JP2004356535A - Fingerprint sensor - Google Patents

Abstract

An object of the present invention is to stably provide a fingerprint sensor that is low in cost, resistant to static electricity and the like, and excellent in resolution and contrast.
A fingerprint sensor for optically detecting a fingerprint includes an optical sensor (11) including a plurality of photoelectric conversion elements, and a member disposed above the optical sensor (11) and making contact with a finger. A silicon substrate 13 having a refractive index higher than the above and having conductivity is used, and the silicon substrate 13 is bonded to the optical sensor 11 via an adhesive 12 which is an insulator. The silicon substrate 13 is grounded. The member is a semiconductor substrate. A thin film for preventing contamination is formed on the surface of the member.
[Selection diagram] Fig. 1

Description

で表すことができる。この値は表１に示したように、０．７９５３３３３２５である。
【００１３】
【表１】

【００１４】
また上記２）の部材中での透過率は下記の式
Ｔｈ２＝ｅｘｐ（−αｔ） ・・・（４）
で表すことができる。単結晶シリコンのλ＝８５０ｎｍでのαの値は、０．６ｄＢ／μｍ程度である。従ってｔ＝１００μｍの板厚では、Ｔｈ２の値は−６０ｄＢ、０．００１になる。
以上から、光学センサに入射する山部の光量Ｐｈは、
Ｐｈ＝∫（Ｐｉ×Ｔｈ１×Ｔｈ２）ｄｌ ・・・（５）
となる。この値は、２×Ｐｉ×ｒ×０．７９５３３３３２５×０．００１であるから、表１に示したように、
２×Ｐｉ×ｒ×０．０００７９５３３となる。
一方、谷部での入射光量Ｐｖｉは、
Ｐｖｉ＝∫Ｐｉｄｌ ・・・（６）
ｄｌ：積分長さ
であり、積分範囲は半円柱の表面６３に沿った角度πの円弧である。
【００１５】
また谷部で問題とするファクタは、
３）指−空気界面（内部６５とエアギャップ６４との界面）での界面反射
４）空気−部材界面（エアギャップ６４と部材６２との界面）での界面反射
５）部材６２中での減衰
である。
上記３）の指−空気界面では、入射光は界面に対して垂直に入射する。従って透過率Ｔｖ１は、
Ｔｖ１＝（２／（１＋１／１・３））^２×（１／１．３） ・・・（７）
と表される。この値は、表１に示したように０．９８２９８６７６７である。
また上記４）の空気−部材界面の透過率Ｔｖ２は入射角θ１と屈折角θ２に依存し、下記のように書ける。

ここでｔｓはｓ波の、ｔｐはｐ波のそれである。
ここでは入射光は、ｓ波とｐ波が半々に含まれると仮定している。
ｔｓ、ｔｐはそれぞれ

と表される。
【００１６】
表１中ではＴｖ２（θ１，θ２）の値は、θ１の値のπ／１００毎に０からπ／２まで計算している。
上記５）の部材６２中の減衰は、透過率Ｔｖ３
Ｔｖ３＝ｅｘｐ（−αｔ（θ２）） ・・・（１１）
で表される。ここでｔ（θ２）は屈折のために延長された光路長であり、
ｔ（θ２）＝ｔ／ｃｏｓθ２ ・・・（１２）
である。
また（１１）式と（１２）式を用いて適当に変形すると、以下のように簡単化される。
Ｔｖ３＝（０．００１）^{（１／ｃｏｓ}θ^２） ・・・（１３）
谷部の全体の透過率Ｔｖを
Ｔｖ＝Ｔｖ１×Ｔｖ２×Ｔｖ３ ・・・（１４）
で定義すると、前記光学センサに入射する谷部の光量Ｐｖは、

となる。Ｔｖ１からＴｖの値を、表１に示す。
【００１７】
本散乱モデルの傾向を図に示すと、図８から図１０のようになる。
まず入射角度θ１に対する透過率Ｔｖ（図８）であるが、ＴｖはＴｈに比べると小さく、θ１＝０の垂直入射の時が最大でＴｈの値の８６％程度である。以下順次低減してθ１＝π／２の時に０となる。
これは谷部に入射する入射光では、部材に垂直に入射する光の寄与が大きいということである。但しその低減の仕方は緩やかであるため、かなりの角度の光まで谷部の画像形成に寄与している。
【００１８】
次に、入射光が光学センサに入射する位置（受光位置）ｘ（図９）であるが、入射光は垂直入射の場合には真下（ｘ＝０μｍ）に入射し、以下、順次谷部の中心ｘ＝０から３０μｍの間に入射している。これは指紋のピッチ（２ｒ×２、４００μｍ程度）に比べるとかなり小さな値である。
この原因は、部材の材質に高屈折率物質である単結晶シリコンを用いたことと比較的薄い板厚（ｔ＝１００μｍ）を採用したことに有る。
いま仮に指紋のピッチを山部、谷部２００μｍ（ｒ＝１００μｍ）とすると、本散乱モデルでは３０−７０μｍの間には光が入射しないことになる。（図１０）それどころか谷部の中央には多くの光量が集中して受光されることになる。これは前記山部と山部の丁度中間に、偽山部、偽隆線を発生させてしまう。
【００１９】
以上の結果を元に、前記散乱モデルのコントラストとＭＴＦを議論する。
まず山部と谷部のコントラストを比較する。
山部に入射する光量は前述のようにＰｈであるから、２×Ｐｉ×ｒ×０．０００７９５３３である。
ここでＰｉ＝１Ｗ／μｍ、ｒ＝１００μｍとすると、Ｐｈ＝０．１５９０６６Ｗである。
それに対してＰｖは、（１５）式で表される。
今ここで、前記積分値を以下の近似式で代用すると、

