いほど大きくなる。以上の理由から、光散乱損失を低減するには、結合が完了するまでの伝搬距離を短くする必要がある。
【００７０】
そこで、本発明では、窒化ケイ素膜（２１，２２）がある場合に伝搬距離を短くするために、光導波路層４３からの窒化ケイ素膜を通して不純物拡散領域５の受光部９への光の透過率の向上を図っている。具体的には、本実施形態３の導波光検出器では、第１窒化ケイ素膜２１及び第２窒化ケイ素膜２２の膜厚を上記膜厚ｔ_１＝９５ｎｍ、ｔ_２＝４００ｎｍに設定することにより、光導波路層４３からの窒化ケイ素膜を通して不純物拡散領域５の受光部９への光の透過率を向上させている。
【００７１】
次に、図５及び図６に基づき、この透過率向上効果が得られる理由について説明する。ここで、図５はテーパ部２６の一部を抜き出し、多層膜とみなしたときの図である。光導波路層４３を主に伝搬する光１０の受光領域（光結合可能な領域）での入射角（図５中にθ_ｉで表記）は、伝搬光のカットオフ付近の実効屈折率Ｎがおよそ誘電体層３の屈折率ｎ_ｂであることと、光導波路層の第１誘電体層４１の屈折率がｎ_ｇ１であることから、下記（１）式で表される。
【００７２】
θ_ｉ＝ｓｉｎ^−１（ｎ_ｂ ／ｎ_ｇ１） …（１）
上記（１）式にｎ_ｂ ＝１．４５を代入してθ_ｉを計算すると、７１．４゜となる。よって、光導波路層４３を主に伝搬する光１０の受光領域での入射角θ_ｉは約７０゜付近と予想される。
【００７３】
ここで、図６（ａ）、（ｂ）は、本発明者等によるシミュレーション結果を示すものであり、テーパ部２６で伝搬光のカットオフとなる付近の構造を、入射光が入る媒質を光導波路層４３の導波層（コーニング社の商品名＃７０５９ガラス層）４１とした多層膜とみなしたときの反射率を示している。図６（ａ）は誘電体層（ＰＳＧ層）３の部分の膜厚ｂ_ｐが２００ｎｍの場合、図６（ｂ）は誘電体層（ＰＳＧ層）３の部分の膜厚ｂ_ｐが１００ｎｍの場合をそれぞれ表す。
【００７４】
図６（ａ）、（ｂ）では、反射率が小さいほど透過率が高いことを意味しており、いずれの図でも入射角θ_ｉが約７０゜付近で反射率がほぼ０になることがわかる。つまり、第１窒化ケイ素膜２１、第２窒化ケイ素膜２２が、半導体基板（Ｓｉ基板）１と、誘電体層（ＰＳＧ層）３と、光導波路層４３の第１誘電体層（コーニング社の商品名＃７０５９ガラス層）４１との間で入射角θ_ｉが約７０゜付近に対する反射防止膜を形成していることがわかる。
【００７５】
この反射防止膜としての機能が、光導波路層４３から不純物拡散領域５の受光部９への光の透過率向上につながる。テーパ斜度適正化の効果に上記透過率向上効果も加わって、本実施形態３導波路検出器では、光導波路層４３から受光部９への結合効率が図４に示すような高光結合効率となっている。
【００７６】
光導波路層４３の部分については、第１誘電体層４１、第２誘電体層４２の２層構成だけでなく、光導波路層４３は３層以上の多層構造でもよい。この理由は、光導波路層４３の最下層の誘電体層（図３の例では第１誘電体層４１）が、窒化ケイ素膜（２１，２２）に対する垂線を基準にした入射角θ_ｉが定義される、入射側媒質と見なせる点にある。
【００７７】
また、窒化ケイ素膜は、窒化ケイ素膜に対する垂線を基準にした入射角θ_ｉが約７０゜付近に対する反射防止膜としての機能が満たされればよいので、第１窒化ケイ素膜２１と第２窒化ケイ素膜２２というように成膜方法等の違う２層以上の構成でもよいし、１層だけとなってもよい。更に、図３の構成のように、第１窒化ケイ素膜２１と第２窒化ケイ素膜２２とでそれぞれの屈折率が異なってもよい。
【００７８】
従って、図７及び図８に示す構成例、また図２に示す構成例についても、本実施形態３に示した方法で高光結合効率を享受することができる。但し、図７は光導波路層が１層、窒化ケイ素膜が２層構造の導波光検出器を示し、図８は光導波路層が２層構造、窒化ケイ素膜が１層の導波光検出器を示す。なお、図３と対応する部分には同一の符号を付してあり、具体的な説明については省略する。
【００７９】
次に、図９（ａ）、（ｂ）に基づき、図３の構成の導波光検出器において、窒化ケイ素膜の厚さの変化に対する光結合特性の変化について説明する。なお、図９（ａ）、（ｂ）は、第１窒化ケイ素膜（図中のＳｉＮ下層）として屈折率ｎ_１が２．０のＳｉＮを用い、第２窒化ケイ素膜として屈折率ｎ_２が１．８６のＳｉＮを用いた場合のシミュレーション結果を示している。
【００８０】
図９（ａ）、（ｂ）では、光導波路層４３の第１誘電体層４１の厚さｄが５７０ｎｍであり、バッファ層３であるＰＳＧ層のテーパ形状が表１に示すような形状であるとき、屈折率ｎ_１＝２．０のＳｉＮ膜の厚さｔ_１、屈折率ｎ_２＝１．８６のＳｉＮ膜の厚さｔ_２をそれぞれ変化させた場合を示しており、図９（ａ）はＴＥモードに対する光結合特性の変化を示し、図９（ｂ）はＴＭモードに対する光結合特性の変化を示している。
【００８１】
図９（ａ）、（ｂ）において、光結合特性は規格化伝送パワー（光結合前の光導波路層４３の伝送パワーを１として、光電変換素子への光結合によって減少していく光導波路層４３の伝送パワーの相対値を示したもの）のテーパ部の終端Ｅからの距離ｌに対する変化をもって示される。
【００８２】
同図（ａ）、（ｂ）から、ＴＥモード、ＴＭモード双方に対する光結合特性は、第１窒化ケイ素膜２１と第２窒化ケイ素膜２２を合わせた厚さｔ_１＋ｔ_２が、４５０ｎｍ〜５５０ｎｍの間で変化しても、あまり変化しないことがわかる。また、誘電体層３には、屈折率ｎ_ｂがほぼ１．４５の材料がよく使われ、第１誘電体層４１には屈折率ｎ_ｇ１がほぼ１．５３の材料がよく使われることから、窒化ケイ素膜の厚さを４５０ｎｍ〜５５０ｎｍの範囲内に設定すれば、高光結合効率の導波光検出器を実現できることがわかる。
【００８３】
（実施形態４）
図１０及び図１１は本発明導波光検出器の実施形態４を示す。本実施形態４の導波光検出器は、光が伝搬して来る側のテーパ部の受光部上にあるテーパ終端から光の伝搬方向に誘電体層の厚さが０となる領域を２０μｍ以上形成し、これにより吸収効率を向上させ、結果的にテーパ終端通過後の伝搬距離を短縮し、高光結合効率を達成する構成を採用している。但し、図１０に示す導波光検出器は、光導波路層が２層構造、窒化ケイ素膜が１層の導波光検出器を示し、図１１に示す導波光検出器は、光導波路層が２層構造、窒化ケイ素膜が２層構造の導波光検出器を示す。
【００８４】
ここで、光結合のためのテーパ部はエッチング等の技術で作製されることが多いため、本発明の導波光検出器としては、図１０、図１１に示す構成になることが多い。ここで、図１０及び図１１に示す本実施形態４の導波光検出器は、光電変換素子を形成した後の半導体基板５１に窒化ケイ素膜（図１０では５２、図１１では５２ａ、５２ｂ）を形成した後、導波光検出器に必要なテーパ部５６を形成しており、光電変換素子の部分（図中窒化ケイ素膜５２より下の部分）は実施形態１〜実施形態３と同様である。
【００８５】
また、図示例では、いずれも光導波路層５９が、第１誘電体層５４と第２誘電体層５５からなる２層構造のものを示しているが、一層のものでも可能である。なお、図１０及び図１１では電極を省略しているが、電極が形成される領域は上記実施形態の場合と同様である。
【００８６】
まず、図１０に示す導波光検出器の構成を製造プロセスと共に説明する。半導体基板５１の表面に受光部（不純物拡散領域）５７を形成する。次に、その上に窒化ケイ素膜５２を積層することになるが、不純物拡散の際に半導体基板５１上に酸化ケイ素膜ができる。従って、ここでは半導体基板５１上にできた酸化ケイ素膜をまず除去し、その後、改めて窒化ケイ素膜５２を積層している。その後、実施形態１で説明したように、ＩＣと集積化された光電変換素子形成時に生じたＳｉＯ_２段差６１よりも厚く誘電体層５３となる誘電体層を積層（バッファ層として機能させる）し、エッチング、研磨処理を施すことでテーパ部５６を形成する。そして最後に、第１誘電体層５４と第２誘電体層５５からなる２層構造の光導波路層５９をこの順に形成する。これにより、図１０に示す構成の導波光検出器が作製される。なお、ここで説明した製造プロセスは、上記実施形態１〜３の導波光検出器にも同様に適用することができる。
【００８７】
ここで、光導波路層５９から光電変換素子の受光領域（不純物拡散領域）５７への高光結合効率の光結合特性を達成するためには、テーパ終端Ｅ１を通過した後に数１０μｍ程度ある伝搬距離を短くすることが必要である。即ち、伝搬距離を短くすると、テーパ終端Ｅ１を通過した後の光散乱損失を低減でき、高光結合効率が得られるからである。伝搬距離を短くすることができる理由を以下に説明する。
【００８８】
