JP3952666B2

JP3952666B2 - Surface acoustic wave device

Info

Publication number: JP3952666B2
Application number: JP2000194887A
Authority: JP
Inventors: 健吾浅井; 光孝疋田; 敦礒部
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2000-06-23
Filing date: 2000-06-23
Publication date: 2007-08-01
Anticipated expiration: 2020-06-23
Also published as: JP2002009584A

Description

【０００１】
【発明の属する技術分野】
本発明は携帯電話等に用いられる弾性表面波を用いる素子およびその基板の製造方法に関する。
【０００２】
【従来の技術】
携帯電話等に用いられる弾性表面波素子は、例えば、電子情報通信学会論文誌Ａ，Ｊ７６巻−Ａ，２号、１８５−１９２頁（１９９３年２月）に示されているように、タンタル酸リチウム基板、ニオブ酸リチウム基板および四ホウ酸リチウム基板などの単結晶圧電基板上に金属薄膜の櫛形交差電極を形成して構成されている。
【０００３】
携帯電話等の高性能化に伴い、それらに用いる弾性表面波素子用基板の遅延時間温度係数を改善させた報告がなされている。例えば、特開平１１−５５０７０号に示されているように単結晶圧電基板とガラス基板を直接接合させた事例がある。さらに、第２０回超音波シンポジウム予稿集５１頁（１９９９年１１月）に示されているように単結晶圧電基板とマイナス膨張ガラスを紫外線硬化型樹脂で接合させた事例がある。
【０００４】
【発明が解決しようとする課題】
携帯電話等は、近年の急速な市場拡大から、送受信の各周波数帯域がより拡大される傾向にあり、送信帯域と受信帯域の周波数間隔が非常に狭いシステムも存在している。このことから携帯電話等に内蔵される各種デバイスに対しても、より一層の高性能化が要求されている。特にタンタル酸リチウム基板あるいはニオブ酸リチウム基板等の単結晶圧電基板上に金属薄膜の櫛形交差電極を形成する従来の弾性表面波素子では、遅延時間温度係数が大きい場合、帯域間減衰量が十分に取れないため重大な課題となる。
【０００５】
弾性表面波素子の遅延時間温度係数は、単結晶圧電基板の線熱膨張係数と弾性表面波伝搬速度の温度係数との差によって決定される。これらの値は単結晶圧電基板固有の値であり、線熱膨張係数に関して言えば、例えばＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板のＸ軸、すなわち弾性表面波伝搬方向では約１６．１ｐｐｍ／℃、またＸ軸を中心にＹ軸からＺ軸方向に６４°の角度で回転された面方位を持つニオブ酸リチウム基板のＸ軸すなわち弾性表面波伝搬方向では約１５．４ｐｐｍ／℃と大きい。今後、弾性表面波素子の性能向上を図る上でこの点が障害となっている。
【０００６】
上記の課題を解決する方法として、単結晶圧電基板に線熱膨張係数が小さいガラス基板を直接接合した複合圧電基板を用いる方法がある。しかし、上記複合圧電基板は材質の異なる基板を接合しているため、特に基板接合界面でのバルク波反射の影響が大きく、弾性表面波素子の特性を劣化させる問題（フィルタでは例えば帯域内リップル、あるいは帯域外のスプリアス応答等）がある。
【０００７】
また、基板接合方法に関しては、前記直接接合以外に、接着剤等を用いる方法もあるが、適用できる接着剤に耐熱性がなく、デバイスを形成する過程での加熱処理時に問題が生じるおそれがある。
【０００８】
本発明は、上記のような問題を考慮し、弾性表面波を励振伝搬させる単結晶圧電基板の線熱膨張係数を改善することによって、遅延時間温度係数が向上できる弾性表面波素子用基板、およびその弾性表面波素子用基板上に弾性表面波素子を実現することを目的とする。
【０００９】
すなわち、単結晶圧電基板の接合法に関しては、直接接合法において基板接合界面でのバルク波反射の影響を抑えた良好な弾性表面波伝搬特性を実現することを目的とし、また接着層を介して基板接合を行う方法において基板接合後の櫛形交差電極の製造プロセス工程に対して十分な耐熱性および耐薬品性を示す基板接合を実現することを目的とする。
【００１０】
【課題を解決するための手段】
上記目的を達成するために、本発明による弾性表面波素子は、単結晶圧電基板である第１の基板と、第１の基板に接合された第２の基板と、第１の基板の第２の基板との接合面と反対側の面上に形成され弾性表面波を励振伝搬する櫛型交差電極とを備えた構造において、第１の基板の弾性表面波伝搬方向を第２の基板の接合面内で最も線熱膨張係数の小さい方向と平行にすることを特徴とする。
【００１１】
上記において、第１と第２の基板が実質的に接合層を介さず、直接接合される構成の場合、上記第１と第２の基板の材質は、同じ材質であることが好ましい。また、本発明による上記第１と第２の基板が異種材料を接合した構成であるときには、耐熱性および耐薬品性の問題を解決した基板接合を可能とするために、基板の接合界面に塗布ガラスを主成分とする接着層を介することが好ましい。
【００１２】
【発明の実施の形態】
図１は本発明による弾性表面波素子の第１の実施例を示す斜視図である。図の１は単結晶圧電基板、２は上記基板１に接合された第２の基板、３は上記基板１の、基板２との接合面と反対側の面上に形成された櫛型交差電極である。本実施例において、基板２の材質は基板１と同じであるが、基板１の弾性表面波の伝搬方向（矢印４）における基板２の線熱膨張係数は、基板１の同方向の線熱膨張係数より小さくなるように接合されている。
【００１３】
本実施例における弾性表面波素子では、基板１と基板２とが直接接合によって接合され、接合した基板を弾性表面波素子用基板５として用いる。基板１上に形成された櫛型交差電極３により励振された弾性表面波は基板１上を伝搬し、弾性表面波素子として機能している。櫛形交差電極３の電極指は基板１のＸ軸に対して垂直方向に形成されているため、弾性表面波は基板１のＸ軸に対して平行な方向に伝搬する。
【００１４】
基板１上に金属薄膜の櫛形交差電極３を形成した弾性表面波素子において遅延時間温度係数は、基板１の弾性表面波伝搬方向４の線熱膨張係数と弾性表面波伝搬速度の温度係数との差によって決定する。これらの値は単結晶圧電基板固有の値であり、例えば、Ｘ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板の弾性表面波伝搬方向４（Ｘ軸方向）の線熱膨張係数は約１６．１ｐｐｍ／℃と良好な数値ではない。
【００１５】
現在、弾性表面波素子に使用されている単結晶圧電基板において、遅延時間温度係数が良好なものとしては水晶基板がある。水晶基板の場合、弾性表面波伝搬方向４の線熱膨張係数は約１３．７１ｐｐｍ／℃と、けして良好な値ではないが、弾性表面波伝搬速度の温度係数がタンタル酸リチウム基板やニオブ酸リチウム基板などとは逆に正の値となる性質を持っているため、線熱膨張係数の値が弾性表面波伝搬速度の温度係数の値によって相殺され、遅延時間温度係数が小さな値を示す。しかしながら、水晶基板は電気機械結合係数が小さく、十分な周波数帯域幅を得ることができないという欠点がある。電気機械結合係数と遅延時間温度係数の両方がともに良好な単結晶圧電基板は、現在のところ発見されていない。
【００１６】
本実施例では、電気機械結合係数が大きい単結晶圧電基板を用いて、遅延時間温度係数が小さい弾性表面波素子を実現するために、単結晶圧電基板である基板１の弾性表面波伝搬方向４と第２の基板２の線熱膨張係数の小さい方向とを平行にして接合する。これにより、基板２の線熱膨張係数によって基板１の線熱膨張係数が抑制され、遅延時間温度係数が改善される。
【００１７】
図２は本実施例による基板１の面方位の一例を示したものであり、図３は本実施例による基板２の面方位の一例を示したものである。図３の矢印６は、第２の基板の熱膨張係数が最も小さい方向を示す。ここでは基板１としてＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板を用い、基板１と同じ材質からなる基板２としてＹ軸方向の面方位を持つタンタル酸リチウム基板を用いる。
【００１８】
図４は、基板１と基板２を接合させる場合の接合方向を示した図である。ここで、基板１および基板２の線熱膨張係数を考える。基板１であるＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板では、弾性表面波の伝搬方向４であるＸ軸方向の線熱膨張係数が約１６．１ｐｐｍ／℃である。これに対して、基板２であるＹ軸方向の面方位を持つタンタル酸リチウム基板の熱膨張係数が非常に小さい方向（矢印６で示す。ここでは弾性表面波の伝搬方向であるＸ軸方向に対して直交するＺ軸方向）の線熱膨張係数は約４．１ｐｐｍ／℃と、この面内で最も小さい。
【００１９】
本発明によると図４に示すように、基板１の弾性表面波伝搬方向４であるＸ軸方向と、基板２の線熱膨張係数が非常に小さいＺ軸方向６を平行にして接合することにより、基板１の線熱膨張係数が基板２の線熱膨張係数によって抑制されるため、弾性表面波伝搬方向４の線熱膨張係数を改善することができる。ただし、基板１の線熱膨張係数がそのまま基板２の線熱膨張係数となるわけではなく、基板１と基板２の熱膨張差によって接合面に生じる熱応力に準じた数値となるため、基板１と基板２の基板厚さが重要となる。検討した結果、基板２の厚さが基板１の厚さの３倍以上となるように基板１を薄板化することにより、接合した弾性表面波素子用基板５において弾性表面波伝搬方向の線熱膨張係数をより顕著に改善できることが分かった。
【００２０】
ここでは、基板１であるＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板の板厚を９０μm、基板２であるＹ軸方向の面方位を持つタンタル酸リチウム基板の板厚を２７０μmとすることにより、Ｙ軸方向の面方位を持つタンタル酸リチウム基板の線熱膨張係数が支配的となり、線熱膨張係数が改善される。この場合の遅延時間温度係数を測定した結果、２４ｐｐｍ／℃であった。基板接合を行わない従来の弾性表面波素子の遅延時間温度係数は３３ｐｐｍ／℃であるから、本発明により９ｐｐｍ／℃の改善効果があった。また、基板１の板厚をより一層薄くすることで、より大きい効果が得られる。
【００２１】
また、本実施例によれば、接合された基板１と基板２が同じ材質からなる構造、すなわち接合界面における格子定数が同じとなる構造であるため、単結晶圧電基板とガラス基板に代表されるような異種材料基板の接合と比較して、より強力な接着力が実現できる。すなわち、Ｘ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板とＹ軸方向の面方位を持つタンタル酸リチウム基板は同じ材質であることから、非常に強力な接着力の実現が可能である。