この値は表１に示したように、Ｐｖ＝０．１７５１３５１６５Ｗである。
ここでコントラストを山部全体が示す光量／谷部全体が示す光量と定義すると、
コントラスト＝Ｐｈ／Ｐｖ ・・・（１７）
となる。
【００２０】
この量は本モデルでは１以下の、０．９０８である。即ち本散乱モデルでは、通常の実験結果が示すＰｖ＜Ｐｈという関係ではなく、Ｐｖ＞Ｐｈという実験事実とは反する反対の結論が導かれる。
勿論これはモデルの精度であるが、「薄板型指紋センサは、ＦＯＰを使用したそれと比べるとコントラストが悪い」という事実を証明するそれでもある。現に我々は類似の系の試作評価結果で、コントラスト１．３という値を得ている。
次にＭＴＦであるが、本散乱モデルでは谷の中心によって生じる指紋ピッチと同じピッチのλ＝４００μｍ（谷部中心間距離）の偽信号と、谷部の境界に存在する２つの暗部（図１０）に起因するλ＝１３０μｍ（同一の谷の暗部間距離）とλ＝２７０μｍ（隣り合う谷の暗部間距離）との偽信号を発生させる。
前者は指紋ピッチと位相の異なる偽信号であり、また後者はより高周波の偽信号であり、前記指紋画像の画質を悪化させる。即ち、本散乱光を用いた薄板型指紋センサのＭＴＦ特性は不満足である。
【００２１】
次に、図７は指紋／指構造に垂直光が入射した時のモデルである。指紋構造や部材の構造は、図６に示した散乱光モデルと同一である。
図７において、７３は指紋の谷部を表す半径ｒの半円柱の表面である。その上方は指の内部７５である。指の内部７５の屈折率には、水と大略同じで有るとして値１．３を用いている。その下方は谷部のエアギャップ７４であり、屈折率１．０の空気が満たされた空洞である。指紋の山部は部材７２と接している。
７２は厚さｔを有する部材であり、その屈折率は単結晶シリコンと同じ３．４４８である。７１は絶縁物、７８は指の上方の空間（空気）を示す。θ１は部材７２の法線方向と光入射部での表面７３の法線方向とのなす角度、θ２は指の内部７５からエアギャップ７４への光の入射角（光入射部での表面７３の法線方向に対する入射光７６の角度）、θ３はエアギャップ７４での屈折角（表面７３の法線方向に対する屈折光の角度）、θ４はエアギャップ７４から部材７２への光の入射角（部材７２の法線方向に対する入射光の角度）、θ５は部材７２での屈折角（部材７２の法線方向に対する屈折光７７の角度）である。ｔは部材７２の厚さ、ｘは半径ｒの半円柱の円弧の中心と部材７２の光の出射位置との距離、ｙは半径ｒの半円柱の円弧の中心と表面７３への光の入射位置との距離である。
本モデルでは指の内部７５から円柱の表面６３に、部材７２の法線方向と平行に、均一な照明光が入射するとしている。
【００２２】
まず山部に入射する入射光であるが、これはすでに図６を用いて説明した散乱光モデルと同一である。
次に谷部であるが、考慮するファクタは、
３′）指−空気界面（内部７５とエアギャップ７４との界面）での反射
４′）空気−部材界面（エアギャップ７４と部材７２との界面）での反射
５′）部材７２内部での減衰
である。
上記３′）の指−空気界面では、透過率Ｔｖ１は、

上記４′）の空気−部材界面では、透過率Ｔｖ２は、

上記５′）の部材６２中の減衰では、透過率Ｔｖ３は、
Ｔｖ３＝（０．００１）^{（１／ｃｏｓ}θ^５） ・・・（２０）
で表される。
これらの計算結果を表２に示す。
【００２３】
【表２】