まず、誘電体層５３がテーパ化されても、不純物拡散領域の受光部５７上に誘電体層５３が残った場合には、テーパ終端Ｅ１を通過した後の伝搬光の電磁界分布は、誘電体層がない場合に比べて、半導体基板５１から光導波路層５９側に強度中心がシフトする。このとき、テーパ終端Ｅ１を通過した後の伝搬光６０の吸収効率は、逆にテーパ終端Ｅ１通過後の電磁界分布の強度中心が半導体基板５１に近いほど高くなるので、誘電体層５３の厚さを０にすることで、伝搬光の電磁界分布の強度中心を半導体基板５１側に近付けることで吸収効率が向上する。この結果、テーパ終端Ｅ１通過後の伝搬距離を短縮することできる。
【００８９】
以上の点から、導波光検出器作製の際も、不純物拡散後に生じる酸化ケイ素膜を除去して窒化ケイ素膜を積層する操作が有効になる。
【００９０】
一方、図１１に示す構成の導波光検出器は、半導体基板５１の表面に不純物拡散領域の受光部５７を形成した後、不純物拡散の際に半導体基板５１上にできた酸化ケイ素膜を除去し、その後、窒化ケイ素膜を形成する。光電変換素子とＩＣを集積化する場合には、先に保護層として図１１のように第１窒化ケイ素膜５２ａを形成し、その後、他のプロセスを経て、反射防止膜の機能を持たせるために第２窒化ケイ素膜５２ｂを形成する。
【００９１】
なお、その後の製造プロセスは、図１０の導波光検出器のそれと同じである。図１１の導波光検出器も、光導波路層５９から光電変換素子の受光部（不純物拡散領域）５７への高光結合効率を達成するために、テーパ部５６を持つ誘電体層５３は不純物拡散領域の受光部５７上で膜厚が０になるように形成し、伝搬光のテーパ終端Ｅ１通過後の吸収効率を高めている。
【００９２】
以下、図１１の導波光検出器の具体的な構成例について説明する。一例として、図３の導波光検出器同様に、各層の構成材料として、次のものに選ぶ。即ち、半導体基板５１はＳｉ基板（ｎ_ｓ＝３．６８−ｊ０．１）であり、第１窒化ケイ素膜５２は屈折率ｎ_１＝２．０、厚さｔ_１が９５ｎｍのＳｉＮ膜、第２窒化ケイ素膜５２’は屈折率ｎ_２＝１．８６、厚さｔ_２が４００ｎｍ のＳｉＮ膜である。また、誘電体層５３は、屈折率ｎ_ｂ＝１．４５のＰＳＧ層であり、光導波路層の第１誘電体層５４は屈折率ｎ_ｇ１＝１．５３、厚さｄが５７０ｎｍのコーニング社の商品名＃７０５９ガラスであり、第２誘電体層５５は屈折率ｎ_ｇ２＝１．４３、厚さｓが１４０ｎｍのＳｉＯ_２層である。
【００９３】
更に、テーパ部５６の形状も、テーパ長（Ｌで表記）が７０ｎｍであり、テーパ化する誘電体層５３の最大膜厚ｂ_ｍａｘが３．６ｎｍであり、テーパ部終端Ｅ１からの距離をｌで表すとき、ｌと厚さｂとの関係が表１のようになる形状である。
【００９４】
従って、この具体的構成例で高光結合効率を得るためには、誘電体層５３の膜厚が０になる領域５８が、図４の結果が示すように２０μｍ以上必要である。即ち、図１１（図１０でも同様）において、Ｅ１〜Ｅ２が２０μｍ以上必要である。また、図９（ａ）、（ｂ）のように反射防止膜の機能がほぼ満たされるときは、光結合特性に大きな変化を生じないことから、図１０及び図１１双方の違いである窒化ケイ素膜の層数に関係なく高光結合効率を得るためには、誘電体層５３の膜厚が０になる領域５８は２０μｍ以上必要である。
【００９５】
【発明の効果】
以上の本発明導波光検出器は、光結合可能な受光領域を、光導波路層より屈折率が低い誘電体層を光の伝搬方向に厚みを徐々に薄くすることで形成されたテーパ部とテーパ部通過後の光導波路層の一部で形成する構成をとるので、テーパ部によって、光導波路層から光電変換素子への光結合の際に光結合損失の最大要因となる構造変化を徐々に起こすことができる。このため、光結合効率を向上できる。
【００９６】
加えて、このテーパ部と光の伝搬方向に対してテーパ部の前後に位置する光導波路層を不純物拡散領域の形成時に形成される段差部に囲まれた領域の範囲内に設ける構成をとるので、段差によって生じる大きな光散乱損失を排除することができる。このため、この点においても、光結合効率を向上できる。
【００９７】
また、特に、研磨処理で滑らかな形状のテーパ部を形成する構成とすれば、ＩＣとの集積プロセスで生じた突出部等がある場合であっても、結果的にこの突出部を平坦化してテーパ部が形成されることになるので、光結合損失の最大要因となる構造変化を徐々に起こすことができる。従って、このような構成によっても、導波光検出器の光結合効率を向上できる。
【００９８】
加えて、テーパ部の最大傾斜度を１０°以下に設定する構成によれば、放射損失、即ち、光導波路層の表面凹凸による光散乱を除くテーパ部から空気中への放射による損失を理論的にほぼなくすことができるので、高光結合効率の導波光検出器を実現できる。
【００９９】
また、特に、受光領域において、窒化ケイ素膜、光導波路層、この光導波路層より屈折率の低い誘電体層及び半導体基板で多層膜構造をなし、これが光導波路層の屈折率をｎ_g1、誘電体層の屈折率をｎ_bとした場合に、θ_i＝ｓｉｎ^-1（ｎ_b/ｎ_g1）で定義され、かつ窒化ケイ素膜に対する垂線を基準とした入射角θ_iに対して反射防止膜として機能させる構成とすれば、上記した理由により、高光結合効率の光結合が達成される。
【０１００】
また、特に、窒化ケイ素膜の厚さを４５０ｎｍ〜５５０ｎｍの間に設定する構成とすれば、高光結合効率での光結合という目的に対してより好ましい条件となる導波光検出器を実現できる。
【０１０１】
また、特に、不純物拡散領域の先端位置での誘電体層の厚さが、バッファ層として十分機能する値であり、なおかつ、その不純物拡散領域内にその厚みが０となるテーパ終端を持つ構成とすれば、テーパ終端を通過した後の伝搬距離を短くすることができる。この結果、光散乱損失を低減できるので、高光結合効率が得られる。
【０１０２】
また、特に、誘電体層の厚さが０である領域が２０μｍ以上続く構成とすれば、高光結合効率での光結合を達成できる。
【０１０３】
また、本発明の導波光検出器の製造方法によれば、上記のような効果を奏することができる導波光検出器を容易に作製することができる。
【図面の簡単な説明】
【図１】本発明の実施形態１を示す、導波光検出器の断面図。
【図２】本発明の実施形態２を示す、導波光検出器の断面図。
【図３】本発明の実施形態３を示す、導波光検出器の断面図。
【図４】本発明の実施形態３を示す、導波光検出器の光結合特性を示すグラフ。
【図５】本発明の実施形態３を示す、反射防止膜機能を説明するための一部を拡大して示す導波光検出器の断面図。
【図６】本発明の実施形態３を示す、（ａ）、（ｂ）は導波光検出器のテーパ部を多層膜構造と見なした場合の反射特性を示すグラフ。
【図７】本発明の実施形態３を示す、光導波路層が１層、窒化ケイ素膜が２層構造の導波光検出器の断面図。
【図８】本発明の実施形態３を示す、光導波路層が２層構造、窒化ケイ素膜が１層の導波光検出器の断面図。
【図９】本発明の実施形態３を示す、（ａ）はＴＥモードを例にとって、窒化ケイ素膜の厚さ変化に対する光結合特性の変化を示すグラフ、（ｂ）はＴＭモードを例にとって、窒化ケイ素膜の厚さ変化に対する光結合特性の変化を示すグラフ。
【図１０】本発明の実施形態４を示す、光導波路層が２層構造、窒化ケイ素膜が１層の導波光検出器の断面図。
【図１１】本発明の実施形態４を示す、光導波路層が２層構造、窒化ケイ素膜が２層構造の導波光検出器の断面図。
【図１２】テーパ導波路を用いた導波光検出器の従来例を示す断面図。
【図１３】（ａ）はＩＣと集積化された導波光検出器の一従来例を示す断面図、（ｂ）はＩＣと集積化された導波光検出器の他の従来例を示す断面図。
【図１４】ＩＣと集積化された導波光検出器の場合における段差の発生理由を説明するための製造プロセス図。
【符号の説明】
１、５１ 半導体基板
２、５２ 窒化ケイ素膜
３、５３ 誘電体層（バッファ層）
４、４３、５９ 光導波路層
５、５７ 不純物拡散領域
６、１６、２６、５６ テーパ部
７ 受光領域
８、６１ ＳｉＯ_２段差
９ 受光部
１０、６０ 導波光
１１ 電極
１２ 電極保護膜
２１ 第１窒化ケイ素膜
２２ 第２窒化ケイ素膜
４１、５４ 第１誘電体層
４２、５５ 第２誘電体層
５２ａ 第１窒化ケイ素膜
５２ｂ 第２窒化ケイ素膜
５８ 誘電体層の膜厚が０になる領域[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a guided light detector that guides guided light to a photodetector, and more specifically, to a guided light detector that can efficiently perform optical coupling of guided light from an optical waveguide (optical waveguide layer) to a photoelectric conversion element. The present invention relates to a wave light detector and a method for manufacturing the same.