【００２２】
図５を用いて本実施例による基板接合界面のバルク波反射の影響を説明する。基板２の厚さが基板１の厚さの３倍以上となるように基板１の板厚を薄板化すると、基板１の表面と基板接合界面とが接近するために（ａ）に示すようにバルク波７の基板接合界面からの反射波８の影響がより大きくなる。しかしながら、本実施例によれば（ｂ）に示すように、接合した基板１と基板２が同じ材質からなる構造であるため、異種材料基板を接合した場合と比較して、バルク波７の基板接合界面からの反射波８の影響が小さくなる。
【００２３】
すなわち、Ｘ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板とＹ軸方向の面方位を持つタンタル酸リチウム基板は、同じ材質であることから接合界面での反射による影響が小さく、この構造を有する本実施例の弾性表面波では接合界面からのバルク波反射による素子特性の劣化を小さくすることができる。
【００２４】
また、異種材料基板どうしを直接接合する場合には、接合基板の線熱膨張係数の差やボイド部と接合部との熱応力の不均一などにより、基板破損の問題が生じやすいが、本実施例によれば、接合された基板１と２が同じ材質であるため、異種材料基板の直接接合と比較して基板破損の問題が生じにくい。
【００２５】
つぎに本発明の弾性表面波素子の製造方法の一例を図６により説明する。例えば基板１として、Ｘ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つ鏡面研磨されたタンタル酸リチウム基板を用意する。また、基板２としてはＹ軸方向の面方位を持つ鏡面研磨されたタンタル酸リチウム基板を用意する。上記両者を接合する前処理として３００℃以上の温度で１時間以上の熱処理を行う。これは基板１および基板２の表面に付着しているガスや有機物を除去する目的で行う。この処理を怠ると基板接合後に接合界面にボイドが発生する可能性がある。
【００２６】
次いで、接合する２枚のタンタル酸リチウム基板を、過酸化水素（Ｈ₂Ｏ₂）とアンモニア水溶液（ＮＨ₄ＯＨ）と純水（Ｈ₂Ｏ）を混合した溶液に約１０分程度浸漬させた後、純水によるリンスを行う。これは基板１および基板２の表面に親水性を持たせ、基板接合時に基板表面に吸着されている水分子間に働くファンデルワース力により結合させる効果がある。
【００２７】
その後、２枚のタンタル酸リチウム基板を乾燥させた後、室温、空気雰囲気中で基板接合を行う。ここではパーティクルフリーの接合界面を得ることが特に重要であり、前記洗浄後、クラス１０以上のクリーン度を持つクリーンルームで基板接合を行うことが望ましい。また、接合直前に洗浄を行うことによりパーティクルフリーの界面と親水性を持った界面を両立させることができる。
【００２８】
その後、接合された２枚のタンタル酸リチウム基板は基板２であるＹ軸方向の面方位を持つタンタル酸リチウム基板の線熱膨張係数が支配的となるように、基板１であるＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板の薄板化を行う。基板研磨装置を用いて、Ｘ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板の板厚を、Ｙ軸方向の面方位を持つタンタル酸リチウム基板の板厚に対して３分の１以下となるように研磨する。
【００２９】
研磨工程は粗研磨から仕上げ研磨を段階的に行い、鏡面研磨を実現する。このとき、ここに示したように基板接合後の研磨工程によって薄板化するのではなく、あらかじめＹ軸方向の面方位を持つタンタル酸リチウム基板に対して３分の１以下の板厚となるＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板を用意してから接合してもよく、基板１の板厚が基板２の板厚に対して３分の１以下の板厚であれば製法は特に問わない。
【００３０】
基板１を薄板化した後、２５０℃の温度で約２時間の熱処理を行うことにより２枚のタンタル酸リチウム基板は完全に接合される。その後、図７に示すような櫛形交差電極３を、基板２に接合された基板１上に通常の電極作製工程を行って作製する。このとき櫛形交差電極３により励振伝搬される弾性表面波が基板１の弾性表面波伝搬方向（Ｘ軸方向）と一致するように櫛形交差電極３を配置する。
【００３１】
上記、第１の実施例においては、基板１としてＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板、同じ材質からなる基板２としてＹ軸方向の面方位を持つタンタル酸リチウム基板を用いた例について説明したが、基板２としてＸ軸方向の面方位を持つタンタル酸リチウム基板を用いた場合も同様の効果がある。
【００３２】
同様に、基板１としてＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板を用い、同じ材質からなる基板２として基板１と同じ面方位を持つタンタル酸リチウム基板を用い、基板１のＸ軸方向が基板２のＸ軸方向と直交するように接合した場合も同様の効果がある。
【００３３】
同様に、基板１としてＸ軸方向の面方位を持つタンタル酸リチウム基板を用い、同じ材質からなる基板２としてＹ軸方向もしくはＸ軸方向の面方位を持つタンタル酸リチウム基板を用い、基板１の弾性表面波伝搬方向４であるＹ軸からＺ軸方向に１１２°の角度で回転された方向が基板２のＺ軸方向と平行となるように接合した場合も同様の効果がある。
【００３４】
同様に、基板１としてＸ軸を中心にＹ軸からＺ軸方向に４１°〜６４°の角度で回転された面方位を持つニオブ酸リチウム基板を用い、同じ材質からなる基板２としてＹ軸方向もしくはＸ軸方向の面方位を持つニオブ酸リチウム基板を用い、基板１のＸ軸方向が基板２のＺ軸方向と平行するように接合した場合も同様の効果がある。
【００３５】
同様に、基板１としてＸ軸を中心にＹ軸からＺ軸方向に４１°〜６４°の角度で回転された面方位を持つニオブ酸リチウム基板を用い、同じ材質からなる基板２として基板１と同じ面方位を持つニオブ酸リチウム基板を用い、基板１のＸ軸方向が基板２のＸ軸方向と直交するように接合した場合も同様の効果がある。
【００３６】
また、基板１として四ホウ酸リチウム基板を用い、同じ材質からなる基板２として接合面内にｃ軸を有する四ホウ酸リチウム基板を用い、基板１の弾性表面波伝搬方向４が基板２のｃ軸方向と平行となるように接合した弾性表面波素子用基板５においても同様の効果がある。
【００３７】
この場合の基板１および基板２の線熱膨張係数を考えると、基板１である四ホウ酸リチウム基板のａ軸方向の線熱膨張係数が約１３ｐｐｍ／℃であるのに対して、基板２である四ホウ酸リチウム基板のｃ軸の線熱膨張係数は約−１．５ｐｐｍ／℃と負の線熱膨張係数となる。よって、四ホウ酸リチウム基板のａ軸方向と四ホウ酸リチウム基板のｃ軸方向が平行となるように基板接合することにより、ａ軸方向の線熱膨張係数の約１３ｐｐｍ／℃がｃ軸方向の線熱膨張係数の約−１．５ｐｐｍ／℃によって抑制され、接合した弾性表面波素子用基板５において弾性表面波伝搬方向の線熱膨張係数が改善できる。
【００３８】
つぎに、本発明の別の実施例を説明する。図８は本発明による弾性表面波素子の第２の実施例を示す斜視図である。図８に示す弾性表面波素子は単結晶圧電基板である基板１と、基板１に接合された基板２と、基板１の基板２との接合面と反対側の面上に形成され弾性表面波を励振する櫛型交差電極３とを備えた弾性表面波素子であり、基板１と基板２の接合には基板１と基板２の接合界面に塗布ガラス（ＳＯＧ：Spin On Grass）を主成分とする接着層９を有している。
【００３９】
基板１の弾性表面波の伝搬方向４における基板２の線熱膨張係数は、基板１の同方向の線熱膨張係数より小さくなるように接合されている。また、基板２の厚さが基板１の厚さの３倍以上となるように基板１の板厚が薄板化されている。接着層９として塗布ガラスを用いて基板１と基板２を接合させた基板を弾性表面波素子用基板５として用いる。基板１上に形成された櫛型交差電極３により励振された弾性表面波は基板１上を伝搬し、弾性表面波素子として機能する。
【００４０】
接着層９として用いる塗布ガラスは酸化珪素を主成分とする被膜を塗布・焼成法で形成することができるもので、珪素化合物を有機溶剤に溶解させたものである。ここでは基板１としてＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板を用い、基板２として酸化珪素基板を用いる。
【００４１】
本実施例によれば、基板１と基板２の接合において、接着層９として主成分が酸化珪素からなる塗布ガラスを用いることにより、接着層９自体の遅延時間温度係数が小さいことから、例えば紫外線硬化型樹脂等を接着剤として用いた場合との比較において、接着層９による遅延時間温度係数の悪化がないため、接合した弾性表面波素子用基板５の弾性表面波伝搬方向４に対する遅延時間温度係数がより改善される。また、塗布ガラスは主成分が酸化珪素からなるため非常に硬度が高く、基板１の熱膨張による応力が発生した場合にも、例えば紫外線硬化型樹脂等と比べて圧電基板１の伸びを抑制することができ、線熱膨張係数の改善にも効果的である。
【００４２】
基板１と基板２を接合した弾性表面波素子用基板５は基板接合後に弾性表面波素子を作製する製造プロセスとして、前工程においては金属薄膜被着工程、ホトリソグラフィー工程、エッチング工程、さらに後工程においては半田リフロー工程などの熱処理を伴う工程を有するため、耐熱性が重要となる。また、各工程において、有機および無機薬品なども使用されるため耐薬品性も重要となる。よって、接着層９を用いて基板１と基板２を接合させる場合には接着層９に耐熱性および耐薬品性が必須となる。
【００４３】
一例として、接着層９に紫外線硬化型樹脂を用いた場合について説明する。基板２の接合面に紫外線硬化型樹脂を塗布し基板接合を行った後、紫外線を照射するだけで紫外線硬化型樹脂が硬化して基板接合が完了するため、熱処理も不要な非常に簡便な基板接合法である。しかし、紫外線硬化型樹脂の特性として耐薬品性は十分であるが、耐熱性が１２０℃程度と低いため接着層９としての適用は難しい。
【００４４】
別の例として、接着層９に熱硬化型樹脂を用いた場合について説明する。基板２の接合面に熱硬化型樹脂を塗布し、熱処理により溶剤を揮発させ硬化させた後、熱硬化型樹脂が塗布された基板２を再び加熱し、熱硬化型樹脂を軟化させた状態で基板１を接合し、基板接合後に冷却することにより熱硬化型樹脂を硬化させ、接合が完了する。しかし、熱硬化型樹脂の特性としては耐薬品性が脆弱で、さらに基板接合後の再加熱により軟化することもあるため接着層９としての適用は難しい。
【００４５】