【００２４】
また図１１から図１３に、これらの結果をグラフ化して示す。
まず入射位置ｙ−透過率Ｔのグラフ（図１１）であるが、Ｔｖ（ｙ＝０μｍ）の値は、散乱光モデルと同一の値である。（Ｔｈの値の８６％程度）またＴｖはｙ＝０μｍからほぼ一定の値を保ち、８０μｍ近傍でストンと落ちている。これ以上では入射光は指−空気界面で全反射を行うため、光学センサの受光面には到達しない。
【００２５】
図１２は入射位置ｙと受光位置ｘとの対応を表している。これから両者にはほぼ線形の対応が有るものの、谷部の境界付近では受光光は谷部を越えて山部の受光部で検出されることが分かる。その最大の大きさは４０μｍ程度（オフセット、ｘ−ｙは６０μｍ程度）である。
【００２６】
また図１３は受光位置ｘに対応する透過率Ｔであるが、全体になだらか、一定であり、散乱光モデルのように急激なＴの変化は観察されていない。
図の０から１００μｍは谷部を表すデータ（測定量）であり、１００μｍを越える部分は逆の山部を表すデータである。従って谷部のカーブの１００μｍから１４０μｍの透過率Ｔｖは、Ｔｈに寄与する部分である。前記０．０００６５から０．０００４に相当する光量は、前記Ｔｈの０から４０μｍの光量を持ち上げる働きを示し、両者のエッジを強調する働きを示す。
【００２７】
以上の結果から、同様にコントラストとＭＴＦを議論する。
まずコントラスト、Ｐｈ／Ｐｖであるが、これは表２から０．１５９０６６／０．１０１５５０２７１＝１．５７という値が得られている。
この値は前記散乱光モデルよりも高コントラストであり、垂直入射の照明光を採用するとコントラストが向上することを示している。また現実に我々の試作評価結果においても、「直進性の高い遠距離に有る照明光源を用いた方がコントラストが向上する」という実験結果が得られている。
【００２８】
またＭＴＦであるが、図１３に示したグラフが物語るように、散乱光モデルで見られたような不連続な画像特性（図９）は見られない。また前述の１００から１４０μｍにかけてのＴｖの漏れこみも前記ＭＴＦを悪化させるが、幸いなことにピッチがλ＝２４０μｍ（同一谷部の両側の距離）とλ＝１６０μｍ（隣接する谷部間の距離）と指紋ピッチに近いことと、エッジ強調の働きを有していることである。従ってこれによるＭＴＦの悪化は致命的ではない。
【００２９】
以上説明したように、コントラスト、ＭＴＦ向上のためには、照明に用いる光は散乱光よりも垂直入射光であることが望ましい。
また指紋の検出に用いる光は、指に入射する外光であることが望ましい。これにより、指を例えばＬＥＤ等で側面から照明した例よりもより垂直に入射する照明光を期待することができる。
【００３０】
また部材は、半導体基板であることが望ましい。半導体基板は本質的に誘電体基板であり、高い屈折率を期待することができる。また高純度であり、化学的な安定性や機械的な強度も期待することができる。半導体基板は、特にシリコン基板であることが望ましい。シリコン基板の屈折率は前述のように３．４４８とかなり高く、理想的である。
【００３１】
また部材の表面には、汚れ防止用の薄膜が形成されていることが望ましい。これにより部材表面に残留する残留指紋の影響を最小にすることができる。汚れ防止用の薄膜は、シリコン系の化合物やセラミック等を用いることができる。これにより充分な硬度と安定性を得ることができる。また半導体プロセス技術の援用も容易である。
【００３２】
また光学センサは、外部電界、静電気等によって部材に誘起される電荷から前記センサを保護するために、その内部にシールドプレートが設けられていることが望ましい。これにより前記電荷等の影響をより小さくすることができる。
また部材に導電性を持たせるには、部材に低抵抗基板を用いることが挙げられる。
【００３３】
また低抵抗基板の抵抗率は、０．１Ω−ｃｍ以下であることが望ましく、これにより前述の通り低抵抗な電気的導通が確保される。
また部材の電気抵抗を低減する工夫には、部材の表面が光学的に透明でない金属薄膜であることが挙げられる。金属薄膜の厚さは、指紋を検出する光の波長のλ／１０以下であることが望ましい。これにより金属薄膜を前記検出する光が部分的に透過可能となり、指紋／光学センサが実現される。
また部材の電気抵抗を低減する工夫には、部材の表面に高濃度不純物拡散層を設けることが挙げられる。これにより部材の電気抵抗を低減することができる。
また部材の電気抵抗を低減する工夫には、部材の表面にパターニングされた金属薄膜を形成することがあげられ、その上方には汚れ防止用の薄膜が形成されていることが望ましい。これにより、前記電気抵抗低減と指の油脂による汚れ、残留指紋を防止することができる。
【００３４】
また部材は、半導体プロセス技術を利用して形成されていることが望ましい。これにより前記薄膜の蒸着やパターニングを安定して安価に製造可能である。
また絶縁物は、半導体プロセスで形成されていないことが望ましい。塗布法、スピンオン法、印刷等により比較的厚膜な絶縁物を形成可能である。
絶縁物は、単位面積当りの抵抗値が１０^１２Ω以上であることが望ましい。これにより前記光学センサ動作にとって有害である前記絶縁物を貫通するリーク電流を、ｐＡ台に押さえ込むことができる。
【００３５】
【実施例】
以下、本発明の実施例について図面を用いて説明する。
［第一実施例］
図１に本発明の第一実施例である、指紋検出センサの概略断面図を示す。図１において、１１は大きさ２ｃｍ×１ｃｍの単結晶シリコン基板上に形成された光学センサである。光学センサは２次元アレー状に配列された大きさ３０μｍ角の単位画素を、６００×３００＝１８００００個有している。
１２は抵抗率１０^１６Ω−ｃｍ、厚さ１０μｍの半導体グレードの接着剤であり、接着剤１２により、基板１１に、ｐ型、抵抗率０．０１Ω−ｃｍ、厚さ１００μｍの単結晶シリコンからなる薄板１３を貼り合わせている。
貼り合わされた薄板１３は、指紋センサの末端でアース電位に接地されている。また薄板１３の表面には、指紋等による汚れを防止するための厚さ２０００Åのシリコン窒化膜１４が形成されている。１５は指紋を検出するための指であり、シリコン窒化膜１４上に載せられる。
【００３６】
本指紋センサの分光感度を図１４に示す。図から分かるように、本センサでは波長１０３０ｎｍの赤外光に感度のピークを有し、波長８００ｎｍから１２００ｎｍの赤外光に感度を有している。このグラフの左側の感度低下は薄板１３を赤外光が透過しないために生じ、右側のそれは光学センサ１１の光電変換特性が低下するために生じている。
【００３７】
本実施例の指紋検出のための照明光１６には、例えば大略垂直光からなる外光を使用する。外光には主に太陽からの屋外光と、蛍光灯等の照明からの室内光等がある。太陽光のスペクトルはブロードであり連続であるが、図１４に示したように本実施例の指紋検出センサにおいてはバンドパスフィルタのような狭い分光感度しか有していないため、実質は波長８００ｎｍから１２００ｎｍの光を照明したことに等しい。また後者の蛍光灯の光質は、主に可視光領域に存在する離散スペクトルである。
【００３８】
本実施例の指紋センサの、室内光での指紋撮像例を図１５に示す。
図１５から分かるように画像は指紋の山谷だけでなく汗腺の穴までをも撮像出来ており、空間解像度的は充分である。また山部（明部）と谷部（暗部）の明るさの比は１．５程度であり、これも前述したモデルで説明可能なコントラスト値となっている。
【００３９】
本発明に用いる薄板の材質はｐ型単結晶シリコン基板以外に、ｎ型、ｉ型、その他の高誘電性物質のいずれかで良い。特に半導体基板は本質的に高誘電率であるので好適である。Ｇｅやダイヤモンド等は人体に悪影響を与えず好適に用いることができる。
【００４０】
また汚れ防止用の薄膜は、前記シリコン窒化膜以外にシリコン酸化膜等のシリコン系のガラス、セラミック等が適当である。
これらは半導体基板上に公知の半導体プロセス技術を援用することにより、簡単安価に製造可能である。
また指紋の照明に用いる光は、室内光のように大略垂直光であれば良く、例えば専用の赤外光源を用意しても良い。その場合には、赤外光を発する赤外ＬＥＤを筒状の反射容器の奥底に埋め込んで使用するのが、指紋のＭＴＦとコントラストを上げるうえで望ましい。
【００４１】
［第二実施例］
図２に本発明の第二実施例である、指紋検出センサの概略断面図を示す。
２１は光学センサ、２２は接着剤、２３は抵抗率１００Ω−ｃｍの単結晶シリコン基板、２４は厚さ５０Åの金の薄膜である。光学センサ２１、接着剤２２は第一実施例と同様のものを用いることができる。
薄膜２４の膜厚は指紋を検出するための照明光の波長１０３０Åの１／２０以下であり、充分に前記照明光を透過する。また薄膜２４のシート抵抗は５Ω程度であるので、指紋センサは充分に低い値で接地可能である。
本実施例によれば、薄板２４の抵抗率が多少高めであっても、指紋センサに用いることができる。本実施例に用いる薄膜の材質は、低抵抗でありまた化学的にも安定な銀、銅等の貴金属が好適である。またチタンやタングステン等の高融点金属もその候補である。
【００４２】
［第三実施例］
図３に本発明の第三実施例である、指紋検出センサの概略断面図を示す。
３１は光学センサ、３２は接着剤、３３は単結晶シリコン基板であり、その表面近傍には公知の半導体プロセス技術によって深さ１μｍのＡｌからなるｐ型不純物拡散層３６が形成されている。
また拡散層３６は、センサの末端で電気的に接地されている。拡散層３６の上方には、同様に半導体プロセス技術によって厚さ１μｍのシリコン酸化膜３４が形成されている。酸化膜３４は同様に、指による前記センサの汚れを防止する。
本実施例によれば、電気的に低抵抗な不純物拡散層３６によって薄板３３の導電性を向上可能である。
拡散層を形成する不純物は、ドーパントである３価（Ｂ，Ａｌ，Ｇａ）や５価（Ｐ，Ａｓ）の原子、あるいは導電性の金属原子等でよい。
【００４３】
［第四実施例］
図４に本発明の第四実施例である、指紋検出センサの概略断面図を示す。
４１は光学センサが形成された半導体基板であり、４７はそのセンサを形成する大きさ５０μｍ角の単位画素である。４２は基板４１と高屈折率を有する厚さ１５０μｍの薄板４３とを接着するための接着剤である。４３は屈折率４．０９２の、半導体ゲルマニウム基板である。
４６は薄板４３上に公知の半導体プロセス技術によって積層、パターニングされた、厚さ１０００Åのアルミ膜である。またアルミ膜４６のパターンは、開口大きさ４０μｍ角、開口ピッチ５０μｍである。
アルミ膜４６のパターンと光学センサの画素４７との位置は対応するように、接着は行われる。その結果、アルミ膜４６の開口から入射する指紋画像光は、画素４７へと無駄なく導かれるようになっている。
４４は前記アルミ膜４６上に形成された、汚れ防止のためとアルミ膜パターンの指の摩擦による磨耗を防ぐための、シリコン窒化膜からなる厚さ２０００Åのオーバーコートである。
【００４４】
本実施例によれば薄板４３の抵抗を、ＩＴＯ等の透明金属を用いずに下げることが可能である。
パターニングされた薄膜に要求される品質は、例えば良好な密度と接着性である。
薄膜化することによって密度が低下するような疎な膜は、指の摩擦に耐えることが出来ない。また下地やオーバーコートとの接着性が悪い場合には、パターンは容易に剥離変形を生じ、指紋画像の品質の低下を生じさせてしまう。
このような特性を満足する膜としては、アルミ以外にタングステンやモリブデン等の高融点金属が挙げられる。
【００４５】
また本実施例においては、他の実施例とは異なり、全面に低抵抗な導体層が設けられていない。その為静電気には問題が無くとも、入射する電界、電磁波に対しては万全ではない。
従って前述したパターニングされた導体以外に、光学センサ内部に電気的なシールド、シールドプレートを設けることも有効である。
シールドプレートは、前記指紋画像光が入射する画素部だけでなく、その周辺の電気回路部（周辺回路部）にも設けることが望ましい。
【００４６】
【発明の効果】
以上説明したように、本発明によれば、低コストで、静電気等に強い、解像度やコントラストに優れた指紋センサを安定して提供することができる。
【図面の簡単な説明】
【図１】本発明の第一実施例である、指紋検出センサの概略断面図である。
【図２】本発明の第二実施例である、指紋検出センサの概略断面図である。
【図３】本発明の第三実施例である、指紋検出センサの概略断面図である。
【図４】本発明の第四実施例である、指紋検出センサの概略断面図である。
【図５】本発明の指紋センサの部材と絶縁物の部分を模式化した図である。
【図６】指紋構造に散乱光が入射した時のモデルである。
【図７】指紋／指構造に垂直光が入射した時のモデルである。
【図８】散乱モデルの入射角−透過率特性を示す図である。
【図９】散乱モデルの谷部入射光の入射角と光学センサに入射する位置との関係を示す図である。
【図１０】散乱モデルの受光位置と透過率との関係を示す図である。
【図１１】垂直入射モデルの入射位置と透過率との関係を示す図である。
【図１２】入射位置ｙと受光位置ｘとの対応を表す図である。
【図１３】垂直入射モデルの受光位置と透過率との関係を示す図である。
【図１４】指紋センサの分光感度を示す図である。
【図１５】本実施例の指紋センサの、室内光での指紋撮像例を示す図である。
【符号の説明】
１１、２１、３１、４１ 基板
１２、２２、３２、４２ 接着剤
１３、２３、３３、４３ 薄板
１４、２４、３４、４４ 薄膜
１５、２５、３５、４５ 指
１６ 照明光
３６ 拡散層
４６ パターン
５１ 薄板
５２ 抵抗
５３ 抵抗
５４ 指側端子
５５ 指
５６ 光学センサ側端子
５７ 接地
６１、７１ 光学センサ
６２、７２ 薄板
６３、７３ 界面
６４、７４ 空洞
６５、７５ 指の内部
６６、７６ 入射光
６７、７７ 屈折光[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fingerprint sensor for detecting a fingerprint, and more particularly to an optical fingerprint sensor.
[0002]
[Prior art]
Conventional fingerprint sensors include an optical sensor that detects a peak and a valley of a fingerprint in an image using a prism or the like, a capacitance sensor that detects a capacitance difference caused by a gap difference between the fingerprint and a valley, and a potential of a finger. There are an electric field intensity sensor for detecting the intensity of the fixed electric field, a temperature sensor for detecting an increase in temperature caused by contact with a finger, and the like.
Among them, the optical sensor has many variations due to its simple principle. For example, a fingerprint sensor using an FOP (fiber optical plate) instead of a prism is disclosed in Patent Document 1 and the like. In addition, as an example in which light is incident from the back side of the silicon substrate on which the optical sensor is formed, there is, for example, a back-illuminated CCD for detecting infrared rays, and these have already been commercialized. Further, an example in which another optical member or thin plate is used instead of the FOP is disclosed in, for example, Patent Document 2.
In a fingerprint sensor, since a semiconductor integrated circuit forming the sensor is arranged close to a finger, the fingerprint sensor is generally very weak to static electricity. Since a large amount of static electricity accumulates in a finger or a human body that makes direct contact, a device is usually designed so that an ESD (electrostatic discharge) does not directly occur from the finger to the sensor.
[0003]
For example, in a capacitance sensor or an electric field strength sensor, a metal electrode for taking a potential of a finger is formed on the outermost surface of the sensor.
In other sensors, the sensor is not directly exposed to the human body for detecting the fingerprint, a shutter is provided above the sensor, and a finger is touched on the shutter to remove static electricity and then touch the fingerprint sensor again. And
In the case of an optical sensor, a transparent metal thin film such as ITO (indium tin oxide film) is used as an electrode for removing static electricity, as described in Japanese Patent Application Laid-Open No. H11-216, as an electrode. It is also widely formed.
[0004]
[Patent Document 1]
Patent No. 3045629
[Patent Document 2]
JP-A-2003-6627
[0005]
[Problems to be solved by the invention]
However, the above-mentioned optical fingerprint sensor has the following problems.
1) Prisms and FOP substrates are expensive
2) The prism and FOP board are bulky (large mounting size)
3) When thin plates are stacked on the optical sensor, the resolution is deteriorated.
4) When the thin plate is laminated on the optical sensor, the contrast is also reduced.
5) Simply forming a thin plate on an optical sensor is susceptible to static electricity and does not prevent noise generation
6) The formation of the transparent electrode is expensive, and the restrictions on the semiconductor process are great.