[0002]
[Prior art]
Recently, tapered waveguides have been frequently used as optical coupling means for guided light propagating from an optical waveguide layer to a photoelectric conversion element. That is, when such a tapered waveguide is used, there is an advantage that the optical coupling efficiency can be increased as compared with the case where optical coupling is performed using a diffraction grating or the like.
[0003]
A conventional example of a guided light (waveguide light) detector using such a tapered waveguide is disclosed in Japanese Patent Application Laid-Open No. 5-291608. FIG. 12 shows this waveguide photodetector. In this waveguide photodetector, a dielectric layer 142 composed of a silicon oxide film layer laminated between an optical waveguide layer (waveguide layer) 141 and a semiconductor substrate 147 has a tapered shape, that is, a light indicated by an arrow in the drawing. The optical waveguide 141 guides the guided light to the photodetector (photoelectric conversion element) by forming the tapered shape such that the thickness gradually decreases in the propagation direction.
[0004]
More specifically, in this waveguide photodetector, the dielectric layer 142 that separates the optical waveguide layer 141 and the semiconductor substrate 147 is formed directly on the diffusion region 143 that is a light receiving unit. In addition, in this waveguide photodetector, when the guided light is detected by the photodetector, as a channel stopper for reducing a leak current generated at the time of reverse bias, the conductivity is opposite to the conductivity type of the outermost surface of the light receiving portion. A mold diffusion region 144 is formed around the diffusion region 143 which is a light receiving portion.
[0005]
Here, the above-described leakage current is caused by the generation of negative fixed charges in the dielectric layer 142 made of a silicon oxide film layer and the optical waveguide layer 141 laminated on the semiconductor substrate 147.
[0006]
In FIG. 12, reference numeral 145 denotes N ⁻ Epitaxial layer, 146 is N ⁺ It is a silicon substrate.
[0007]
[Problems to be solved by the invention]
By the way, recent waveguide photodetectors are required to have high-speed response and high integration because of their applications. To meet such demands, recently, an external control circuit IC has been integrated with the waveguide photodetector. A complicated structure manufactured in the above has been developed.
[0008]
FIG. 13A shows an example of a waveguide photodetector having such a configuration. A metal layer 203 serving as a mask at the time of impurity diffusion and serving as a wiring in an IC (IC part), a silicon nitride film 204 for preventing reflection, a silicon nitride film 205 serving as a wiring insulating film, and A protective layer 206 for the IC and the metal wiring is formed. For this reason, a step of several μm is generated as a whole between the light receiving unit 201 and the IC around it. In FIG. 13A, the structure of the electrode lead portion is omitted.
[0009]
Here, in the above configuration, the protective layer 206 and the metal layer 203 can be omitted, but the silicon nitride film 204 for anti-reflection is indispensable from its function, ₂ The film 202 is indispensable for protecting the PN junction. For this reason, as shown in FIG. 13 (b), even if the simplest structure in which the above-mentioned optional parts are removed is considered, ₂ A step of about 1 μm due to the layer is inevitable. However, in general use, the light for signal detection is incident from free space, so this level difference does not matter at all.
[0010]
Next, the above SiO ₂ The reason why the step due to the layer is generated will be specifically described based on the manufacturing steps shown in FIGS. First, as shown in FIG. 14A, a thermally oxidized SiO 2 ₂ A film 232 is formed. As the formation method, dry oxidation by heating in an oxygen stream and steam oxidation by heating in an oxygen stream containing steam are known. The thermally oxidized SiO thus obtained ₂ The film 232 is patterned using a photoresist or the like, and is etched to form a thermally oxidized SiO 2 layer as shown in FIG. ₂ A mask 232a using the film 232 can be formed. Thereafter, a diffusion process is performed on the silicon substrate 231 from the etched portion to form an impurity diffusion region 231a as shown in FIG. When this high-temperature treatment is performed, a thermal oxide film 233 as shown in FIG. 14C is newly formed.
[0011]
In this waveguide photodetector, a light-receiving portion, that is, a photoelectric conversion element and the like are formed simultaneously with the IC. Therefore, as shown in FIG. ₂ The film 234 is further laminated. After that, etching is performed, and SiO ₂ An opening 235 as shown in FIG. 14E is formed in the film 234 and the thermal oxide film 233.
[0012]
Here, in the above-described first and second etching patterns, a shift of 2 to 3 μm occurs due to mask positioning accuracy, etching accuracy, and the like, and thereby a step portion 236 occurs.
[0013]
Now, in the configuration shown in FIG. 13, light absorption from the optical waveguide layer to the semiconductor substrate other than the optical waveguide layer and the buffer layer, that is, the portion other than the optical coupler, is simply placed on the semiconductor substrate on which the IC and the photoelectric conversion element are integrated. After forming a tapered portion by laminating a silicon oxide film functioning as a dielectric layer having a sufficient thickness to prevent the formation of a tapered portion, an attempt is made to produce a waveguide photodetector as shown in FIGS. 13 (a) and 13 (b). Thus, the optical waveguide layer is directly laminated on the structure having the step. For this reason, the optical waveguide layer has a discontinuous portion due to a step close to the wavelength order. As a result, since the propagating light is almost scattered by the discontinuous portion, it hardly functions as an optical waveguide layer. Therefore, even if the tapered portion is formed, optical coupling from the optical waveguide layer to the photoelectric conversion element hardly occurs.
[0014]
Further, even if it is assumed that the problem of the step is solved, there are new problems described below. That is, when a silicon oxide film serving as a buffer layer and an optical waveguide layer are laminated on the photoelectric conversion element of FIG. 13 to form a tapered waveguide (tapered portion) and a waveguide photodetector is manufactured, the example of FIG. When the thickness of the silicon nitride film 204 is set to the thickness of the antireflection film when light is incident from the air, the optical waveguide layer, the silicon oxide film (buffer layer), the silicon nitride film 204, The propagation length from the optical waveguide layer to the complete photoelectric coupling to the photoelectric conversion element becomes longer due to the effect of multiple reflection at the boundary.
[0015]
Here, an increase in the propagation length until the optical coupling is completed means that the amount of scattered light also increases. For this reason, as a result, the optical loss increases, and the optical coupling efficiency from the optical waveguide layer to the photoelectric conversion element decreases.
[0016]
In the waveguide photodetector having the configuration shown in FIG. 12, the refractive index is n. ₁ , N ₂ , N ₃ (Subscripts 1, 2, and 3 are added in order from the bottom layer of the film) n is the minimum condition under which a multilayer film composed of three layers functions as an antireflection film. ₁ <N ₂ <N ₃ Does not satisfy That is, the refractive index n of the optical waveguide layer 141 _g And the refractive index n of the silicon oxide film (buffer layer) 142 _b And the refractive index n of the semiconductor substrate 147 _s Are n in order to function as an optical waveguide. _b <N _g <N _s Therefore, the multilayer film composed of these three layers does not satisfy the above minimum condition for functioning as an antireflection film. As a result, in the waveguide photodetector having the configuration shown in FIG. 12, it is very difficult to reduce the reflection at the interlayer boundary during the optical coupling to the photoelectric conversion element, so that the optical coupling efficiency is reduced.