さらに別の例として、接着層９に接着用ワックスを用いた場合について説明する。ホットプレートなどで加熱した基板２の接合面に接着用ワックスを塗り、接着用ワックスが溶けた状態で基板１を接合した後、冷却することにより接着用ワックスを硬化させ、接合が完了するという非常に簡便な基板接合方法である。しかしながら、接着用ワックスの特性としては耐熱性が低いことにくわえて、アルコールでも溶けるほど耐薬品性がないため接着層９としての適用は難しい。
【００４６】
本実施例において、基板１と基板２を接合する際に接着層９として用いる主成分が酸化珪素からなる塗布ガラスは、４００℃以上の熱処理においても十分な耐熱性を示し、また耐薬品性に関しても酸化珪素に準じた高い耐性を示すため、前記紫外線硬化型樹脂、熱硬化型樹脂、接着用ワックス等を接着層に使用した場合と比較するまでもなく、櫛形交差電極３の製造プロセス工程、半田リフロー工程等に対しても十分な耐熱性、耐薬品性を示し、強力な接着力が維持できる。
【００４７】
弾性表面波素子の遅延時間温度係数は、前述のとおり単結晶圧電基板の弾性表面波伝搬方向４の線熱膨張係数と弾性表面波伝搬速度の温度係数との差によって決定する。ここで弾性表面波伝搬速度の温度係数に着目すると、タンタル酸リチウム基板やニオブ酸リチウム基板等では負の値となる性質を持っているため、線熱膨張係数との差により決まる遅延時間温度係数はより悪化する。
【００４８】
これに対して、本実施例において接着層９として用いる塗布ガラスは主成分が酸化珪素からなるため、弾性表面波伝搬速度の温度係数が正の値となり、線熱膨張係数との差により決まる遅延時間温度係数は向上する。塗布ガラスが有するこの性質を利用することにより、基板１の線熱膨張係数の値を接着層９の塗布ガラスが有する弾性表面波伝搬速度の温度係数の値によって相殺することが可能である。
【００４９】
つまり、塗布ガラスを主成分とする接着層９が有する弾性波伝搬速度の温度係数が、基板１の弾性波表面波伝搬方向４の熱膨張係数を相殺する値となるように、接着層９の膜厚を最適化することにより、接合した弾性表面波素子用基板５の弾性表面波伝搬方向４の遅延時間温度係数が改善できることになる。
【００５０】
また本実施例として、基板１と基板２の接合界面に塗布ガラスを主成分とする接着層９を有する弾性表面波素子用基板５において、基板２として接合面内にｃ軸を有する四ホウ酸リチウム基板を用い、基板１の弾性表面波伝搬方向４が基板２のｃ軸と平行となるように接合することにより、接合した弾性表面波素子用基板５において弾性表面波伝搬方向４の線熱膨張係数が改善できる。
【００５１】
四ホウ酸リチウム基板のｃ軸の線熱膨張係数は前述のように約−１．５ｐｐｍ／℃と負の線熱膨張係数を示すため、基板１の線熱膨張係数がより大きく改善できるためである。
【００５２】
また本実施例の別の実施形態として、単結晶圧電基板である基板１と、基板１に塗布ガラスを主成分とする接着層９により接合された基板２と、基板１の基板２との接合面と反対側の面上に形成され弾性表面波を励振する櫛型交差電極３とを備えた弾性表面波素子において、基板２として弾性表面波伝搬速度が非常に高速であるダイアモンド基板を用いると、接合した基板１上に形成された弾性表面波素子において励振伝搬される弾性表面波の伝搬速度が速くなるため、高周波化に対して効果がある。さらに、基板２に用いたダイアモンド基板には熱伝導性が非常に高いという性質もあるため、弾性表面波素子の熱伝導率が高くなり、櫛形交差電極３の耐電力性も向上できる。
【００５３】
つぎに本実施例の弾性表面波素子の製造方法の一例を図９により説明する。例えば基板１として用いる鏡面研磨されたＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板と、基板２として用いる鏡面研磨されたダイアモンド基板を、接合の前処理として３００℃以上の温度で１時間以上の熱処理を行う。
【００５４】
次いで、接合するタンタル酸リチウム基板とダイアモンド基板を過酸化水素（Ｈ₂Ｏ₂）とアンモニア水溶液（ＮＨ₄ＯＨ）と純水（Ｈ₂Ｏ）を混合した溶液に約１０分程度浸漬させた後、純水によるリンスを行う。２枚の基板を乾燥させた後、接着層９として塗布ガラスを介して基板接合する工程を行う。まずダイアモンド基板の接合面に塗布ガラスを回転塗布する。
【００５５】
その後、塗布ガラスを塗布したダイアモンド基板を８０℃程度に加熱したホットプレート上で５分程度加熱する。これは塗布ガラスの溶媒である有機溶剤を蒸発させるために行なう。５分間程度加熱した後、ホットプレート上でタンタル酸リチウム基板の接合面とダイアモンド基板の塗布ガラス塗布面とを接合させる。ここではパーティクルフリーの接合界面を得ることが特に重要であり、クラス１０以上のクリーン度を持つクリーンルームで基板接合を行うことが望ましい。
【００５６】
基板接合後、タンタル酸リチウム基板とダイアモンド基板に圧力をかけることで基板接合界面の気泡を完全に除去する。その後、接合された弾性表面波素子用基板５は、ダイアモンド基板の線熱膨張係数が支配的となるようにタンタル酸リチウム基板１の薄板化を行う。基板研磨装置（図示せず）を用いて、タンタル酸リチウム基板１の板厚をダイアモンド基板２の板厚に対して３分の１以下となるように研磨する。上記研磨工程は、粗研磨から仕上げ研磨を段階的に行い、鏡面研磨を実現する。なお、基板の薄膜化に関しては前記の方法にこだわるものではなく、基板１の板厚が基板２の板厚に対して３分の１以下の板厚であれば製法は特に問わない。
【００５７】
タンタル酸リチウム基板を薄板化した後、１５０℃の温度で２０分の熱処理を行い、さらに２００℃の温度で約１時間程度の熱処理を行なうことにより、２枚の基板は完全に接合される。
【００５８】
その後、図１０に示すような櫛形交差電極３を、塗布ガラスによる接着層９を介してダイアモンド基板２に接合されたタンタル酸リチウム基板１上に、通常の電極作製工程を行って作製する。このとき櫛形交差電極３により励振伝搬される弾性表面波が基板１の弾性表面波伝搬方向４（Ｘ軸方向）と一致するように櫛形交差電極３を配置する。
【００５９】
上記第２の実施例は、基板１としてＸ軸を中心にＹ軸からＺ軸方向に３６°〜４６°の角度で回転された面方位を持つタンタル酸リチウム基板について説明したが、基板１としてＸ軸を面方位とするタンタル酸リチウム、もしくはＸ軸を中心にＹ軸からＺ軸方向に４１〜６４°の範囲の角度で回転された面方位を有するニオブ酸リチウム基板を用いた場合も同様の効果がある。
【００６０】
また上記第２の実施例は、基板２として酸化珪素基板、ダイアモンド基板および四ホウ酸リチウム基板について説明したが、窒化アルミニウム、珪素、窒化珪素、硼素、酸化硼素、窒化硼素、タンタル酸リチウム、ニオブ酸リチウム、またはそれらの複合材料による基板においても同様な効果がある。
【００６１】
【発明の効果】
以上に説明したように、本発明において弾性表面波を励振伝搬させる第１の基板の弾性波表面波伝搬方向と、第１の基板と同じ材料からなる第２の基板の接合面内で最も熱膨張係数の小さい方向とを平行にして接合する構造を提案した。これにより、線熱膨張係数が改善され、遅延時間温度係数が小さい弾性表面波素子の作製が可能となる。
【００６２】
また、接合した第１の基板と第２の基板が同じ材質からなる構造であることから、非常に強力な接着力の実現でき、さらには接合界面でのバルク波反射の影響が小さい弾性表面波素子の作製が可能となる。また、同種材料基板どうしを直接接合することにより、異種材料基板を直接接合する場合と比較して基板破損の発生が減少するという効果もある。
【００６３】
また、本発明において、第１の基板と第２の基板の接合に、塗布ガラスを接着層として用いる方法を提案した。塗布ガラスを用いることにより、耐熱性、耐薬品性を有する基板接合が簡便かつ安価な方法により実現が可能となり、線熱膨張係数の小さい基板、弾性表面波伝搬速度の速い基板、および熱伝導率が高い基板など、あらゆる特性を持つ基板を第２の基板として用いることができるため弾性表面波素子の特性改善が可能となる。
【図面の簡単な説明】
【図１】本発明の第１の実施例による弾性表面波素子の斜視図。
【図２】本発明の第１の実施例による第１の基板の面方位の一例を示す説明図。
【図３】本発明の第１の実施例による第２の基板の面方位の一例を示す説明図。
【図４】本発明の第１の実施例による弾性表面波素子用基板の接合方向を示す説明図。
【図５】弾性表面波素子用基板の接合界面でのバルク波反射を示す説明図。
【図６】本発明の第１の実施例による弾性表面波素子用基板の製造工程を示す断面図。
【図７】本発明の第１の実施例による弾性表面波素子の断面図。
【図８】本発明の第２の実施例による弾性表面波素子の斜視図。
【図９】本発明の第２の実施例による弾性表面波素子用基板の製造工程を示す断面図。
【図１０】本発明の第２の実施例による弾性表面波素子の断面図。
【符号の説明】
１…第１の基板、２…第２の基板、３…櫛形交差電極、４…第１の基板の弾性表面波伝搬方向、５…弾性表面波素子用基板、６…第２の基板の熱膨張係数が最も小さい方向、７…バルク波、８…反射波、９…接着層。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an element using a surface acoustic wave used for a mobile phone or the like and a method of manufacturing the substrate.
[0002]
[Prior art]
A surface acoustic wave element used for a mobile phone or the like is, for example, tantalum acid as shown in IEICE Transactions A, J76-A, 2, 185-192 (February 1993). A comb-shaped cross electrode of a metal thin film is formed on a single crystal piezoelectric substrate such as a lithium substrate, a lithium niobate substrate, or a lithium tetraborate substrate.
[0003]
