JP2004172991A - Surface wave instrument and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface wave instrument of excellent frequency temperature characteristics, a large electromechanical coupling coefficient and small propagation loss. <P>SOLUTION: The surface wave device 1 is provided with a piezoelectric substrate 2 composed of 17°-58°rotated Y plate X propagation LiTaO<SB>3</SB>, an IDT 3 formed on the piezoelectric substrate 2 and composed of tantalum, for which a standard film thickness H/λ is in the range of 0.004-0.055 when defining a film thickness as H and the wavelength of surface waves as λ, and an SiO<SB>2</SB>film formed on the piezoelectric substrate 2 so as to cover the IDT 3, for which the standard film thickness Hs/λ is in the range of 0.10-0.40. <P>COPYRIGHT: (C)2004,JPO

Description

【００３３】
【表２】

【００３４】
すなわち、表１及び表２から明らかなように、タンタルよりなる電極の膜厚Ｈ／λが、０．０１〜０．０５５の場合、温度特性を改善するために、ＳｉＯ_２膜の規格化膜厚を０．１〜０．４の範囲とした場合、ＬｉＴａＯ_３のオイラー角におけるθは、１０７°〜１４８°の範囲、すなわち、回転角で１７°〜５８°の範囲、より好ましくは、ＳｉＯ_２の膜厚に応じて表１に示すオイラー角を選択すればよいことがわかる。
【００３５】
同様に、表２から明らかなように、タンタル膜からなる電極の規格化膜厚が０．０１６〜０．０４５であり、周波数温度特性を改善するために、ＳｉＯ_２膜の膜厚を０．１〜０．４の範囲とした場合には、ＬｉＴａＯ_３基板のオイラー角は１０９°〜１４４°の範囲とすればよく、より好ましくはＳｉＯ_２膜の膜厚に応じて表２のオイラー角を選択すればよいことがわかる。
【００３６】
ＬｉＴａＯ_３のオイラー角の範囲は、減衰定数が０．０５以下の範囲を規定したものである。また、表１及び表２におけるＬｉＴａＯ_３のオイラー角のより好ましい範囲は、減衰定数が０．０２５以下に規定したものである。また、タンタルからなる電極膜の規格化膜厚が０．０１２、０．０１５、０．０４２、０．０５３である場合のＳｉＯ_２膜の膜厚とオイラー角の関係は、図８〜図１１に示すタンタルからなる電極膜の規格化膜厚から換算して求めて、表１及び表２のＳｉＯ_２膜の膜厚とオイラー角の値を求めている。
【００３７】
また、図１４（ａ），（ｂ），（ｃ）は、上記実施例の弾性表面波フィルタにおける表面の走査型電子顕微鏡写真である。ここでは、Ｈ／λ＝０．０２５の規格化膜厚のタンタルからなるＩＤＴ上に、規格化膜厚Ｈｓ／λ＝０．３のＳｉＯ_２膜が形成されている前後の場合の結果が示されている。図１４（ｂ）の成膜後の写真から明らかなように、ＳｉＯ_２膜の表面にクラックは見られず、従って、クラックによる特性の劣化も生じ難いことがわかる。Ａｌ電極に比べ、タンタル電極は薄い膜厚で大きな電気機械結合係数と反射係数が得られる。そのため、薄いタンタル電極の上にＳｉＯ_２が成膜されても図１４（ｂ），（ｃ）に示すようにＳｉＯ_２に大きな段差やクラックが生じないという利点がある。
【００３８】