[0017]
Further, in the tapered waveguide as shown in FIG. 12, even after passing through the end of the tapered portion, light propagates for several tens of μm, so that the refractive index n of the optical waveguide layer 141 is n. _g Is the refractive index n of the silicon oxide film (buffer layer) 142 _b The higher the reflection at the interlayer boundary, the worse the absorption efficiency of the propagating light, and the longer the propagation distance. When the propagation distance becomes longer, the influence of light loss due to scattering at the surface of the optical waveguide and at each layer boundary of the waveguide photodetector increases, and as a result, the optical coupling efficiency deteriorates.
[0018]
Therefore, for example, even if the silicon nitride film is laminated between the silicon oxide film 142 and the optical waveguide layer 141 in the waveguide photodetector having the configuration shown in FIG. In the state where the silicon oxide film 142 is present, the absorption efficiency after passing through the end of the tapered portion is poor, and there is a problem that the optical coupling efficiency is reduced due to the influence of the scattered light loss.
[0019]
On the other hand, the technology of the waveguide photodetector using the tapered waveguide shown in FIG. 12 is integrated with a photoelectric conversion element that requires higher stability of characteristics than the waveguide photodetector shown in FIG. 12, that is, an IC. In the case where the photoelectric conversion element is used for optical coupling of guided light to a photoelectric conversion element having a high-speed response characteristic, silicon oxide is present as a buffer layer on the diffusion region which is a light receiving portion. The function described above is insufficient for preventing the entry of moisture and the like over the entire semiconductor substrate including the light receiving portion of the waveguide photodetector, and there is also a problem that the element characteristics are degraded.
[0020]
The present invention has been made in view of such a situation, and can solve the above-mentioned problems of the prior art, can improve the optical coupling efficiency, integrate an IC as an external control circuit, and provide a high-speed response and high performance. An object of the present invention is to provide a waveguide photodetector suitable as a waveguide photodetector capable of enjoying integration and a method of manufacturing the same.
[0021]
[Means for Solving the Problems]
The waveguide photodetector of the present invention is on the same semiconductor substrate. , Photoelectric conversion element having impurity diffusion region, and IC for processing output from photoelectric conversion element When Is provided, at least one silicon nitride film is provided above the semiconductor substrate, and further, at least one optical waveguide for light propagation is provided above the silicon nitride film. Wherein the silicon nitride film has a refractive index lower than that of the silicon nitride film. A dielectric layer having a lower refractive index than the lowermost layer of the optical waveguide layer The dielectric layer is formed such that the thickness of the dielectric layer is gradually reduced in the light propagation direction in the optical waveguide layer so that the thickness at the end becomes zero. The optical waveguide layer has a flat region of a certain thickness located on the opposite side to the light propagation direction, the optical waveguide layer has a front portion located on the flat region of the dielectric layer, A tapered portion positioned on the tapered region of the dielectric layer; and a rear portion provided on the silicon nitride film continuously to the tapered portion, wherein the tapered portion and the rear portion are formed in the impurity diffusion region. Is formed in a region surrounded by a step portion formed on the semiconductor substrate at the time of formation of the semiconductor substrate, and a portion of the tapered portion in the optical waveguide layer and a portion continuous to the tapered portion in the impurity diffusion region. A light-receiving region capable of optical coupling is provided for a part of the rear part, The thickness of the nitride silicon film located below the contributing light receiving portion to the photoelectric conversion of the propagation light in the optical waveguide layer is characterized that it is 450nm~550nm in pure object diffusion in the region, Thereby, the above object is achieved.
[0027]
Further, the method for manufacturing a waveguide photodetector of the present invention, Said In a method of manufacturing a waveguide photodetector, a step of forming an impurity diffusion region for forming a photoelectric conversion element on a semiconductor substrate, and the step of forming an impurity diffusion region occurs at the time of impurity diffusion. On the semiconductor substrate Removing the oxide film; forming at least one silicon nitride film on the semiconductor substrate from which the oxide film has been removed; and forming a dielectric material having a lower refractive index on the silicon nitride film than the lowermost layer of the optical waveguide layer. Laminating a body layer, processing the dielectric layer into a tapered shape, and forming at least one optical waveguide layer on the tapered dielectric layer. Therefore, the above object is achieved.
[0028]
Hereinafter, the operation of the present invention will be described.
[0029]
A light-receiving region capable of optical coupling, a tapered portion formed by gradually reducing the thickness of a dielectric layer having a lower refractive index than the optical waveguide layer in the light propagation direction, and a part of the optical waveguide layer after passing through the tapered portion. According to the above-described configuration, the tapered portion can gradually cause a structural change which is the largest factor of optical coupling loss at the time of optical coupling from the optical waveguide layer to the photoelectric conversion element. Can be improved.
[0030]
In addition, when the tapered portion and the optical waveguide layers located before and after the tapered portion with respect to the light propagation direction are provided within a region surrounded by a step formed when the impurity diffusion region is formed, The resulting large light scattering losses can be eliminated. Therefore, also in this respect, the optical coupling efficiency can be improved.
[0031]
Further, in such a waveguide photodetector, when there is a protrusion or the like generated in an integration process with an IC, a structural change which is the largest factor of optical coupling loss becomes very large. According to a configuration in which a tapered portion having a smooth shape is formed by polishing, as a result, the protruding portion is flattened to form a tapered portion, so that the structural change which is the largest factor of optical coupling loss is gradually reduced. Can wake up. Therefore, even with such a configuration, the optical coupling efficiency of the waveguide photodetector can be improved.
[0032]
According to the configuration in which the maximum inclination of the tapered portion is set to 10 ° or less, radiation loss, that is, loss due to radiation from the tapered portion to the air excluding light scattering due to surface irregularities of the optical waveguide layer is theoretically reduced. Since it can be almost eliminated, a waveguide photodetector with high optical coupling efficiency can be realized. The specific reason will be described in an embodiment described later.
[0033]
In the light receiving region, a silicon nitride film, an optical waveguide layer, a dielectric layer having a lower refractive index than the optical waveguide layer, and a semiconductor substrate form a multilayer structure, and the refractive index of the optical waveguide layer is n. _g1 , The refractive index of the dielectric layer is n _b And θ _i = Sin ^-1 (N _b / N _g1 ) And the incident angle θ with respect to the normal to the silicon nitride film _i According to the configuration functioning as an antireflection film, optical coupling with high optical coupling efficiency is achieved. The reason will be described below.
[0034]
Optical coupling mainly occurs near the cutoff (however, the cutoff refers to a phenomenon in which propagating light stops being in a guided mode, that is, radiation to the semiconductor substrate side starts to increase). Here, the effective refractive index N of the propagating light near the cutoff, that is, the phase constant of the propagating light is represented by the wave number (k ₀ = 2π / λ, where λ is the wavelength and λ = 780 nm) is the refractive index n of the dielectric layer. _b It will be a value near. That is, N ≒ n _b Is established.
[0035]
From this, when light travels from the optical waveguide layer toward the impurity diffusion region, that is, the light receiving portion, the traveling angle θ with respect to the normal to the silicon nitride film is referred to. _i Is θ _i = Sin ^-1 (N _b / N _g1 ), A series of multilayer structure from the optical waveguide layer to the dielectric layer, the silicon nitride film and the semiconductor substrate is represented by θ _i = Sin ^-1 (N / n _g1 If an antireflection film is formed around the incident angle that satisfies (1), optical coupling with high optical coupling efficiency can be achieved.
[0036]
Here, the refractive index of the optical waveguide layer is usually around 1.53 due to its material. The refractive index of the dielectric layer having a lower refractive index than that of the optical waveguide layer is usually around 1.45. For this reason, for example, when the thickness of the silicon nitride film is between 450 nm and 550 nm, a series of multilayer film structures from the optical waveguide layer to the dielectric layer, the silicon nitride film, and the semiconductor substrate become θ _i = Sin ^-1 = (N _b / N _g1 It is considered that the film almost functions as an anti-reflection film in the vicinity of an incident angle satisfying the condition (1). Therefore, when the thickness of the silicon nitride film is set to 450 nm to 550 nm, the condition becomes more preferable for the purpose of optical coupling with high optical coupling efficiency.
[0037]
In addition to the above, when the above-described tapered portion is formed in the optical waveguide layer, the light propagates several tens of μm as described above even after passing through the tapered end. The thickness of the dielectric layer at the leading end of the region is a value that sufficiently functions as a buffer layer, and the impurity diffusion region has a tapered end with a thickness of 0 in the impurity diffusion region. The subsequent propagation distance can be shortened. As a result, light scattering loss can be reduced, and high optical coupling efficiency can be obtained. The reason will be described below.