7) Depending on the material of the thin plate, an optical afterimage occurs due to the adhesion of finger oil or fat.
However, prisms and FOPs are originally expensive optical components. Although there is a technique of substituting a high-precision glass prism with, for example, plastic or the like, it is not easy to produce a high-precision glass prism required for a fingerprint sensor. In addition, since the FOP is made by bundling glass fibers, even if the thickness of the FOP is reduced, it is not as inexpensive as expected. Since both have a size on the order of mm, for example, when a fingerprint sensor is mounted on a portable device, the size cannot be ignored.
In addition, the thin-plate fingerprint sensor disclosed in Patent Document 2 is liable to deteriorate in resolution and contrast as compared with the FOP-type fingerprint sensor. The reason is that the FOP type is a contact / non-contact sensor that detects the unevenness and mechanical shape of the finger, while the thin plate type is an essentially optical sensor that detects the amount of transmitted light as an image. Because there is. Therefore, the former is 10 ⁴ While the contrast is close, the latter usually has only about 1 to 10 contrasts.
In the case of the FOP type, most of the light for detecting a fingerprint passes through the glass fiber without mixing and attenuating when transmitting through a thick FOP, whereas in the case of the thin type, an optical device is used. Light simply diffuses and passes through a thin plate with no voids. Therefore, in the former, the spatial resolution is not substantially deteriorated, whereas in the latter, the spatial resolution is largely deteriorated.
[0006]
Japanese Patent Application Laid-Open No. H11-163873 describes some design guidelines for the refractive index and thickness of a thin plate, but they simply multiply the light scattering formula at two interfaces, a finger-air gap and an air gap-thin plate interface. It's just that, and lacks the accuracy of theory. More specifically, the lack of geometrical considerations of fingerprints, the elimination of ray tracing, and the like. Although it mentions the contrast of a fingerprint image, it does not mention the spatial resolution, which is the most problematic in fingerprint detection. In addition, it does not mention the quality of light for illuminating a finger, and Patent Document 2 treats illumination light as diffused light passing through the finger.
Further, the thin plate type is weaker against static electricity and an external electric field because the plate type is thinner than the FOP type. When a material having a high refractive index, a high dielectric property, and a high conductivity such as silicon is used as the material of the thin plate, its electrical handling becomes a problem.
Forming a transparent electrode such as ITO to prevent static electricity and an external electric field is generally expensive and difficult because the method of forming the transparent electrode is not compatible with the conventional semiconductor IC process. For this reason, in the past, the transparent metal was more likely to be formed on the thinner side than on the semiconductor IC side where the optical sensor was formed. The adverse effects of static electricity and an external electric field are not limited to only the pixel portion where the fingerprint image light is incident.
[0007]
[Means for Solving the Problems]
The fingerprint sensor of the present invention, in a fingerprint sensor that optically detects a fingerprint, includes an optical sensor including a plurality of photoelectric conversion elements, and a member that is disposed above the optical sensor and makes contact with the finger, The member has a higher refractive index than the finger and has conductivity, and the member is attached to the optical sensor via an insulator.
The material with a high refractive index is
n = √ (ε × μ) (1)
n: refractive index ε: dielectric constant μ: magnetic permeability
Is essentially a high dielectric substance. Therefore, it is a substance that easily generates electric charge and electric noise in a member due to the influence of static electricity and an external electric field.
[0008]
FIG. 5 is a diagram schematically illustrating a member and an insulator portion of the fingerprint sensor of the present invention. Reference numeral 51 denotes a combination of the member of the fingerprint sensor and the insulator, and reference numeral 52 (Rh) denotes a resistance representing the sheet resistance of the member. Also, 53 (Rv) is a resistance representing the resistance per unit area of the insulator.
[0009]
Reference numeral 54 denotes an electrical contact that contacts the finger 55, and reference numeral 56 denotes a contact that contacts the upper part of the optical sensor forming the fingerprint sensor. The resistor 52 (Rh) is connected to the ground potential as is known at the end of the member.
In the present invention, for example, a substrate having a resistivity of 0.1 Ω-cm and a thickness of about 100 μm is used for the member. Therefore, the value of Rh is 0.1 / 0.01 = 10Ω. Since the size of the area for detecting the fingerprint is approximately 2 cm × 1 cm, the resistance value of the entire rectangular parallelepiped is 10Ω × 2 = 20Ω.
In contrast, insulators have a resistivity of 10 ¹⁶ Since an adhesive having a resistance of about Ω-cm and a thickness of about 10 μm is used, the value of Rv is 10 ¹⁶ × 0.001 = 10 ^Thirteen Ω has a very large value as compared with Rv.
[0010]
In the present invention, the member has conductivity (consideration is made to reduce the electric resistance), so that the potential at any place in the member is substantially the same as the external potential at which the potential is fixed. Become. For this reason, the charges and the like generated in the member are quickly transferred to and removed from the outside where the potential is fixed. In addition, since the conductive member and the optical sensor are sufficiently electrically separated by the interposition of the insulator, electric charge and the like do not adversely affect the optical sensor.
In addition, when the potential of the member is fixed, there is a concern about electric leakage with the electric circuit of the optical sensor and short circuit. However, as described above, since the resistance value of the insulator is sufficiently high, the value of the leak current leaks is It can be suppressed to a value that is not a problem.
In addition, the light used for detecting a fingerprint is light that is incident on a finger substantially vertically.
The following describes how important the quality of illumination light for illuminating the finger is with a simple model.
[0011]
FIG. 6 is a model when scattered light enters the fingerprint structure.
In FIG. 6, reference numeral 63 denotes a surface of a semi-cylinder having a radius r representing a valley of a fingerprint. Above that is the inside 65 of the finger. The value of 1.3 is used for the refractive index of the inside 65 of the finger because it is almost the same as that of water. Below that is an air gap 64 at the valley, which is a cavity filled with air having a refractive index of 1.0. The peak of the fingerprint is in contact with the member 62.
Reference numeral 62 denotes a member having a thickness t, the refractive index of which is 3.448, which is the same as that of single crystal silicon. 61 indicates an insulator. θ1 is the incident angle of the light 66 from the air gap 64 to the member 62 (the angle of the incident light 66 with respect to the normal direction of the member 62), and θ2 is the refraction angle at the member 62 (the refraction light 67 with respect to the normal direction of the member 62). Angle). t is the thickness of the member 62, and x is the distance between the light incident position of the member 62 (the center of the arc of the semi-cylindrical column having a radius r) and the light emitting position of the member 62.
[0012]
In order to simplify the problem, the model assumes that uniform illumination light is incident perpendicularly to the surface 63 of the cylinder from the inside 65 of the finger (actually, the scattered light impinges light perpendicular to the surface). Including light at any angle of incidence about the center). Further, it is assumed that the pitch of the peaks and valleys of the fingerprint is the same (2 × r).
First, the amount of light Phi incident on the peak portion of the fingerprint is represented by the following equation, where Pi is the incident light density per unit length.
Phi = ∫Pidl (2)
dl: integral length
Is represented by This integration range is (-r, + r).
Eventually, the value of the incident light amount Phi at the peak is 2 × Pi × r.
Factors that matter in the mountains are:
1) Interface reflection at the finger-member interface (interface between interior 65 and member 62)
2) Damping in the member 62
It is.
The transmittance at the finger-member interface in the above 1) is expressed by the following equation.