A report has been made that the delay time temperature coefficient of the surface acoustic wave device substrate used in the cellular phone or the like has been improved along with the improvement in performance of the mobile phone or the like. For example, there is an example in which a single crystal piezoelectric substrate and a glass substrate are directly bonded as disclosed in JP-A-11-55070. Furthermore, there is an example in which a single crystal piezoelectric substrate and a negative expansion glass are bonded with an ultraviolet curable resin as shown in the 20th ultrasonic symposium symposium, page 51 (November 1999).
[0004]
[Problems to be solved by the invention]
In mobile phones and the like, due to the rapid market expansion in recent years, each frequency band for transmission and reception tends to be further expanded, and there are systems in which the frequency interval between the transmission band and the reception band is very narrow. For this reason, even higher performance is required for various devices built in cellular phones and the like. In particular, in a conventional surface acoustic wave device in which a metal thin film comb-shaped cross electrode is formed on a single crystal piezoelectric substrate such as a lithium tantalate substrate or a lithium niobate substrate, when the delay time temperature coefficient is large, the interband attenuation is sufficient. Since it cannot be taken, it becomes a serious problem.
[0005]
The delay time temperature coefficient of the surface acoustic wave element is determined by the difference between the linear thermal expansion coefficient of the single crystal piezoelectric substrate and the temperature coefficient of the surface acoustic wave propagation velocity. These values are specific values of the single crystal piezoelectric substrate, and in terms of the linear thermal expansion coefficient, for example, it has a plane orientation rotated from the Y axis to the Z axis direction by an angle of 36 ° to 46 ° around the X axis. Lithium tantalate substrate X-axis, that is, about 16.1 ppm / ° C. in the direction of surface acoustic wave propagation, and lithium niobate having a plane orientation rotated about the X-axis at an angle of 64 ° from the Y-axis to the Z-axis The X axis of the substrate, that is, the surface acoustic wave propagation direction is as large as about 15.4 ppm / ° C. In the future, this is an obstacle to improving the performance of the surface acoustic wave device.
[0006]
As a method for solving the above problem, there is a method using a composite piezoelectric substrate in which a glass substrate having a small linear thermal expansion coefficient is directly bonded to a single crystal piezoelectric substrate. However, since the composite piezoelectric substrate is bonded to substrates of different materials, the influence of bulk wave reflection at the substrate bonding interface is particularly large, and the characteristics of the surface acoustic wave device are degraded (for example, in-band ripple, Or spurious response out of band).
[0007]
As for the substrate bonding method, there is a method using an adhesive or the like in addition to the direct bonding, but the applicable adhesive has no heat resistance, and a problem may occur during heat treatment in the process of forming a device. .
[0008]
In consideration of the above problems, the present invention provides a surface acoustic wave element substrate capable of improving the delay time temperature coefficient by improving the linear thermal expansion coefficient of a single crystal piezoelectric substrate that excites and propagates a surface acoustic wave, and It is an object of the present invention to realize a surface acoustic wave element on the surface acoustic wave element substrate.
[0009]
That is, with respect to the bonding method of the single crystal piezoelectric substrate, the direct bonding method is intended to realize good surface acoustic wave propagation characteristics that suppress the influence of bulk wave reflection at the substrate bonding interface, and through an adhesive layer. It is an object of the present invention to realize substrate bonding that exhibits sufficient heat resistance and chemical resistance for the manufacturing process steps of the comb-shaped cross electrode after substrate bonding in the method of substrate bonding.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a surface acoustic wave device according to the present invention includes a first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a second substrate of the first substrate. In a structure including a comb-shaped crossing electrode formed on a surface opposite to the bonding surface of the first substrate and exciting and propagating the surface acoustic wave, the surface acoustic wave propagation direction of the first substrate is changed to the bonding of the second substrate. It is characterized by being parallel to the direction having the smallest linear thermal expansion coefficient in the plane.