[0038]
First, before the position where the taper starts, the dielectric layer functioning as a buffer layer is tapered, and when the dielectric layer remains on the impurity diffusion region, the electromagnetic field of the propagation light after passing through the tapered end In the distribution, the intensity center shifts from the semiconductor substrate to the optical waveguide layer side as compared with the case where there is no dielectric layer. Here, the absorption efficiency after passing through the tapered end increases as the intensity center of the electromagnetic field distribution after passing through the tapered end approaches the semiconductor substrate. Therefore, when the thickness of the buffer layer is set to 0, the center of intensity of the electromagnetic field distribution of the propagating light can be closer to the semiconductor substrate side, so that the absorption efficiency can be improved. Therefore, as a result, the propagation distance after passing through the tapered end can be reduced.
[0039]
As described above, if the propagation distance after passing through the tapered end is reduced, the light scattering loss can be reduced. Therefore, according to the above configuration having the tapered end having a thickness of 0 on the light receiving portion of the impurity diffusion region, High optical coupling efficiency can be obtained.
[0040]
As described above, when light propagates through the tapered optical waveguide layer, optical coupling is not completely completed even if the thickness of the dielectric layer having a lower refractive index than the optical waveguide layer becomes zero. More specifically, in the region where optical coupling is possible, a region where the thickness of the dielectric layer is 0 continues at least 20 μm at the tip of the tapered waveguide layer at the light receiving portion of the impurity diffusion region, thereby achieving optical coupling. Is completed. Therefore, according to the above configuration in which the region where the thickness of the dielectric layer is 0 continues for 20 μm or more, optical coupling with high optical coupling efficiency can be achieved.
[0041]
The waveguide photodetector having the above-described operation can be easily manufactured by the manufacturing method of the present invention including the above manufacturing steps.
[0042]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.
[0043]
(Embodiment 1)
FIG. 1 shows Embodiment 1 of the waveguide photodetector of the present invention. First, the basic structure of the waveguide photodetector of the first embodiment will be described.
[0044]
A silicon nitride film 2 is laminated on a semiconductor substrate 1 having an impurity diffusion region 5 formed in a surface layer portion, and a dielectric layer 3 serving as a buffer layer is formed thereon. The right side portion of the dielectric layer 3 in the drawing is formed in a tapered shape whose thickness becomes thinner in the light propagation direction. An optical waveguide layer (waveguide layer) 4 is laminated on the dielectric layer 3. Here, the refractive index of the dielectric layer 3 is lower than the refractive index of the optical waveguide layer 4.
[0045]
The polarity of the impurity diffusion region 5 is P ⁺ And P in FIG. ⁺ , N, N ⁺ Indicates the polarity of each part of the semiconductor substrate 1, and P ⁺ Region and N ⁺ A photoelectric conversion element is formed between the region and the region. And P ⁺ A portion contributing to the photoelectric conversion of the propagation light 10 in the impurity diffusion region 5 is defined as a light receiving portion 9 (a portion indicated by oblique lines in the drawing), and a tapered portion 6 is formed in the light receiving portion 9 to function as a guided light detector. Light receiving region 7 capable of optical coupling is formed.
[0046]
Here, P ⁺ When the impurity diffusion region 5 is formed, a SiO 2 ₂ In the first embodiment, the tapered portion 6 and the optical waveguide portions X and Y before and after the tapered portion 6 as shown in FIG. X, Y) ⁺ It is formed inside a step 8 generated by the formation of the impurity diffusion region 5. For this reason, according to the structure of the waveguide photodetector of the first embodiment, the large light scattering loss caused by the step can be reduced, so that the optical coupling efficiency can be improved.
[0047]
Next, the function of each layer will be described with reference to FIG. The optical waveguide layer 4 is a layer for confining light in a thin film, and light mainly propagates through this layer. Further, the electric field or the magnetic field of the propagating light 10 in the optical waveguide layer 4 has a skirt from the optical waveguide layer 4 toward the semiconductor substrate 1, that is, the skirt portion of the electric and magnetic lines of force is applied to the semiconductor substrate 1. If the thickness of the dielectric layer 3 in X is not sufficient, a foot of an electric or magnetic field is applied to the semiconductor substrate 1, and a light absorption phenomenon occurs here. Therefore, in the waveguide photodetector of the first embodiment, the thickness of the dielectric layer 3 is sufficiently ensured to suppress the light absorption phenomenon.
[0048]
Further, when the optical waveguide layer 4 is formed on the light receiving section 9 integrated with an IC (not shown) for processing an electric signal converted by the photoelectric conversion element, unevenness of the dielectric layer 3 is removed. The dielectric layer 3 of FIG. 1 functions as a buffer layer so that the dielectric layer 3 of FIG. 1 is thicker than the protrusion such as a step so as to cover the protrusion such as a step generated due to the integration process with the IC. It is the minimum condition to do. Therefore, in the first embodiment, the dielectric layer 3 having a sufficient thickness is formed.
[0049]
In the drawing, ψ indicates the degree of inclination of the tapered portion 6.
[0050]
(Embodiment 2)
FIG. 2 shows Embodiment 2 of the waveguide photodetector of the present invention. The waveguide light detector of the second embodiment has a configuration in which the light scattering loss is suppressed by forming the tapered portion 16 into a smooth shape.
[0051]
That is, when there is unevenness in the propagation of the propagation light 10, the structural change which is the largest factor of the optical coupling loss becomes large, and the light scattering loss becomes very large before detecting the guided light in the light receiving region 7. Therefore, a configuration is adopted in which the light scattering loss is suppressed by forming the tapered portion 16 by smoothing this portion.
[0052]
More specifically, a polishing process is performed in the process of forming the tapered portion 16, thereby forming a smooth tapered portion 16. As a result, the shape of the tapered portion 16 of the second embodiment is not the linear shape of the first embodiment, but a shape in which the film thickness smoothly decreases as shown in FIG.
[0053]
FIG. 2 also shows the electrode arrangement when configuring the element, and the photoelectric conversion element of the waveguide photodetector is connected to the IC by the electrode wiring 11 and is protected by being covered with a wiring protection film 12 made of polyimide. Is done.
[0054]
For the reasons described above, the waveguide photodetector of the second embodiment has an advantage that the light scattering loss can be further suppressed as compared with the waveguide photodetector of the first embodiment.
[0055]
(Embodiment 3)
3 to 9 show Embodiment 3 of the waveguide photodetector of the present invention. The waveguide photodetector of the third embodiment has a two-layer structure of a silicon nitride film and an optical waveguide layer. The structure will be described below with reference to FIG. However, in FIG. 3, a step portion and a portion on the right side of the optical waveguide Y are omitted.
[0056]
P on the surface of the semiconductor substrate 1 ⁺ After the impurity diffusion region 5 is formed, a silicon nitride film having a two-layer structure composed of the first silicon nitride film 21 and the second silicon nitride film 22 is laminated thereon in this order. Subsequently, the dielectric layer 3 is laminated thereon, and a tapered portion 26 whose thickness becomes thinner in the light propagation direction 30 is formed on the right side in the figure. The left portion of the dielectric layer 3 on the flat surface in the figure is a buffer layer in the optical waveguide X. Subsequently, an optical waveguide layer having a two-layer structure including the first dielectric layer 41 and the second dielectric layer 42 is formed in this order.
[0057]
Here, in Embodiment 3, the two-layer structure of the silicon nitride film is formed by stacking the first silicon nitride film 21 as a protective film during the integration process of the photoelectric conversion element and an IC (not shown). Through the process and then through other manufacturing processes, the second silicon nitride film 22 is laminated under different conditions than when the first silicon nitride film 21 is laminated, in order to achieve high optical coupling efficiency of the waveguide photodetector. It is obtained by doing.
[0058]
Next, the effect of the taper slope (taper slope) of the tapered portion 26 in the above configuration and the function of the silicon nitride films (21, 22) will be described.
[0059]
However, in the third embodiment, the refractive index and the thickness of each layer are described as follows. That is, the refractive index of the semiconductor substrate 1 is n _s , The refractive index of the first silicon nitride film 21 is n ₁ , The refractive index of the second silicon nitride film 22 is n ₂ , The refractive index of the dielectric layer 3 is n _b , The refractive index of the first dielectric layer (waveguide layer) 41 of the optical waveguide layer is set to n. _g1 , The refractive index of the second dielectric layer 42 is n _g2 And
[0060]
Further, the thickness of the first silicon nitride film 21 is set to t. ₁ , The thickness of the second silicon nitride film 22 is t ₂ The thickness of the dielectric layer 3 in the optical waveguide X is b _max The thickness of the first dielectric layer 41 of the optical waveguide layer 43 is d, and the thickness of the second dielectric layer 42 is s.