Can be represented by This value is 0.795333325 as shown in Table 1.
[0013]
[Table 1]

[0014]
The transmittance in the member of the above 2) is expressed by the following equation.
Th2 = exp (−αt) (4)
Can be represented by The value of α at λ = 850 nm of single crystal silicon is about 0.6 dB / μm. Therefore, at a plate thickness of t = 100 μm, the value of Th2 is −60 dB and 0.001.
From the above, the light quantity Ph of the peak incident on the optical sensor is
Ph = ∫ (Pi × Th1 × Th2) dl (5)
It becomes. Since this value is 2 × Pi × r × 0.795333325 × 0.001, as shown in Table 1,
It becomes 2 × Pi × r × 0.0079533.
On the other hand, the incident light amount Pvi at the valley is
Pvi = ∫Pidl (6)
dl: integral length
Where the integration range is an arc of angle π along the surface 63 of the semi-cylinder.
[0015]
The factor that matters in the valley is
3) Interface reflection at the finger-air interface (interface between interior 65 and air gap 64)
4) Interface reflection at air-member interface (interface between air gap 64 and member 62)
5) Damping in the member 62
It is.
At the finger-air interface of the above 3), the incident light is perpendicularly incident on the interface. Therefore, the transmittance Tv1 is
Tv1 = (2 / (1 + 1 / 1.3)) ² × (1 / 1.3) (7)
It is expressed as This value is 0.982986767 as shown in Table 1.
Further, the transmittance Tv2 of the above-mentioned 4) at the air-member interface depends on the incident angle θ1 and the refraction angle θ2, and can be written as follows.

Here, ts is that of the s-wave and tp is that of the p-wave.
Here, it is assumed that the incident light contains the s-wave and the p-wave in half.
ts and tp are respectively

It is expressed as
[0016]
In Table 1, the value of Tv2 (θ1, θ2) is calculated from 0 to π / 2 for each π / 100 of the value of θ1.
The attenuation in the member 62 of the above 5) is due to the transmittance Tv3
Tv3 = exp (−αt (θ2)) (11)
Is represented by Here, t (θ2) is an optical path length extended due to refraction,
t (θ2) = t / cos θ2 (12)
It is.
Further, when appropriately modified using the equations (11) and (12), the following simplification is achieved.
Tv3 = (0.001) ^{(1 / cos} θ ²⁾ ... (13)
The total transmittance Tv of the valley
Tv = Tv1 × Tv2 × Tv3 (14)
The light quantity Pv of the valley incident on the optical sensor is defined as