[0011]
In the above, in the case where the first and second substrates are directly bonded without substantially interposing the bonding layer, the first and second substrates are preferably made of the same material. In addition, when the first and second substrates according to the present invention have a configuration in which different materials are bonded, in order to enable substrate bonding that solves the problems of heat resistance and chemical resistance, it is applied to the bonding interface of the substrates. It is preferable to pass through an adhesive layer mainly composed of glass.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a perspective view showing a first embodiment of a surface acoustic wave device according to the present invention. In the figure, 1 is a single crystal piezoelectric substrate, 2 is a second substrate bonded to the substrate 1, and 3 is a comb-shaped cross electrode formed on the surface of the substrate 1 opposite to the bonding surface with the substrate 2. It is. In this embodiment, the material of the substrate 2 is the same as that of the substrate 1, but the linear thermal expansion coefficient of the substrate 2 in the surface acoustic wave propagation direction (arrow 4) of the substrate 1 is the same as that of the substrate 1. Joined to be smaller than the coefficient.
[0013]
In the surface acoustic wave element according to the present embodiment, the substrate 1 and the substrate 2 are directly bonded to each other, and the bonded substrate is used as the substrate 5 for the surface acoustic wave element. The surface acoustic wave excited by the comb-shaped crossing electrode 3 formed on the substrate 1 propagates on the substrate 1 and functions as a surface acoustic wave element. Since the electrode fingers of the comb-shaped cross electrode 3 are formed in a direction perpendicular to the X axis of the substrate 1, the surface acoustic wave propagates in a direction parallel to the X axis of the substrate 1.
[0014]
In a surface acoustic wave element in which a metal thin film comb-shaped cross electrode 3 is formed on a substrate 1, the delay time temperature coefficient is the linear thermal expansion coefficient in the surface acoustic wave propagation direction 4 of the substrate 1 and the temperature coefficient of the surface acoustic wave propagation velocity. Determine by difference. These values are specific to the single crystal piezoelectric substrate, for example, the elastic surface of the lithium tantalate substrate having a plane orientation rotated about the X axis from the Y axis in the Z axis direction by an angle of 36 ° to 46 °. The coefficient of linear thermal expansion in the wave propagation direction 4 (X-axis direction) is about 16.1 ppm / ° C., which is not a good value.
[0015]
Among the single crystal piezoelectric substrates currently used for surface acoustic wave elements, a quartz substrate is one having a good delay time temperature coefficient. In the case of a quartz substrate, the coefficient of linear thermal expansion in the surface acoustic wave propagation direction 4 is about 13.71 ppm / ° C., which is not a very good value, but the temperature coefficient of the surface acoustic wave propagation velocity is a lithium tantalate substrate or niobic acid substrate. Since it has a property of being a positive value contrary to a lithium substrate or the like, the value of the linear thermal expansion coefficient is offset by the value of the temperature coefficient of the surface acoustic wave propagation velocity, and the delay time temperature coefficient shows a small value. However, the quartz substrate has a drawback that the electromechanical coupling coefficient is small and a sufficient frequency bandwidth cannot be obtained. At present, no single crystal piezoelectric substrate having a good both electromechanical coupling coefficient and delay time temperature coefficient has been found.
[0016]
In this example, in order to realize a surface acoustic wave element having a small delay time temperature coefficient using a single crystal piezoelectric substrate having a large electromechanical coupling coefficient, the surface acoustic wave propagation direction 4 of the substrate 1 which is a single crystal piezoelectric substrate is used. And the second substrate 2 are bonded in parallel with a direction having a small linear thermal expansion coefficient. Thereby, the linear thermal expansion coefficient of the substrate 1 is suppressed by the linear thermal expansion coefficient of the substrate 2, and the delay time temperature coefficient is improved.
[0017]
FIG. 2 shows an example of the plane orientation of the substrate 1 according to this embodiment, and FIG. 3 shows an example of the plane orientation of the substrate 2 according to this embodiment. An arrow 6 in FIG. 3 indicates a direction in which the thermal expansion coefficient of the second substrate is the smallest. Here, a lithium tantalate substrate having a plane orientation rotated by an angle of 36 ° to 46 ° in the Z-axis direction from the Y axis as the substrate 1 is used as the substrate 1, and the substrate 2 made of the same material as the substrate 1 is used as the substrate 2. A lithium tantalate substrate having an axial orientation is used.
[0018]
FIG. 4 is a diagram showing a bonding direction when the substrate 1 and the substrate 2 are bonded. Here, the linear thermal expansion coefficients of the substrate 1 and the substrate 2 are considered. In a lithium tantalate substrate having a plane orientation rotated from the Y axis in the Z-axis direction by an angle of 36 ° to 46 ° around the X axis as the substrate 1, the surface acoustic wave propagation direction 4 is in the X-axis direction. The linear thermal expansion coefficient is about 16.1 ppm / ° C. On the other hand, the thermal expansion coefficient of the lithium tantalate substrate having the surface orientation in the Y-axis direction, which is the substrate 2, is indicated by a very small coefficient (indicated by the arrow 6. Here, in the X-axis direction, which is the propagation direction of the surface acoustic wave) The coefficient of linear thermal expansion in the Z-axis direction perpendicular to the surface is about 4.1 ppm / ° C., which is the smallest in this plane.
[0019]
According to the present invention, as shown in FIG. 4, the X-axis direction, which is the surface acoustic wave propagation direction 4 of the substrate 1, and the Z-axis direction 6 having a very small linear thermal expansion coefficient of the substrate 2 are joined in parallel. Since the linear thermal expansion coefficient of the substrate 1 is suppressed by the linear thermal expansion coefficient of the substrate 2, the linear thermal expansion coefficient in the surface acoustic wave propagation direction 4 can be improved. However, since the linear thermal expansion coefficient of the substrate 1 does not directly become the linear thermal expansion coefficient of the substrate 2, it becomes a numerical value according to the thermal stress generated in the bonding surface due to the thermal expansion difference between the substrate 1 and the substrate 2. The substrate thickness of the substrate 2 is important. As a result of the examination, the substrate 1 is thinned so that the thickness of the substrate 2 is three times or more of the thickness of the substrate 1, whereby linear heat in the surface acoustic wave propagation direction in the bonded surface acoustic wave element substrate 5 is obtained. It was found that the expansion coefficient can be improved more significantly.
[0020]
Here, the plate thickness of the lithium tantalate substrate having a plane orientation rotated from the Y axis to the Z axis direction by an angle of 36 ° to 46 ° around the X axis as the substrate 1 is 90 μm, and the Y axis as the substrate 2 By setting the thickness of the lithium tantalate substrate having the plane orientation in the direction to 270 μm, the linear thermal expansion coefficient of the lithium tantalate substrate having the plane orientation in the Y-axis direction becomes dominant, and the linear thermal expansion coefficient is improved. . As a result of measuring the delay time temperature coefficient in this case, it was 24 ppm / ° C. Since the temperature coefficient of delay time of a conventional surface acoustic wave element that does not perform substrate bonding is 33 ppm / ° C., the present invention has an improvement effect of 9 ppm / ° C. Further, by further reducing the thickness of the substrate 1, a greater effect can be obtained.
[0021]
In addition, according to this embodiment, the bonded substrate 1 and the substrate 2 have a structure made of the same material, that is, a structure in which the lattice constant at the bonded interface is the same, and therefore, a single crystal piezoelectric substrate and a glass substrate are representative. Compared with the joining of such dissimilar material substrates, stronger adhesion can be realized. That is, the lithium tantalate substrate having a plane orientation rotated around the X axis by an angle of 36 ° to 46 ° from the Y axis in the Z axis direction and the lithium tantalate substrate having a plane orientation in the Y axis direction are made of the same material. Therefore, it is possible to realize a very strong adhesive force.
[0022]
The influence of bulk wave reflection at the substrate bonding interface according to this embodiment will be described with reference to FIG. When the thickness of the substrate 1 is reduced so that the thickness of the substrate 2 is three times or more the thickness of the substrate 1, the surface of the substrate 1 and the substrate bonding interface come close to each other as shown in FIG. The influence of the reflected wave 8 from the substrate bonding interface of the bulk wave 7 becomes larger. However, according to the present embodiment, as shown in (b), since the bonded substrate 1 and the substrate 2 are made of the same material, the substrate of the bulk wave 7 is compared with the case where different material substrates are bonded. The influence of the reflected wave 8 from the joint interface is reduced.