[0061]
Next, constituent materials of each of the above layers, numerical examples of the refractive index, and the like will be described. The semiconductor substrate 1 is a Si substrate and has a refractive index n _s = 3.68-j0.1. Here, the portion after j indicates an absorption coefficient when light propagates in a medium (in this case, Si), and the notation itself means a notation of a refractive index including an absorption coefficient, a complex refractive index ( (Refractive index represented by a complex number).
[0062]
The first silicon nitride film 21 has a refractive index n ₁ = 2.0, thickness t ₁ Is 95 nm, the second silicon nitride film 22 has a refractive index n ₂ = 1.86, thickness t ₂ Is a 400 nm SiN film. The dielectric layer 3 has a refractive index n _b = 1.45 PSG (Phospho-Silicate Glass) layer. The first dielectric layer 41 of the optical waveguide layer has a refractive index n _g1 = 1.53, Corning Co. # 7059 glass with a thickness d of 570 nm, and the second dielectric layer 42 has a refractive index n _g2 = 1.43, 140 nm thick s SiO ₂ Layer.
[0063]
Here, if PSG is used as a constituent material of the dielectric layer 3, the film stress at the time of stacking the element structure can be reduced. Therefore, there is an advantage that occurrence of cracks and change in characteristics of semiconductor elements on the IC can be prevented. In addition, a BPSG (Boron-doped Phospho-Silicate Glass) film can be used as a constituent material of the dielectric layer 3. Furthermore, NSG (undoped CVD-SiO ₂ ) It is also possible to use a film. In this case, there is an advantage that the effect of absorbing the guided light by the doped material can be prevented in addition to the relaxation of the film stress. In addition, as the first dielectric layer 41, SiON can be used.
[0064]
The shape of the tapered portion 26 of the third embodiment is a smooth shape having no inflection point as shown in Table 1 below, has a taper length (denoted by L) of 70 μm, and has a tapered dielectric layer (buffer). Maximum thickness b of layer 3) _max Is a tapered portion of 3.6 μm. When the distance from the end of the tapered portion 26 (indicated by reference numeral E in FIG. 3 and the thickness of the dielectric layer 3 becomes 0) is represented by l (μm), l and the thickness (μm ) Are as shown in Table 1 below, and the tapered portion 26 has a shape in which each point in Table 1 is smoothly connected.
[0065]
[Table 1]

[0066]
In the taper portion 26 having the shape shown in FIG. 3, the slope is strictly equal to the gradient of the taper portion 6 shown in FIG. 1 (the inclination angle with respect to the boundary between the silicon nitride film 2 and the dielectric layer 3 (buffer layer)). Although φ cannot be defined, the maximum slope is 10 ° or less as shown in Table 1.
[0067]
Next, the optical coupling characteristics of the waveguide photodetector of the third embodiment will be described with reference to FIG. However, the horizontal axis x in FIG. 4 indicates the distance between the tapered end E and the starting point (indicated by the symbol O in FIG. 3) of the impurity diffusion region 5, and the symbol + indicates that the light receiving portion 9 and the tapered portion 26 The case where they overlap is shown.
[0068]
In FIG. 4, attention is first focused on the maximum value of the optical coupling efficiency. Regarding the experimental value of the optical coupling efficiency in FIG. 4, when the distance x from the edge E of the tapered portion 26 exceeds +10 μm, the change in the optical coupling efficiency becomes small and approaches a constant value. When the distance x reaches +20 μm or more, the optical coupling efficiency reaches 90% or more. On the other hand, the theoretical value of the optical coupling efficiency becomes substantially constant when the distance x from the edge E of the tapered portion 26 exceeds 10 μm, and becomes 100% when x reaches +20 μm or more. That is, the change in both the theoretical value and the experimental value means that the optical coupling is almost 100% completed when x reaches +20 μm or more. Although the experimental results show that the optical coupling efficiency is only 95% even when x is +30 μm, the remaining 5% loss is due to the light scattering loss, and the air at the tapered portion 26 which is another factor of the loss. Radiation loss to the side is sufficiently suppressed. That is, the radiation loss to the air side is eliminated by the waveguide photodetector having the structure shown in the specific example of FIG. The effect of reducing the radiation loss to the air side by restricting the taper slope is described in an experiment of investigating the radiation characteristics of a tapered waveguide in a tapered waveguide without a silicon nitride film (1995 Autumn Meeting of the Japan Society of Applied Physics 28a-SQ −29; Sharp Corp., Minami et al.). As is clear from the experimental results in FIG. 4, even in the presence of the silicon nitride film, the tapered shape changes despite the change of the radiation condition due to the influence of the silicon nitride film. When the maximum inclination is 10 ° or less, the effect of reducing radiation to the air side is exhibited, and high optical coupling efficiency can be achieved.
[0069]
However, in an actually manufactured device, light scattering loss often occurs due to the influence of unevenness at each layer boundary. The effect of the loss increases as the distance from the optical waveguide layer 43 to the completion of optical coupling to the light receiving section 9 of the photoelectric conversion element increases. For the above reasons, in order to reduce the light scattering loss, it is necessary to shorten the propagation distance until the coupling is completed.
[0070]
Therefore, in the present invention, in order to shorten the propagation distance when there is a silicon nitride film (21, 22), the transmittance of light from the optical waveguide layer 43 to the light receiving portion 9 of the impurity diffusion region 5 through the silicon nitride film is reduced. Is being improved. Specifically, in the waveguide photodetector of the third embodiment, the thickness of the first silicon nitride film 21 and the second silicon nitride film 22 is set to the thickness t. ₁ = 95 nm, t ₂ = 400 nm, the transmittance of light from the optical waveguide layer 43 to the light receiving portion 9 of the impurity diffusion region 5 through the silicon nitride film is improved.
[0071]
Next, the reason why the transmittance improving effect is obtained will be described with reference to FIGS. Here, FIG. 5 is a diagram when a part of the tapered portion 26 is extracted and is regarded as a multilayer film. The incident angle of the light 10 mainly propagating through the optical waveguide layer 43 in the light receiving region (region capable of optical coupling) (θ in FIG. 5) _i ) Indicates that the effective refractive index N near the cutoff of the propagating light is approximately the refractive index n of the dielectric layer 3. _b And that the refractive index of the first dielectric layer 41 of the optical waveguide layer is n _g1 Therefore, it is expressed by the following equation (1).
[0072]
θ _i = Sin ^-1 (N _b / N _g1 …… (1)
In the above equation (1), n _b = 1.45 and θ _i Is calculated to be 71.4 °. Accordingly, the incident angle θ of the light 10 mainly propagating through the optical waveguide layer 43 in the light receiving region. _i Is expected to be around 70 °.
[0073]
Here, FIGS. 6A and 6B show simulation results by the present inventors and the like. The structure near the cutoff of the propagating light at the tapered portion 26 is changed to a medium through which the incident light enters. It shows the reflectance when the waveguide layer 43 is regarded as a multilayer film having a waveguide layer (commercial name: # 7059 glass layer) # 41. FIG. 6A shows the thickness b of the dielectric layer (PSG layer) 3. _p Is 200 nm, FIG. 6B shows the film thickness b of the dielectric layer (PSG layer) 3 portion. _p Is 100 nm.
[0074]
6 (a) and 6 (b), the smaller the reflectance is, the higher the transmittance is, and in both figures, the incident angle θ _i It can be seen that the reflectivity becomes almost 0 around 70 °. That is, the first silicon nitride film 21 and the second silicon nitride film 22 are formed of the semiconductor substrate (Si substrate) 1, the dielectric layer (PSG layer) 3, and the first dielectric layer of the optical waveguide layer 43 (Corning Corporation). (Commercial name # 7059 glass layer) 41 and incident angle θ _i Shows that an antireflection film is formed around 70 °.
[0075]
This function as an antireflection film leads to an improvement in the transmittance of light from the optical waveguide layer 43 to the light receiving section 9 of the impurity diffusion region 5. In addition to the effect of improving the transmittance in addition to the effect of optimizing the taper inclination, in the waveguide detector according to the third embodiment, the coupling efficiency from the optical waveguide layer 43 to the light receiving section 9 is high, as shown in FIG. Has become.
[0076]
As for the portion of the optical waveguide layer 43, not only the two-layer structure of the first dielectric layer 41 and the second dielectric layer 42, but also the optical waveguide layer 43 may have a multilayer structure of three or more layers. The reason for this is that the lowermost dielectric layer (the first dielectric layer 41 in the example of FIG. 3) of the optical waveguide layer 43 has an incident angle θ with respect to a normal to the silicon nitride films (21, 22). _i Is defined, and can be regarded as an incident side medium.
[0077]
The silicon nitride film has an incident angle θ with respect to a normal to the silicon nitride film. _i It is sufficient that the function as an anti-reflection film in the vicinity of about 70 ° is satisfied, so that two or more layers having different film forming methods such as a first silicon nitride film 21 and a second silicon nitride film 22 may be used, It may be only one layer. Further, as in the configuration of FIG. 3, the first silicon nitride film 21 and the second silicon nitride film 22 may have different refractive indexes.