It becomes. Table 1 shows the values of Tv1 to Tv.
[0017]
FIGS. 8 to 10 show the tendency of the present scattering model.
First, the transmittance Tv with respect to the incident angle θ1 (FIG. 8) is smaller than Th, and is approximately 86% of the value of Th at the time of perpendicular incidence at θ1 = 0 at the maximum. Thereafter, it is sequentially reduced and becomes 0 when θ1 = π / 2.
This means that, in the incident light incident on the valley, the contribution of light incident perpendicularly to the member is large. However, since the reduction method is gradual, even a light at a considerable angle contributes to the image formation of the valley.
[0018]
Next, the position where the incident light is incident on the optical sensor (light receiving position) x (FIG. 9). In the case of vertical incidence, the incident light is incident immediately below (x = 0 μm). The light is incident between the center x = 0 and 30 μm. This value is considerably smaller than the fingerprint pitch (2r × 2, about 400 μm).
This is due to the fact that single-crystal silicon, which is a high-refractive-index substance, was used for the material of the member, and that a relatively thin plate thickness (t = 100 μm) was employed.
If the pitch of the fingerprint is 200 μm (r = 100 μm), the light does not enter between 30 and 70 μm in the present scattering model. (FIG. 10) On the contrary, a large amount of light is concentrated and received at the center of the valley. This causes a false mountain part and a false ridge just in the middle of the mountain part.
[0019]
Based on the above results, the contrast and MTF of the scattering model will be discussed.
First, the contrast between the peak and the valley is compared.
Since the amount of light incident on the peak is Ph as described above, it is 2 × Pi × r × 0.0079533.
If Pi = 1 W / μm and r = 100 μm, then Ph = 0.159066 W.
On the other hand, Pv is expressed by equation (15).
Now, if the above integral value is substituted by the following approximate expression,

This value is Pv = 0.175135165W as shown in Table 1.
Here, if the contrast is defined as the amount of light indicated by the entire peak / the amount of light indicated by the entire valley,
Contrast = Ph / Pv (17)
It becomes.
[0020]
This amount is 0.908, which is 1 or less in the present model. That is, in the present scattering model, the opposite conclusion that is contrary to the experimental fact that Pv> Ph is derived, not the relation of Pv <Ph shown by the ordinary experimental result, is derived.
Of course, this is the accuracy of the model, but it also proves the fact that "thin-plate fingerprint sensors have poor contrast compared to those using FOP". As a matter of fact, we have obtained a value of 1.3 in contrast in the evaluation results of prototypes of similar systems.
Next, regarding the MTF, in this scattering model, a false signal of λ = 400 μm (distance between valley centers) having the same pitch as the fingerprint pitch generated by the center of the valley, and two dark portions existing at the boundary of the valley (FIG. 10) ) And a false signal of λ = 270 μm (distance between dark parts of adjacent valleys) and λ = 270 μm (distance between dark parts of the same valley).
The former is a false signal having a different phase from the fingerprint pitch, and the latter is a higher frequency false signal, which deteriorates the image quality of the fingerprint image. That is, the MTF characteristics of the thin fingerprint sensor using the scattered light are unsatisfactory.
[0021]
Next, FIG. 7 is a model when vertical light is incident on the fingerprint / finger structure. The fingerprint structure and the structure of the member are the same as those of the scattered light model shown in FIG.
In FIG. 7, reference numeral 73 denotes a surface of a semi-cylinder having a radius r representing a valley of a fingerprint. Above it is the interior 75 of the finger. The value of 1.3 is used for the refractive index of the inside 75 of the finger, as it is almost the same as that of water. Below that is an air gap 74 in the valley, which is a cavity filled with air having a refractive index of 1.0. The peak of the fingerprint is in contact with the member 72.
Reference numeral 72 denotes a member having a thickness t, the refractive index of which is 3.448, which is the same as that of single crystal silicon. 71 denotes an insulator, and 78 denotes a space (air) above the finger. θ1 is the angle between the normal direction of the member 72 and the normal direction of the surface 73 at the light incident portion, and θ2 is the incident angle of light from the inside 75 of the finger to the air gap 74 (the surface angle of the surface 73 at the light incident portion). Θ3 is the angle of refraction at the air gap 74 (the angle of the refracted light with respect to the normal direction of the surface 73), and θ4 is the angle of incidence of the light from the air gap 74 to the member 72 (the member). The angle of the incident light with respect to the normal direction of 72) and θ5 are the refraction angles at the member 72 (the angles of the refracted light 77 with respect to the normal direction of the member 72). t is the thickness of the member 72, x is the distance between the center of the arc of the semi-cylinder of radius r and the light emission position of the member 72, and y is the incidence of light on the center of the arc of the semi-circle of radius r and the surface 73. This is the distance from the position.
In this model, it is assumed that uniform illumination light is incident on the surface 63 of the cylinder from the inside 75 of the finger in parallel with the normal direction of the member 72.
[0022]
First, the incident light incident on the peak portion is the same as the scattered light model already described with reference to FIG.
Next is the valley, but the factors to consider are:
3 ') Reflection at finger-air interface (interface between interior 75 and air gap 74)
4 ') Reflection at air-member interface (interface between air gap 74 and member 72)
5 ′) Damping inside the member 72
It is.
At the finger-air interface of 3 ′), the transmittance Tv1 is

At the air-member interface in 4 ′) above, the transmittance Tv2 is

In the attenuation in the member 62 of the above 5 ′), the transmittance Tv3 becomes
Tv3 = (0.001) ^{(1 / cos} θ ⁵⁾ ... (20)
Is represented by
Table 2 shows the results of these calculations.
[0023]
[Table 2]