[0023]
That is, the lithium tantalate substrate having a plane orientation rotated about the X axis from the Y axis by an angle of 36 ° to 46 ° from the Y axis and the lithium tantalate substrate having a plane orientation in the Y axis direction are the same material. Therefore, the influence of the reflection at the bonding interface is small, and the surface acoustic wave of this embodiment having this structure can reduce the deterioration of the element characteristics due to the bulk wave reflection from the bonding interface.
[0024]
Also, when dissimilar substrates are directly joined together, the problem of substrate damage is likely to occur due to differences in the linear thermal expansion coefficients of the joined substrates and uneven thermal stress between the voids and joined parts. According to the example, since the bonded substrates 1 and 2 are made of the same material, the problem of substrate damage is less likely to occur compared to direct bonding of dissimilar material substrates.
[0025]
Next, an example of a method for manufacturing the surface acoustic wave device of the present invention will be described with reference to FIG. For example, as the substrate 1, a mirror-polished lithium tantalate substrate having a plane orientation rotated around the X axis from the Y axis in the Z axis direction by an angle of 36 ° to 46 ° is prepared. As the substrate 2, a mirror-polished lithium tantalate substrate having a surface orientation in the Y-axis direction is prepared. As a pretreatment for joining the two, heat treatment is performed at a temperature of 300 ° C. or higher for 1 hour or longer. This is performed for the purpose of removing gases and organic substances adhering to the surfaces of the substrate 1 and the substrate 2. If this process is neglected, voids may occur at the bonding interface after substrate bonding.
[0026]
Next, the two lithium tantalate substrates to be bonded are combined with hydrogen peroxide (H ₂ O ₂ ) And aqueous ammonia (NH _Four OH) and pure water (H ₂ After immersing in the solution mixed with O) for about 10 minutes, rinsing with pure water is performed. This has an effect of imparting hydrophilicity to the surfaces of the substrate 1 and the substrate 2 and bonding them by van der Waals force acting between water molecules adsorbed on the substrate surface at the time of substrate bonding.
[0027]
Then, after drying two lithium tantalate substrates, substrate bonding is performed at room temperature in an air atmosphere. Here, it is particularly important to obtain a particle-free bonding interface, and it is desirable to perform substrate bonding in a clean room having a cleanness of class 10 or higher after the cleaning. In addition, the particle-free interface and the hydrophilic interface can be made compatible by performing cleaning immediately before bonding.
[0028]
After that, the two lithium tantalate substrates bonded to each other are centered on the X axis which is the substrate 1 so that the linear thermal expansion coefficient of the lithium tantalate substrate having the plane orientation in the Y axis direction which is the substrate 2 becomes dominant. Then, a lithium tantalate substrate having a plane orientation rotated at an angle of 36 ° to 46 ° in the Z-axis direction from the Y-axis is thinned. Using a substrate polishing apparatus, the thickness of the lithium tantalate substrate having a plane orientation rotated from the Y axis to the Z axis direction by an angle of 36 ° to 46 ° around the X axis, and the plane orientation in the Y axis direction. Polishing is performed so that the thickness of the lithium tantalate substrate is 1/3 or less.
[0029]
The polishing process performs rough polishing to finish polishing step by step to realize mirror polishing. At this time, as shown here, the thickness is not reduced by a polishing process after bonding the substrates, but the thickness is less than one third of that of a lithium tantalate substrate having a surface orientation in the Y-axis direction in advance. A lithium tantalate substrate having a plane orientation rotated by an angle of 36 ° to 46 ° in the Z-axis direction from the Y axis around the axis may be prepared and bonded. The manufacturing method is not particularly limited as long as the thickness is 1/3 or less of the thickness.
[0030]
After the substrate 1 is thinned, the two lithium tantalate substrates are completely joined by performing a heat treatment at a temperature of 250 ° C. for about 2 hours. Thereafter, a comb-shaped cross electrode 3 as shown in FIG. 7 is produced by performing a normal electrode production process on the substrate 1 bonded to the substrate 2. At this time, the comb-shaped cross electrode 3 is arranged so that the surface acoustic wave excited and propagated by the comb-shaped cross electrode 3 coincides with the surface acoustic wave propagation direction (X-axis direction) of the substrate 1.
[0031]
In the first embodiment, as the substrate 1, a lithium tantalate substrate having a plane orientation rotated about the X axis from the Y axis in the Z axis direction by an angle of 36 ° to 46 ° as the substrate 1, a substrate made of the same material Although an example using a lithium tantalate substrate having a surface orientation in the Y-axis direction as 2 has been described, the same effect can be obtained when a lithium tantalate substrate having a surface orientation in the X-axis direction is used as the substrate 2.
[0032]
Similarly, a lithium tantalate substrate having a plane orientation rotated from the Y axis by an angle of 36 ° to 46 ° around the X axis as the substrate 1 is used as the substrate 1, and the substrate 1 made of the same material as the substrate 1 is used. The same effect can be obtained when a lithium tantalate substrate having the same plane orientation is used and bonded so that the X-axis direction of the substrate 1 is orthogonal to the X-axis direction of the substrate 2.
[0033]
Similarly, a lithium tantalate substrate having a plane orientation in the X-axis direction is used as the substrate 1, and a lithium tantalate substrate having a plane orientation in the Y-axis direction or the X-axis direction is used as the substrate 2 made of the same material. The same effect can be obtained when bonding is performed so that the direction rotated at an angle of 112 ° from the Y axis, which is the surface acoustic wave propagation direction 4, to the Z axis direction is parallel to the Z axis direction of the substrate 2.
[0034]
Similarly, a lithium niobate substrate having a plane orientation rotated at an angle of 41 ° to 64 ° in the Z-axis direction from the Y-axis as the substrate 1 is used as the substrate 1, and the Y-axis direction is used as the substrate 2 made of the same material. Alternatively, the same effect can be obtained when a lithium niobate substrate having a plane orientation in the X-axis direction is used and bonded so that the X-axis direction of the substrate 1 is parallel to the Z-axis direction of the substrate 2.
[0035]
Similarly, a lithium niobate substrate having a plane orientation rotated at an angle of 41 ° to 64 ° from the Y axis to the Z axis direction around the X axis is used as the substrate 1, and the substrate 1 is made of the same material as the substrate 1. The same effect can be obtained when a lithium niobate substrate having the same plane orientation is used and bonded so that the X-axis direction of the substrate 1 is orthogonal to the X-axis direction of the substrate 2.
[0036]
Further, a lithium tetraborate substrate is used as the substrate 1, a lithium tetraborate substrate having a c-axis in the joint surface is used as the substrate 2 made of the same material, and the surface acoustic wave propagation direction 4 of the substrate 1 is c of the substrate 2. The same effect can be obtained in the surface acoustic wave element substrate 5 bonded so as to be parallel to the axial direction.
[0037]
Considering the linear thermal expansion coefficients of the substrate 1 and the substrate 2 in this case, the linear thermal expansion coefficient in the a-axis direction of the lithium tetraborate substrate which is the substrate 1 is about 13 ppm / ° C., whereas The linear thermal expansion coefficient of the c-axis of a certain lithium tetraborate substrate is about −1.5 ppm / ° C., which is a negative linear thermal expansion coefficient. Therefore, by bonding the substrates so that the a-axis direction of the lithium tetraborate substrate and the c-axis direction of the lithium tetraborate substrate are parallel, the linear thermal expansion coefficient in the a-axis direction is about 13 ppm / ° C. The linear thermal expansion coefficient in the surface acoustic wave propagation direction can be improved in the bonded surface acoustic wave element substrate 5 by the linear thermal expansion coefficient of about −1.5 ppm / ° C.
[0038]
Next, another embodiment of the present invention will be described. FIG. 8 is a perspective view showing a second embodiment of the surface acoustic wave device according to the present invention. The surface acoustic wave element shown in FIG. 8 is formed on the surface of the substrate 1 that is a single crystal piezoelectric substrate, the substrate 2 bonded to the substrate 1, and the surface of the substrate 1 opposite to the bonding surface of the substrate 2. The surface acoustic wave element includes a comb-shaped cross electrode 3 that excites the substrate, and the substrate 1 and the substrate 2 are bonded to each other with a coating glass (SOG: Spin On Grass) as a main component at the bonding interface between the substrate 1 and the substrate 2. An adhesive layer 9 is provided.
[0039]
The linear thermal expansion coefficient of the substrate 2 in the propagation direction 4 of the surface acoustic wave of the substrate 1 is bonded so as to be smaller than the linear thermal expansion coefficient of the substrate 1 in the same direction. Further, the thickness of the substrate 1 is reduced so that the thickness of the substrate 2 is three times or more the thickness of the substrate 1. A substrate obtained by bonding the substrate 1 and the substrate 2 using coated glass as the adhesive layer 9 is used as the surface acoustic wave element substrate 5. The surface acoustic wave excited by the comb-shaped cross electrode 3 formed on the substrate 1 propagates on the substrate 1 and functions as a surface acoustic wave element.