[0078]
Therefore, the configuration example shown in FIGS. 7 and 8 and the configuration example shown in FIG. 2 can also enjoy high optical coupling efficiency by the method shown in the third embodiment. 7 shows a waveguide photodetector having one optical waveguide layer and two silicon nitride films, and FIG. 8 shows a waveguide photodetector having two optical waveguide layers and one silicon nitride film. Show. Parts corresponding to those in FIG. 3 are denoted by the same reference numerals, and a detailed description thereof will be omitted.
[0079]
Next, based on FIGS. 9A and 9B, changes in optical coupling characteristics with respect to changes in the thickness of the silicon nitride film in the waveguide photodetector having the configuration shown in FIG. 3 will be described. 9A and 9B show the refractive index n as the first silicon nitride film (the lower layer of SiN in the figure). ₁ Is 2.0, and the refractive index n is used as the second silicon nitride film. ₂ Shows the simulation results when 1.86 SiN is used.
[0080]
9A and 9B, the thickness d of the first dielectric layer 41 of the optical waveguide layer 43 is 570 nm, and the PSG layer serving as the buffer layer 3 has a tapered shape as shown in Table 1. At some point, the refractive index n ₁ = SiN film thickness t = 2.0 ₁ , Refractive index n ₂ = 1.86 SiN film thickness t ₂ Are respectively changed, FIG. 9A shows a change in optical coupling characteristics with respect to the TE mode, and FIG. 9B shows a change in optical coupling characteristics with respect to the TM mode.
[0081]
9A and 9B, the optical coupling characteristics are normalized transmission power (the transmission power of the optical waveguide layer 43 before the optical coupling is set to 1, and the optical waveguide layer that decreases by the optical coupling to the photoelectric conversion element). 43 indicating the relative value of the transmission power) with respect to the distance l from the end E of the tapered portion.
[0082]
From FIGS. 6A and 6B, the optical coupling characteristics for both the TE mode and the TM mode are determined by the thickness t of the first silicon nitride film 21 and the second silicon nitride film 22 combined. ₁ + T ₂ However, it can be seen that even if it changes between 450 nm and 550 nm, it does not change much. The dielectric layer 3 has a refractive index n _b Is generally used, and the first dielectric layer 41 has a refractive index n. _g1 It is understood that a waveguide photodetector with high optical coupling efficiency can be realized if the thickness of the silicon nitride film is set in the range of 450 nm to 550 nm, since a material having a ratio of approximately 1.53 is often used.
[0083]
(Embodiment 4)
10 and 11 show Embodiment 4 of the waveguide photodetector of the present invention. In the waveguide photodetector of the fourth embodiment, a region where the thickness of the dielectric layer becomes 0 in the light propagation direction from the taper end on the light receiving portion of the taper portion on the side where light propagates is formed to 20 μm or more. Thus, a configuration is adopted in which the absorption efficiency is improved, and as a result, the propagation distance after passing through the tapered end is shortened to achieve high optical coupling efficiency. However, the waveguide photodetector shown in FIG. 10 shows a waveguide photodetector having a two-layer optical waveguide layer and one silicon nitride film, and the waveguide photodetector shown in FIG. 11 has a two-layer optical waveguide layer. 1 shows a waveguide photodetector having a two-layer silicon nitride film structure.
[0084]
Here, since the tapered portion for optical coupling is often manufactured by a technique such as etching, the waveguide light detector of the present invention often has the configuration shown in FIGS. Here, the waveguide photodetector according to the fourth embodiment shown in FIGS. 10 and 11 has a silicon nitride film (52 in FIG. 10 and 52a and 52b in FIG. 11) formed on the semiconductor substrate 51 after the photoelectric conversion element is formed. After the formation, the tapered portion 56 necessary for the waveguide photodetector is formed, and the portion of the photoelectric conversion element (the portion below the silicon nitride film 52 in the drawing) is the same as in the first to third embodiments.
[0085]
In the illustrated examples, the optical waveguide layer 59 has a two-layer structure including the first dielectric layer 54 and the second dielectric layer 55, but may have a single-layer structure. Although the electrodes are omitted in FIGS. 10 and 11, the regions where the electrodes are formed are the same as in the above embodiment.
[0086]
First, the configuration of the waveguide photodetector shown in FIG. 10 will be described together with the manufacturing process. A light receiving portion (impurity diffusion region) 57 is formed on the surface of the semiconductor substrate 51. Next, a silicon nitride film 52 is laminated thereon, and a silicon oxide film is formed on the semiconductor substrate 51 at the time of impurity diffusion. Therefore, here, the silicon oxide film formed on the semiconductor substrate 51 is first removed, and then, the silicon nitride film 52 is newly laminated. Thereafter, as described in the first embodiment, the SiO generated at the time of forming the photoelectric conversion element integrated with the IC is formed. ₂ A dielectric layer that is to be a dielectric layer 53 thicker than the step 61 is stacked (function as a buffer layer) and subjected to etching and polishing to form a tapered portion 56. Finally, an optical waveguide layer 59 having a two-layer structure including the first dielectric layer 54 and the second dielectric layer 55 is formed in this order. Thus, a waveguide photodetector having the configuration shown in FIG. 10 is manufactured. The manufacturing process described here can be similarly applied to the guided light detectors of the first to third embodiments.
[0087]
Here, in order to achieve optical coupling characteristics with high optical coupling efficiency from the optical waveguide layer 59 to the light receiving region (impurity diffusion region) 57 of the photoelectric conversion element, a propagation distance of about several tens μm after passing through the tapered end E1. It is necessary to shorten it. That is, when the propagation distance is shortened, the light scattering loss after passing through the tapered end E1 can be reduced, and high optical coupling efficiency can be obtained. The reason why the propagation distance can be shortened will be described below.
[0088]
First, even if the dielectric layer 53 is tapered, if the dielectric layer 53 remains on the light receiving portion 57 in the impurity diffusion region, the electromagnetic field distribution of the propagation light after passing through the tapered end E1 becomes The intensity center shifts from the semiconductor substrate 51 to the optical waveguide layer 59 side as compared with the case where there is no body layer. At this time, the absorption efficiency of the propagation light 60 after passing through the tapered end E1 becomes higher as the intensity center of the electromagnetic field distribution after passing through the tapered end E1 is closer to the semiconductor substrate 51. By setting the height to 0, the absorption efficiency is improved by bringing the intensity center of the electromagnetic field distribution of the propagation light closer to the semiconductor substrate 51 side. As a result, the propagation distance after passing through the tapered end E1 can be reduced.
[0089]
From the above points, the operation of removing the silicon oxide film generated after the impurity diffusion and laminating the silicon nitride film becomes effective also in the production of the waveguide photodetector.
[0090]
On the other hand, in the waveguide photodetector having the configuration shown in FIG. 11, after the light receiving portion 57 of the impurity diffusion region is formed on the surface of the semiconductor substrate 51, the silicon oxide film formed on the semiconductor substrate 51 during the impurity diffusion is removed. Then, a silicon nitride film is formed. When the photoelectric conversion element and the IC are integrated, a first silicon nitride film 52a is first formed as a protective layer as shown in FIG. 11 and then subjected to another process to have a function of an antireflection film. Then, a second silicon nitride film 52b is formed.
[0091]
The subsequent manufacturing process is the same as that of the waveguide photodetector in FIG. The waveguide photodetector of FIG. 11 also has a dielectric layer 53 having a tapered portion 56 in order to achieve high optical coupling efficiency from the optical waveguide layer 59 to the light receiving portion (impurity diffusion region) 57 of the photoelectric conversion element. The light-receiving portion 57 is formed so that the film thickness becomes zero, thereby increasing the absorption efficiency of the propagation light after passing through the tapered end E1.
[0092]
Hereinafter, a specific configuration example of the waveguide photodetector in FIG. 11 will be described. As an example, similarly to the waveguide photodetector of FIG. 3, the following materials are selected as constituent materials of each layer. That is, the semiconductor substrate 51 is a Si substrate (n _s = 3.68-j0.1), and the first silicon nitride film 52 has a refractive index n ₁ = 2.0, thickness t ₁ Is a 95 nm SiN film, and the second silicon nitride film 52 ′ has a refractive index n. ₂ = 1.86, thickness t ₂ Is a 400 nm SiN film. The dielectric layer 53 has a refractive index n _b = 1.45, and the first dielectric layer 54 of the optical waveguide layer has a refractive index n _g1 = 1.53, thickness d is 570 nm, made by Corning Corporation under the trade name # 7059 glass, and the second dielectric layer 55 has a refractive index n _g2 = 1.43, 140 nm thick s SiO ₂ Layer.