[0024]
FIGS. 11 to 13 graphically show these results.
First, a graph of the incident position y-transmittance T (FIG. 11), the value of Tv (y = 0 μm) is the same value as the scattered light model. (Approximately 86% of the value of Th) Further, Tv keeps a substantially constant value from y = 0 μm, and drops down near 80 μm. Above this point, the incident light is totally reflected at the finger-air interface and does not reach the light receiving surface of the optical sensor.
[0025]
FIG. 12 shows the correspondence between the incident position y and the light receiving position x. From this, it can be seen that although there is a substantially linear correspondence between the two, the received light near the boundary of the valley crosses the valley and is detected by the light receiving portion at the peak. Its maximum size is about 40 μm (offset, xy is about 60 μm).
[0026]
FIG. 13 shows the transmittance T corresponding to the light receiving position x, which is gentle and constant as a whole, and a sharp change in T is not observed as in the scattered light model.
In the figure, 0 to 100 μm is data representing a valley (measured amount), and a portion exceeding 100 μm is data representing an opposite peak. Therefore, the transmittance Tv of the valley curve from 100 μm to 140 μm is a portion that contributes to Th. The light amount corresponding to 0.00065 to 0.0004 indicates a function of increasing the light amount of Th of 0 to 40 μm, and indicates a function of enhancing both edges.
[0027]
From the above results, the contrast and the MTF will be similarly discussed.
First, the contrast, Ph / Pv, is obtained from Table 2 as 0.159066 / 0.1015550271 = 1.57.
This value is higher in contrast than the scattered light model, and indicates that the use of vertically incident illumination light improves the contrast. Actually, in our prototype evaluation results, there is also obtained an experimental result that "contrast is improved by using an illumination light source located at a long distance with high straightness".
[0028]
As for the MTF, as shown in the graph of FIG. 13, the discontinuous image characteristic (FIG. 9) as seen in the scattered light model is not seen. In addition, the leakage of Tv from 100 to 140 μm described above also deteriorates the MTF. Fortunately, the pitches are λ = 240 μm (distance on both sides of the same valley) and λ = 160 μm (distance between adjacent valleys). ) And close to the fingerprint pitch, and has the function of edge enhancement. Therefore, the deterioration of MTF due to this is not fatal.
[0029]
As described above, in order to improve the contrast and the MTF, it is preferable that the light used for the illumination be vertically incident light rather than scattered light.
The light used for detecting the fingerprint is preferably external light incident on the finger. Thereby, it is possible to expect illumination light that is incident more perpendicularly than the case where the finger is illuminated from the side by, for example, LEDs.
[0030]
Preferably, the member is a semiconductor substrate. The semiconductor substrate is essentially a dielectric substrate, and a high refractive index can be expected. It is also highly pure and can be expected to have chemical stability and mechanical strength. The semiconductor substrate is particularly preferably a silicon substrate. As described above, the refractive index of the silicon substrate is as high as 3.448, which is ideal.
[0031]
Further, it is desirable that a thin film for preventing contamination is formed on the surface of the member. Thereby, the influence of the residual fingerprint remaining on the member surface can be minimized. As the thin film for preventing contamination, a silicon-based compound, ceramic, or the like can be used. Thereby, sufficient hardness and stability can be obtained. It is also easy to use semiconductor process technology.
[0032]
Further, it is preferable that the optical sensor is provided with a shield plate inside the optical sensor in order to protect the sensor from electric charges induced in the member due to an external electric field, static electricity or the like. Thereby, the influence of the charge and the like can be further reduced.
In order to make the member conductive, a low-resistance substrate may be used for the member.
[0033]
Also, the resistivity of the low-resistance substrate is desirably 0.1 Ω-cm or less, thereby ensuring low-resistance electrical conduction as described above.
In order to reduce the electric resistance of the member, the surface of the member may be a metal thin film that is not optically transparent. It is desirable that the thickness of the metal thin film is λ / 10 or less of the wavelength of light for detecting a fingerprint. Thereby, the light for detecting the metal thin film can be partially transmitted, and a fingerprint / optical sensor is realized.
In order to reduce the electric resistance of the member, a high-concentration impurity diffusion layer may be provided on the surface of the member. Thereby, the electric resistance of the member can be reduced.
In order to reduce the electric resistance of the member, a patterned metal thin film may be formed on the surface of the member, and a thin film for preventing contamination is preferably formed above the metal thin film. As a result, it is possible to reduce the electric resistance, prevent the finger from being stained with oil and fat, and prevent the remaining fingerprint.
[0034]
Further, it is desirable that the member is formed using a semiconductor process technology. This makes it possible to stably deposit and pattern the thin film at low cost.
It is desirable that the insulator is not formed by a semiconductor process. A relatively thick insulator can be formed by a coating method, a spin-on method, printing, or the like.
The insulator has a resistance value of 10 per unit area. ¹² It is desirable that it be Ω or more. This makes it possible to suppress the leakage current that penetrates the insulator, which is harmful to the operation of the optical sensor, to the pA level.
[0035]
【Example】
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[First embodiment]
FIG. 1 is a schematic sectional view of a fingerprint detection sensor according to a first embodiment of the present invention. In FIG. 1, reference numeral 11 denotes an optical sensor formed on a single-crystal silicon substrate having a size of 2 cm × 1 cm. The optical sensor has 600 × 300 = 180000 unit pixels of 30 μm square arranged in a two-dimensional array.
12 is resistivity 10 ¹⁶ Ω-cm, a semiconductor-grade adhesive having a thickness of 10 μm. A thin plate 13 made of single-crystal silicon having a thickness of 100 μm and a p-type resistivity of 0.01 Ω-cm is bonded to the substrate 11 by the adhesive 12. ing.
The laminated thin plate 13 is grounded to the ground potential at the end of the fingerprint sensor. A 2000 nm thick silicon nitride film 14 is formed on the surface of the thin plate 13 to prevent contamination by fingerprints or the like. Reference numeral 15 denotes a finger for detecting a fingerprint, which is placed on the silicon nitride film 14.
[0036]
FIG. 14 shows the spectral sensitivity of the fingerprint sensor. As can be seen from the figure, this sensor has a sensitivity peak in the infrared light having a wavelength of 1030 nm, and has a sensitivity in the infrared light having a wavelength of 800 nm to 1200 nm. The sensitivity drop on the left side of this graph is caused by the fact that infrared light does not pass through the thin plate 13, and that on the right side is caused by the deterioration of the photoelectric conversion characteristics of the optical sensor 11.
[0037]
As the illumination light 16 for fingerprint detection in the present embodiment, for example, external light composed of substantially vertical light is used. The external light mainly includes outdoor light from the sun and indoor light from lighting such as a fluorescent lamp. Although the spectrum of sunlight is broad and continuous, as shown in FIG. 14, the fingerprint detection sensor of this embodiment has only a narrow spectral sensitivity such as a band-pass filter, so that the wavelength is substantially from 800 nm. It is equivalent to illuminating light of 1200 nm. The light quality of the latter fluorescent lamp is a discrete spectrum that exists mainly in the visible light region.
[0038]
FIG. 15 shows an example of fingerprint imaging with room light of the fingerprint sensor of this embodiment.
As can be seen from FIG. 15, the image can capture not only the peaks and valleys of the fingerprint but also the holes of the sweat glands, and the spatial resolution is sufficient. Further, the ratio of the brightness between the peaks (bright portions) and the valleys (dark portions) is about 1.5, which is also a contrast value that can be explained by the aforementioned model.
[0039]
The material of the thin plate used in the present invention may be any of n-type, i-type, and other high-dielectric substances other than the p-type single crystal silicon substrate. In particular, a semiconductor substrate is preferable because it has a substantially high dielectric constant. Ge, diamond, and the like can be suitably used without adversely affecting the human body.
[0040]
In addition to the silicon nitride film, a silicon-based glass such as a silicon oxide film, ceramic, or the like is suitable for the thin film for preventing contamination.
These can be easily and inexpensively manufactured on a semiconductor substrate by using a known semiconductor process technology.
The light used for illuminating the fingerprint may be substantially vertical light such as indoor light, and for example, a dedicated infrared light source may be prepared. In that case, it is desirable to use an infrared LED that emits infrared light by embedding it in the bottom of a cylindrical reflecting container in order to increase the MTF of the fingerprint and the contrast.
[0041]
[Second embodiment]
FIG. 2 is a schematic sectional view of a fingerprint detection sensor according to a second embodiment of the present invention.
21 is an optical sensor, 22 is an adhesive, 23 is a single crystal silicon substrate having a resistivity of 100 Ω-cm, and 24 is a gold thin film having a thickness of 50 °. The same optical sensor 21 and adhesive 22 as those in the first embodiment can be used.
The thickness of the thin film 24 is not more than 1/20 of the wavelength of 1030 ° of the illumination light for detecting the fingerprint, and sufficiently transmits the illumination light. Since the sheet resistance of the thin film 24 is about 5Ω, the fingerprint sensor can be grounded at a sufficiently low value.
According to this embodiment, even if the resistivity of the thin plate 24 is slightly higher, it can be used for a fingerprint sensor. The material of the thin film used in this embodiment is preferably a noble metal such as silver or copper which has low resistance and is chemically stable. Refractory metals such as titanium and tungsten are also candidates.
[0042]
[Third embodiment]
FIG. 3 is a schematic sectional view of a fingerprint detection sensor according to a third embodiment of the present invention.
Reference numeral 31 denotes an optical sensor, reference numeral 32 denotes an adhesive, reference numeral 33 denotes a single-crystal silicon substrate, and a p-type impurity diffusion layer 36 of Al having a depth of 1 μm is formed near the surface thereof by a known semiconductor process technique.
The diffusion layer 36 is electrically grounded at the end of the sensor. Above the diffusion layer 36, a silicon oxide film 34 having a thickness of 1 μm is similarly formed by a semiconductor process technique. The oxide film 34 also prevents the sensor from being soiled by a finger.
According to the present embodiment, the conductivity of the thin plate 33 can be improved by the impurity diffusion layer 36 having a low electrical resistance.
The impurity forming the diffusion layer may be a trivalent (B, Al, Ga) or pentavalent (P, As) atom as a dopant, or a conductive metal atom.
[0043]
[Fourth embodiment]
FIG. 4 is a schematic sectional view of a fingerprint detection sensor according to a fourth embodiment of the present invention.
41 is a semiconductor substrate on which an optical sensor is formed, and 47 is a 50 μm square unit pixel forming the sensor. Reference numeral 42 denotes an adhesive for bonding the substrate 41 to a thin plate 43 having a high refractive index and a thickness of 150 μm. Reference numeral 43 denotes a semiconductor germanium substrate having a refractive index of 4.092.
Reference numeral 46 denotes an aluminum film having a thickness of 1000 ° laminated and patterned on the thin plate 43 by a known semiconductor process technique. The pattern of the aluminum film 46 has an opening size of 40 μm square and an opening pitch of 50 μm.
The bonding is performed so that the pattern of the aluminum film 46 and the position of the pixel 47 of the optical sensor correspond to each other. As a result, the fingerprint image light incident from the opening of the aluminum film 46 is guided to the pixel 47 without waste.
Reference numeral 44 denotes an overcoat having a thickness of 2000 mm and made of a silicon nitride film formed on the aluminum film 46 to prevent contamination and prevent abrasion of the aluminum film pattern due to finger friction.
[0044]
According to this embodiment, the resistance of the thin plate 43 can be reduced without using a transparent metal such as ITO.
The quality required for the patterned thin film is, for example, good density and adhesion.
A sparse film whose density is reduced by thinning cannot withstand finger friction. If the adhesion to the base or overcoat is poor, the pattern is easily peeled and deformed, and the quality of the fingerprint image is reduced.
As a film satisfying such characteristics, a metal having a high melting point such as tungsten or molybdenum other than aluminum may be used.
[0045]
In this embodiment, unlike the other embodiments, a low-resistance conductor layer is not provided on the entire surface. Therefore, even if there is no problem with static electricity, it is not perfect against incident electric fields and electromagnetic waves.
Therefore, it is effective to provide an electric shield and a shield plate inside the optical sensor in addition to the above-described patterned conductor.
It is desirable that the shield plate is provided not only in the pixel portion where the fingerprint image light is incident, but also in an electric circuit portion (peripheral circuit portion) around the pixel portion.
[0046]
【The invention's effect】
As described above, according to the present invention, it is possible to stably provide a fingerprint sensor that is low in cost, resistant to static electricity and the like, and excellent in resolution and contrast.
[Brief description of the drawings]
FIG. 1 is a schematic sectional view of a fingerprint detection sensor according to a first embodiment of the present invention.
FIG. 2 is a schematic sectional view of a fingerprint detection sensor according to a second embodiment of the present invention.
FIG. 3 is a schematic sectional view of a fingerprint detection sensor according to a third embodiment of the present invention.
FIG. 4 is a schematic sectional view of a fingerprint detection sensor according to a fourth embodiment of the present invention.
FIG. 5 is a diagram schematically illustrating a member and an insulator portion of the fingerprint sensor of the present invention.
FIG. 6 is a model when scattered light is incident on a fingerprint structure.
FIG. 7 is a model when vertical light is incident on a fingerprint / finger structure.
FIG. 8 is a diagram showing an incident angle-transmittance characteristic of a scattering model.
FIG. 9 is a diagram illustrating a relationship between an incident angle of valley incident light and a position of incidence on an optical sensor in a scattering model.
FIG. 10 is a diagram illustrating a relationship between a light receiving position and a transmittance of a scattering model.
FIG. 11 is a diagram showing the relationship between the incidence position and the transmittance of the normal incidence model.
FIG. 12 is a diagram illustrating a correspondence between an incident position y and a light receiving position x.
FIG. 13 is a diagram illustrating a relationship between a light receiving position and a transmittance of a normal incidence model.
FIG. 14 is a diagram showing the spectral sensitivity of a fingerprint sensor.
FIG. 15 is a diagram illustrating an example of fingerprint imaging with room light of the fingerprint sensor according to the present embodiment.
[Explanation of symbols]
11, 21, 31, 41 substrate
12, 22, 32, 42 adhesive
13,23,33,43 Thin plate
14, 24, 34, 44 thin film
15, 25, 35, 45 fingers
16 Illumination light
36 Diffusion layer
46 patterns
51 Thin plate
52 resistance
53 Resistance
54 finger side terminal
55 fingers
56 Optical sensor side terminal
57 Ground
61, 71 Optical sensor
62, 72 thin plate
63, 73 interface
64, 74 cavities
65, 75 finger inside
66, 76 Incident light
67, 77 Refracted light