[0040]
The coated glass used as the adhesive layer 9 can be formed by a coating / firing method with a film mainly composed of silicon oxide, and is obtained by dissolving a silicon compound in an organic solvent. Here, a lithium tantalate substrate having a plane orientation rotated from the Y axis in the Z-axis direction by an angle of 36 ° to 46 ° around the X axis is used as the substrate 1, and a silicon oxide substrate is used as the substrate 2.
[0041]
According to the present embodiment, in the bonding of the substrate 1 and the substrate 2, by using the coated glass whose main component is silicon oxide as the adhesive layer 9, the adhesive layer 9 itself has a small delay time temperature coefficient. In comparison with the case where a curable resin or the like is used as an adhesive, the delay time temperature coefficient due to the adhesive layer 9 is not deteriorated, so that the delay time temperature with respect to the surface acoustic wave propagation direction 4 of the bonded surface acoustic wave element substrate 5 is reduced. The coefficient is further improved. The coated glass is very hard because the main component is made of silicon oxide, and even when stress due to thermal expansion of the substrate 1 is generated, the elongation of the piezoelectric substrate 1 is suppressed as compared with, for example, an ultraviolet curable resin. This is effective in improving the linear thermal expansion coefficient.
[0042]
The surface acoustic wave element substrate 5 in which the substrate 1 and the substrate 2 are joined is a manufacturing process for producing a surface acoustic wave element after joining the substrates. In the previous process, a metal thin film deposition process, a photolithography process, an etching process, and a subsequent process. Has a process involving a heat treatment such as a solder reflow process, so that heat resistance is important. In addition, chemical resistance is also important because organic and inorganic chemicals are used in each step. Therefore, when the substrate 1 and the substrate 2 are bonded using the adhesive layer 9, the adhesive layer 9 must have heat resistance and chemical resistance.
[0043]
As an example, a case where an ultraviolet curable resin is used for the adhesive layer 9 will be described. After applying the UV curable resin to the bonding surface of the substrate 2 and bonding the substrates, the UV curable resin is cured and the substrate bonding is completed simply by irradiating the ultraviolet rays, so that a very simple substrate that does not require heat treatment. It is a joining method. However, although the chemical resistance is sufficient as a characteristic of the ultraviolet curable resin, since the heat resistance is as low as about 120 ° C., the application as the adhesive layer 9 is difficult.
[0044]
As another example, a case where a thermosetting resin is used for the adhesive layer 9 will be described. After the thermosetting resin is applied to the bonding surface of the substrate 2 and the solvent is volatilized and cured by heat treatment, the substrate 2 to which the thermosetting resin is applied is heated again to soften the thermosetting resin. The substrate 1 is joined, and the thermosetting resin is cured by cooling after joining the substrates, and the joining is completed. However, since the chemical resistance of the thermosetting resin is fragile and it may be softened by reheating after bonding the substrates, application as the adhesive layer 9 is difficult.
[0045]
As another example, a case where an adhesive wax is used for the adhesive layer 9 will be described. The bonding wax is applied to the bonding surface of the substrate 2 heated by a hot plate or the like, the substrate 1 is bonded in a state where the bonding wax is melted, and then the bonding wax is cured by cooling to complete the bonding. This is a simple substrate bonding method. However, in addition to the low heat resistance of the adhesive wax, it is difficult to apply as the adhesive layer 9 because it is not chemically resistant enough to dissolve even in alcohol.
[0046]
In this embodiment, the coated glass made of silicon oxide as a main component used as the adhesive layer 9 when bonding the substrate 1 and the substrate 2 exhibits sufficient heat resistance even at a heat treatment of 400 ° C. or higher, and chemical resistance. In order to show high resistance in accordance with silicon oxide, it is not necessary to compare with the case where the ultraviolet curable resin, thermosetting resin, adhesive wax or the like is used for the adhesive layer. It exhibits sufficient heat resistance and chemical resistance for solder reflow processes, etc., and can maintain a strong adhesive force.
[0047]
The delay time temperature coefficient of the surface acoustic wave element is determined by the difference between the linear thermal expansion coefficient in the surface acoustic wave propagation direction 4 of the single crystal piezoelectric substrate and the temperature coefficient of the surface acoustic wave propagation velocity as described above. Here, focusing on the temperature coefficient of surface acoustic wave propagation velocity, the lithium tantalate substrate and the lithium niobate substrate have negative values, so the delay time temperature coefficient determined by the difference from the linear thermal expansion coefficient Will get worse.
[0048]
On the other hand, since the coating glass used as the adhesive layer 9 in this embodiment is mainly composed of silicon oxide, the temperature coefficient of the surface acoustic wave propagation velocity becomes a positive value, and the delay determined by the difference from the linear thermal expansion coefficient. The time temperature coefficient is improved. By utilizing this property of the coated glass, the value of the linear thermal expansion coefficient of the substrate 1 can be offset by the temperature coefficient of the surface acoustic wave propagation velocity of the coated glass of the adhesive layer 9.
[0049]
That is, the adhesive layer 9 of the adhesive layer 9 has a temperature coefficient of elastic wave propagation velocity that is a value that cancels out the thermal expansion coefficient of the surface acoustic wave propagation direction 4 of the substrate 1. By optimizing the film thickness, the temperature coefficient of delay time in the surface acoustic wave propagation direction 4 of the bonded surface acoustic wave element substrate 5 can be improved.
[0050]
In this example, in the surface acoustic wave element substrate 5 having the adhesive layer 9 mainly composed of coated glass at the bonding interface between the substrate 1 and the substrate 2, tetraboric acid having a c-axis in the bonding surface as the substrate 2 is used. By using a lithium substrate and bonding so that the surface acoustic wave propagation direction 4 of the substrate 1 is parallel to the c-axis of the substrate 2, the linear heat in the surface acoustic wave propagation direction 4 in the bonded surface acoustic wave element substrate 5 is obtained. The expansion coefficient can be improved.
[0051]
Since the linear thermal expansion coefficient of the c-axis of the lithium tetraborate substrate shows a negative linear thermal expansion coefficient of about −1.5 ppm / ° C. as described above, the linear thermal expansion coefficient of the substrate 1 can be greatly improved. is there.
[0052]
As another embodiment of the present example, a substrate 1 which is a single crystal piezoelectric substrate, a substrate 2 bonded to the substrate 1 with an adhesive layer 9 mainly composed of a coated glass, and a substrate 2 of the substrate 1 are bonded. In a surface acoustic wave device including a comb-shaped cross electrode 3 formed on a surface opposite to the surface and exciting surface acoustic waves, a diamond substrate having a very high surface acoustic wave propagation velocity is used as the substrate 2. Since the propagation speed of the surface acoustic wave excited and propagated in the surface acoustic wave element formed on the bonded substrate 1 is increased, it is effective for increasing the frequency. Furthermore, since the diamond substrate used for the substrate 2 has a property of extremely high thermal conductivity, the thermal conductivity of the surface acoustic wave element is increased, and the power durability of the comb-shaped cross electrode 3 can be improved.
[0053]
Next, an example of a method for manufacturing the surface acoustic wave device of this embodiment will be described with reference to FIG. For example, a lithium tantalate substrate having a plane orientation rotated at an angle of 36 ° to 46 ° from the Y axis in the Z-axis direction around the mirror-polished X-axis used as the substrate 1 and the mirror-polished used as the substrate 2 The diamond substrate is heat-treated at a temperature of 300 ° C. or higher for 1 hour or longer as a pretreatment for bonding.
[0054]
Next, the lithium tantalate substrate and the diamond substrate to be bonded are combined with hydrogen peroxide (H ₂ O ₂ ) And aqueous ammonia (NH _Four OH) and pure water (H ₂ After immersing in the solution mixed with O) for about 10 minutes, rinsing with pure water is performed. After the two substrates are dried, a step of bonding the substrates as the adhesive layer 9 through the coated glass is performed. First, coating glass is spin-coated on the bonding surface of the diamond substrate.
[0055]
Thereafter, the diamond substrate coated with the coated glass is heated on a hot plate heated to about 80 ° C. for about 5 minutes. This is performed in order to evaporate the organic solvent that is the solvent of the coated glass. After heating for about 5 minutes, the bonding surface of the lithium tantalate substrate and the coated glass coated surface of the diamond substrate are bonded on a hot plate. Here, it is particularly important to obtain a particle-free bonding interface, and it is desirable to perform substrate bonding in a clean room having a cleanness of class 10 or higher.