[0093]
Further, the shape of the tapered portion 56 also has a taper length (denoted by L) of 70 nm, and the maximum thickness b of the dielectric layer 53 to be tapered. _max Is 3.6 nm, and when the distance from the tapered end E1 is represented by l, the relationship between l and the thickness b is as shown in Table 1.
[0094]
Therefore, in order to obtain high optical coupling efficiency in this specific configuration example, the region 58 where the thickness of the dielectric layer 53 is 0 is required to be 20 μm or more as shown in the results of FIG. That is, in FIG. 11 (the same applies to FIG. 10), E1 to E2 need to be 20 μm or more. Further, when the function of the anti-reflection film is almost satisfied as shown in FIGS. 9A and 9B, there is no significant change in the optical coupling characteristics. In order to obtain high optical coupling efficiency irrespective of the number of layers of the film, the region 58 where the thickness of the dielectric layer 53 is 0 needs to be 20 μm or more.
[0095]
【The invention's effect】
The waveguide light detector of the present invention has a tapered portion and a tapered portion formed by gradually reducing the thickness of a light-receiving region capable of optical coupling in a dielectric layer having a lower refractive index than that of the optical waveguide layer in the light propagation direction. Since it is configured to be formed by a part of the optical waveguide layer after passing through the portion, the tapered portion gradually causes a structural change which is the largest factor of optical coupling loss at the time of optical coupling from the optical waveguide layer to the photoelectric conversion element. be able to. Therefore, the optical coupling efficiency can be improved.
[0096]
In addition, the configuration is such that the optical waveguide layers located before and after the tapered portion and the tapered portion with respect to the light propagation direction are provided within a region surrounded by a step formed when the impurity diffusion region is formed. In addition, a large light scattering loss caused by a step can be eliminated. Therefore, also in this respect, the optical coupling efficiency can be improved.
[0097]
Also, In particular, Configuration to form a smooth taper by polishing given that Even if there is a projection or the like generated in the process of integration with the IC, the projection is eventually flattened and a tapered portion is formed. Change can take place gradually. Therefore, even with such a configuration, the optical coupling efficiency of the waveguide photodetector can be improved.
[0098]
In addition, according to the configuration in which the maximum inclination of the tapered portion is set to 10 ° or less, radiation loss, that is, loss due to radiation from the tapered portion to the air excluding light scattering due to surface irregularities of the optical waveguide layer is theoretically reduced. Therefore, a waveguide photodetector with high optical coupling efficiency can be realized.
[0099]
Also, In particular In the light receiving region, a silicon nitride film, an optical waveguide layer, a dielectric layer having a lower refractive index than the optical waveguide layer, and a semiconductor substrate form a multilayer film structure, which reduces the refractive index of the optical waveguide layer to n. _g1 , The refractive index of the dielectric layer is n _b And θ _i = Sin ^-1 (N _b / n _g1 ) And the incident angle θ with respect to the normal to the silicon nitride film _i To function as an anti-reflection film for given that For the reasons described above, optical coupling with high optical coupling efficiency is achieved.
[0100]
Also, In particular, Configuration in which the thickness of a silicon nitride film is set between 450 nm and 550 nm given that Thus, a waveguide photodetector having more preferable conditions for the purpose of optical coupling with high optical coupling efficiency can be realized.
[0101]
Also, In particular A structure in which the thickness of the dielectric layer at the tip end of the impurity diffusion region is a value that sufficiently functions as a buffer layer, and has a tapered end in the impurity diffusion region where the thickness becomes zero. given that , The propagation distance after passing through the tapered end can be shortened. As a result, light scattering loss can be reduced, and high optical coupling efficiency can be obtained.
[0102]
Also, In particular Where the region where the thickness of the dielectric layer is 0 continues for 20 μm or more given that Optical coupling with high optical coupling efficiency can be achieved.
[0103]
Also, Of the present invention According to the method for manufacturing a waveguide photodetector, a waveguide photodetector having the above-described effects can be easily manufactured.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a waveguide photodetector, showing Embodiment 1 of the present invention.
FIG. 2 is a cross-sectional view of a waveguide photodetector, showing a second embodiment of the present invention.
FIG. 3 is a sectional view of a waveguide photodetector, showing a third embodiment of the present invention.
FIG. 4 is a graph showing optical coupling characteristics of a waveguide photodetector, showing a third embodiment of the present invention.
FIG. 5 is a cross-sectional view of a waveguide photodetector, showing a third embodiment of the present invention, in which a part for explaining an anti-reflection film function is enlarged and shown.
6 (a) and 6 (b) are graphs showing reflection characteristics when a tapered portion of a waveguide photodetector is regarded as a multilayer film structure, showing a third embodiment of the present invention.
FIG. 7 is a cross-sectional view illustrating a waveguide photodetector according to a third embodiment of the present invention, which has a single optical waveguide layer and a two-layer silicon nitride film.
FIG. 8 is a cross-sectional view illustrating a waveguide photodetector according to a third embodiment of the present invention, in which the optical waveguide layer has a two-layer structure and the silicon nitride film has one layer.
9A and 9B show a third embodiment of the present invention, in which FIG. 9A is a graph showing a change in optical coupling characteristics with respect to a change in the thickness of a silicon nitride film, taking a TE mode as an example, and FIG. 4 is a graph showing changes in optical coupling characteristics with respect to changes in the thickness of a silicon nitride film.
FIG. 10 is a cross-sectional view showing a waveguide photodetector according to a fourth embodiment of the present invention, in which the optical waveguide layer has a two-layer structure and the silicon nitride film has one layer.
FIG. 11 is a cross-sectional view showing a waveguide photodetector according to a fourth embodiment of the present invention, in which the optical waveguide layer has a two-layer structure and the silicon nitride film has a two-layer structure.
FIG. 12 is a cross-sectional view showing a conventional example of a waveguide photodetector using a tapered waveguide.
13A is a cross-sectional view showing a conventional example of a waveguide photodetector integrated with an IC, and FIG. 13B is a cross-sectional view showing another conventional example of a waveguide photodetector integrated with an IC. .
FIG. 14 is a manufacturing process diagram for explaining the reason for the occurrence of a step in the case of a waveguide photodetector integrated with an IC.
[Explanation of symbols]
1,51 semiconductor substrate
2,52 silicon nitride film
3,53 dielectric layer (buffer layer)
4, 43, 59 Optical waveguide layer
5, 57 impurity diffusion region
6, 16, 26, 56 Tapered part
7 Light receiving area
8,61 SiO ₂ Step
9 Receiver
10, 60 guided light
11 electrodes
12 Electrode protection film
21 First silicon nitride film
22 Second silicon nitride film
41, 54 First dielectric layer
42, 55 Second dielectric layer
52a first silicon nitride film
52b Second silicon nitride film
58 Region where the thickness of the dielectric layer becomes 0

Claims (2)

On the same semiconductor substrate, a photoelectric conversion element having an impurity diffusion regions, photoelectric conversion is provided an IC for processing the output from the device, at least one layer of silicon nitride film above the semiconductor substrate is provided, further A waveguide light detector provided with an optical waveguide for light propagation comprising at least one layer above the silicon nitride film ,
On the silicon nitride film, a dielectric layer having a lower refractive index than the lowermost layer of the optical waveguide layer having a lower refractive index than the silicon nitride film is provided,
The dielectric layer has a tapered region in which the thickness is gradually reduced in the light propagation direction of the optical waveguide layer so that the thickness at the end becomes zero, and a light transmission of the optical waveguide layer is more than the tapered region. Having a flat region of a certain thickness located on the opposite side to the propagation direction,
The optical waveguide layer has a front portion located on a flat region of the dielectric layer, a tapered portion located on a tapered region of the dielectric layer, and a portion on the silicon nitride film that is continuous with the tapered portion. A rear portion provided, wherein the tapered portion and the rear portion are provided in a region surrounded by a step portion formed on the semiconductor substrate when the impurity diffusion region is formed,
In the impurity diffusion region, a light receiving region that can be optically coupled to a part of the tapered part and a part of the rear part continuous with the tapered part in the optical waveguide layer is provided,
A waveguide photodetector, wherein a thickness of the silicon nitride film located below a light receiving portion contributing to photoelectric conversion of light propagating in an optical waveguide layer in the impurity diffusion region is 450 nm to 550 nm. . The method for manufacturing a waveguide photodetector according to claim 1,
Forming an impurity diffusion region for forming a photoelectric conversion element on a semiconductor substrate,
Removing an oxide film on the semiconductor substrate generated at the time of impurity diffusion;
Forming at least one silicon nitride film on the semiconductor substrate from which the oxide film has been removed;
Laminating a dielectric layer having a lower refractive index than the lowermost layer of the optical waveguide layer on the silicon nitride film;
Processing the dielectric layer into a tapered shape;
Forming at least one optical waveguide layer on the tapered dielectric layer.

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