Claims (18)

In a fingerprint sensor that optically detects a fingerprint,
An optical sensor including a plurality of photoelectric conversion elements, and a member that is disposed above the optical sensor and makes contact with the finger,
A fingerprint sensor, wherein the member has a refractive index higher than that of a finger and has conductivity, and the member is bonded to the optical sensor via an insulator. . The fingerprint sensor according to claim 1, wherein the light used for detecting the fingerprint is light that is incident on the finger substantially perpendicularly. 2. The fingerprint sensor according to claim 1, wherein the light used for detecting the fingerprint is external light incident on the finger. 2. The fingerprint sensor according to claim 1, wherein said member is a semiconductor substrate. 5. The fingerprint sensor according to claim 4, wherein said semiconductor substrate is a silicon substrate. 2. The fingerprint sensor according to claim 1, wherein a thin film for preventing contamination is formed on a surface of said member. 7. The fingerprint sensor according to claim 6, wherein said thin film for preventing contamination is a silicon-based compound or ceramic. 2. The fingerprint sensor according to claim 1, wherein a shield plate is provided inside the optical sensor in order to protect the optical sensor from charges induced on the member due to an external electric field, static electricity, or the like. Characteristic fingerprint sensor. 2. The fingerprint sensor according to claim 1, wherein a low-resistance substrate is used as said member. 10. The fingerprint sensor according to claim 9, wherein the low-resistance substrate has a resistivity of 0.1 Ω-cm or less. 2. The fingerprint sensor according to claim 1, wherein said member has a metal thin film which is not optically transparent on its surface. 12. The fingerprint sensor according to claim 11, wherein a thickness of the metal thin film is equal to or smaller than λ / 10 of a wavelength of light for detecting the fingerprint. The fingerprint sensor according to claim 4, wherein the member has a high-concentration impurity diffusion layer on a surface thereof. 2. The fingerprint sensor according to claim 1, wherein said member has a patterned metal thin film on a surface thereof, and a thin film for preventing contamination is formed above said member. 15. The fingerprint sensor according to claim 14, wherein the member is formed by using a semiconductor process technology. 2. The fingerprint sensor according to claim 1, wherein the insulating film is formed by coating, spin-on, printing, or the like. The fingerprint sensor according to claim 1, wherein the insulator has a resistance per unit area of 10 ¹² Ω or more. 2. The fingerprint sensor according to claim 1, wherein said insulator is an adhesive.

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