[0056]
After the substrate bonding, the bubbles at the substrate bonding interface are completely removed by applying pressure to the lithium tantalate substrate and the diamond substrate. Thereafter, the bonded surface acoustic wave element substrate 5 is thinned so that the linear thermal expansion coefficient of the diamond substrate becomes dominant. Using a substrate polishing apparatus (not shown), the thickness of the lithium tantalate substrate 1 is polished so that it is one third or less of the thickness of the diamond substrate 2. In the polishing step, mirror polishing is realized by performing rough polishing to finish polishing step by step. The method for reducing the thickness of the substrate is not particularly limited to the above-described method, and the manufacturing method is not particularly limited as long as the thickness of the substrate 1 is one third or less of the thickness of the substrate 2.
[0057]
After thinning the lithium tantalate substrate, a heat treatment is performed at a temperature of 150 ° C. for 20 minutes, and further a heat treatment is performed at a temperature of 200 ° C. for about one hour, whereby the two substrates are completely bonded.
[0058]
After that, the comb-shaped cross electrode 3 as shown in FIG. 10 is manufactured by performing a normal electrode manufacturing process on the lithium tantalate substrate 1 bonded to the diamond substrate 2 through the adhesive layer 9 made of coated glass. At this time, the comb-shaped cross electrode 3 is arranged so that the surface acoustic wave excited and propagated by the comb-shaped cross electrode 3 coincides with the surface acoustic wave propagation direction 4 (X-axis direction) of the substrate 1.
[0059]
In the second embodiment, the lithium tantalate substrate having the plane orientation rotated about the X axis from the Y axis in the Z axis direction by an angle of 36 ° to 46 ° as the substrate 1 has been described. The same applies when using lithium tantalate with the X axis as the plane orientation, or a lithium niobate substrate having a plane orientation rotated from the Y axis to the Z axis in the range of 41 to 64 ° around the X axis. There is an effect.
[0060]
In the second embodiment, a silicon oxide substrate, a diamond substrate, and a lithium tetraborate substrate have been described as the substrate 2. However, aluminum nitride, silicon, silicon nitride, boron, boron oxide, boron nitride, lithium tantalate, niobium are used. The same effect can be obtained in a substrate made of lithium acid or a composite material thereof.
[0061]
【The invention's effect】
As described above, in the present invention, the surface acoustic wave propagation direction of the first substrate for exciting and propagating the surface acoustic wave and the most heat in the bonding surface of the second substrate made of the same material as the first substrate. A structure was proposed in which the direction of expansion coefficient was parallel to the joint. Thereby, the linear thermal expansion coefficient is improved, and a surface acoustic wave device having a small delay time temperature coefficient can be manufactured.
[0062]
In addition, since the bonded first substrate and the second substrate are made of the same material, a very strong adhesive force can be realized, and the surface acoustic wave is less affected by bulk wave reflection at the bonded interface. An element can be manufactured. Further, by directly bonding the same kind of material substrates, there is an effect that the occurrence of the substrate damage is reduced as compared with the case of directly bonding the different material substrates.
[0063]
In the present invention, a method has been proposed in which coated glass is used as an adhesive layer for bonding the first substrate and the second substrate. By using coated glass, it becomes possible to realize heat- and chemical-resistant substrate bonding by a simple and inexpensive method, a substrate with a low coefficient of linear thermal expansion, a substrate with a high surface acoustic wave propagation velocity, and thermal conductivity. Since a substrate having all characteristics, such as a substrate having a high value, can be used as the second substrate, the characteristics of the surface acoustic wave element can be improved.
[Brief description of the drawings]
FIG. 1 is a perspective view of a surface acoustic wave device according to a first embodiment of the invention.
FIG. 2 is an explanatory diagram showing an example of the plane orientation of the first substrate according to the first embodiment of the present invention.
FIG. 3 is an explanatory diagram showing an example of a plane orientation of a second substrate according to the first embodiment of the present invention.
FIG. 4 is an explanatory view showing the bonding direction of the surface acoustic wave device substrate according to the first embodiment of the present invention.
FIG. 5 is an explanatory diagram showing bulk wave reflection at a bonding interface of a surface acoustic wave device substrate.
FIG. 6 is a cross-sectional view showing a manufacturing process of the surface acoustic wave device substrate according to the first embodiment of the present invention.
FIG. 7 is a sectional view of a surface acoustic wave device according to a first embodiment of the invention.
FIG. 8 is a perspective view of a surface acoustic wave device according to a second embodiment of the invention.
FIG. 9 is a cross-sectional view showing a manufacturing process of a surface acoustic wave device substrate according to a second embodiment of the present invention.
FIG. 10 is a cross-sectional view of a surface acoustic wave device according to a second embodiment of the invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... 1st board | substrate, 2 ... 2nd board | substrate, 3 ... Comb-shaped cross electrode, 4 ... Surface acoustic wave propagation direction of 1st board | substrate, 5 ... Substrate for surface acoustic wave elements, 6 ... Heat of 2nd board | substrate The direction with the smallest expansion coefficient, 7 ... bulk wave, 8 ... reflected wave, 9 ... adhesive layer.

Claims

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. thermal expansion coefficient of the second substrate in a direction, rather smaller than the thermal expansion coefficient of the same direction of the first substrate,
The first and second substrates are lithium tantalate, the Z-axis of the second substrate is present in the bonding surface of the second substrate, and the propagation direction of the elastic wave of the first substrate is A surface acoustic wave element, wherein the surface acoustic wave element is parallel to a Z-axis of the second substrate .

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium tantalate, and the plane orientation of the first substrate is rotated at an angle in the range of 36 ° to 46 ° from the Y axis to the Z axis direction around the X axis. The surface orientation of the second substrate is the Y-axis direction or the X-axis direction, the propagation direction of the elastic wave of the first substrate is the X-axis direction of the first substrate, and the first substrate The surface acoustic wave device according to claim 1, wherein the X-axis of the substrate is parallel to the Z-axis of the second substrate.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium tantalate, and the plane orientations of the first and second substrates rotate at an angle in the range of 36 ° to 46 ° from the Y axis to the Z axis direction around the X axis. The direction of propagation of the elastic wave of the first substrate is the X-axis direction of the first substrate, and the X-axis of the first substrate is orthogonal to the X-axis of the second substrate. A surface acoustic wave device.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium tantalate, the plane orientation of the first substrate is the X-axis direction, and the plane orientation of the second substrate is the Y-axis direction or the X-axis direction; The propagation direction of the elastic wave of the first substrate is a direction rotated at an angle of 112 ° from the Y axis of the first substrate in the Z axis direction, and the Z axis direction from the Y axis of the first substrate. The surface acoustic wave device is characterized in that the direction rotated at an angle of 112 ° is parallel to the Z-axis of the second substrate.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium niobate, the Z-axis of the second substrate is present in the bonding surface of the second substrate, and the propagation direction of the elastic wave of the first substrate The surface acoustic wave device is parallel to the Z-axis of the second substrate.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium niobate, and the plane orientation of the first substrate is rotated at an angle in the range of 41 to 64 degrees from the Y axis to the Z axis direction around the X axis. The surface orientation of the second substrate is the Y-axis direction or the X-axis direction, the propagation direction of the elastic wave of the first substrate is the X-axis direction of the first substrate, and the first substrate The surface acoustic wave device according to claim 1, wherein the X-axis of the substrate is parallel to the Z-axis of the second substrate.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The first and second substrates are lithium niobate, and the plane orientation of the first and second substrates is an angle in the range of 41 to 64 degrees from the Y axis to the Z axis direction around the X axis. The direction of rotation, the propagation direction of the elastic wave of the first substrate is the X-axis direction of the first substrate, and the X-axis of the first substrate is the X-axis of the second substrate. A surface acoustic wave device characterized by being orthogonal.

Formed on a surface of the first substrate which is a single crystal piezoelectric substrate, a second substrate bonded to the first substrate, and a surface of the first substrate opposite to the bonding surface of the second substrate. In the surface acoustic wave device including the comb-shaped crossing electrode for exciting the elastic wave, the second substrate is a substrate made of the same material as the first substrate, and the propagation of the elastic wave of the first substrate is performed. The thermal expansion coefficient of the second substrate in the direction is smaller than the thermal expansion coefficient in the same direction of the first substrate,
The second substrate is a lithium tetraborate single crystal, and the c-axis of the lithium tetraborate single crystal of the second substrate is present in the bonding surface of the second substrate, and The surface acoustic wave device is characterized in that the propagation direction of the acoustic wave is parallel to the c-axis of the lithium tetraborate single crystal of the second substrate.

The surface acoustic wave device according to any one of claims 1 to 8 ,
The surface acoustic wave device according to claim 1, wherein the thickness of the second substrate is at least three times the thickness of the first substrate.