本発明に係る弾性表面波装置の製造に際しては、回転Ｙ板Ｘ伝搬ＬｉＴａＯ_３基板上にタンタルを主成分とする金属からなるＩＤＴを形成した後、その状態において周波数調整を行い、しかる後減衰定数αを小さくし得る範囲の膜厚のＳｉＯ_２膜を成膜することが望ましい。これを、図１５及び図１６を参照して説明する。図１５は、オイラー角（０°，１２６°，０°）の回転Ｙ板Ｘ伝搬ＬｉＴａＯ_３基板上に、タンタルからなるＩＤＴ及びＳｉＯ_２膜を形成した場合の、タンタルの規格化膜厚Ｈ／λと、ＳｉＯ_２膜の規格化膜厚Ｈｓ／λと、漏洩弾性表面波の音速との関係を示す。また、図１６は、同じオイラー角のＬｉＴａＯ_３基板上に、種々の膜厚のタンタルからなるＩＤＴを形成し、その上に形成されるＳｉＯ_２膜の規格化膜厚を変化させた場合の漏洩弾性表面波の音速の変化を示す。図１５と図１６を比較すれば明らかなように、タンタルの膜厚を変化させた場合の方が、ＳｉＯ_２膜の膜厚を変化させた場合よりも表面波の音速の変化がはるかに大きい。従って、ＳｉＯ_２膜の形成に先立ち、周波数調整が、行われることが望ましく、例えば、レーザーエッチングやイオンエッチングなどによりタンタルからなるＩＤＴを形成した後に周波数調整を行うことが望ましい。
【００３９】
なお、本発明は、上記のように、１７°〜５８°回転Ｙ板Ｘ伝搬ＬｉＴａＯ_３からなる圧電基板、Ｈ／λ＝０．００４〜０．０５５であるタンタルよりなるＩＤＴと、Ｈｓ／λ＝０．１０〜０．４０であるＳｉＯ_２膜とを有することを特徴とするものであり、従って、ＩＤＴの数及び構造等については特に限定されない。すなわち、本発明は、図１に示した表面波装置だけでなく、上記条件を満たす限り、様々な表面波共振子や表面波フィルタ等に適用することができる。
【００４０】
タンタルを主成分という意味は、タンタルと他の電極と積層された場合、厚みの比をいうのではなく、密度と厚みを乗じた重量比で半分以上という意味である。なお、タンタルの上あるいは下に７１００ｋｇ／ｍ^３以上の密度をもつＷ、Ａｕ、Ｐｔ、Ｃｕ、Ａｇ、Ｃｒ等の金属からなる電極と同じ程度の割合で積層してもタンタル電極単層と同じ効果をもつことはいうまでもない。
【００４１】
【発明の効果】
本発明に係る表面波装置では、１７°〜５８°回転Ｙ板Ｘ伝搬ＬｉＴａＯ_３からなる圧電基板上に、規格化膜厚Ｈ／λが０．００４〜０．０５５であり、かつタンタルよりなるＩＤＴが形成されており、ＩＤＴを覆うように、Ｈｓ／λ＝０．１０〜０．４０のＳｉＯ_２膜が形成されている、ＳｉＯ_２膜により周波数温度係数ＴＣＦが改善され、タンタル膜よりなるＩＤＴの膜厚Ｈ／λが上記特定の範囲とされているため、電気機械結合係数と反射係数が大きく、さらにＬｉＴａＯ_３基板の回転角が上記特定の範囲とされているため、減衰定数が小さくされる。よって、周波数温度特性に優れ、大きな電気機械結合係数を有し、かつ伝搬損失が少ない表面波装置を提供することが可能となる。
【００４２】
特に、ＩＤＴの膜厚Ｈ／λが０．１０〜０．５５の範囲、より好ましくは０．０１６〜０．０４５の範囲にある場合には、電気機械結合係数を効果的に高めることができる。
【００４３】
また、上記ＬｉＴａＯ_３からなる圧電基板の回転角が２１°〜５３°の範囲である場合には、減衰定数をより一層小さくすることができる。
また、タンタル電極が薄いため、このタンタル電極ＩＤＴ上にＳｉＯ_２が成膜されてもＳｉＯ_２に大きな段差やクラックができないため、Ａｌ電極の場合に生じるそれらに起因した挿入損失等の特性の劣化もない。
【図面の簡単な説明】
【図１】本発明の一実施例に係る表面波装置を示す模式的平面図。
【図２】オイラー角（０，θ，０）のＬｉＴａＯ_３基板のθと、減衰定数αとの関係を示す図。
【図３】オイラー角（０，θ，０）のＬｉＴａＯ_３基板におけるθと電気機械結合係数Ｋ^２との関係を示す図。
【図４】オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上にタンタルからなる電極膜を形成した構造における電極膜の規格化膜厚Ｈ／λと、電気機械結合係数Ｋ^２との関係を示す図。
【図５】オイラー角（０°，１１３°，０°）、（０°，１２６°，０°）及び（０°，１２９°０，０°）の各ＬｉＴａＯ_３基板上にＳｉＯ_２膜を形成した場合のＳｉＯ_２膜の規格化膜厚Ｈｓ／λと、周波数温度係数ＴＣＦとの関係を示す図。
【図６】オイラー角（０°，１２０°，０°）のＬｉＴａＯ_３基板上に、様々な厚みのＳｉＯ_２膜及び様々な厚みのタンタルからなるＩＤＴを形成した構造における減衰定数αの変化を示す図。
【図７】オイラー角（０°，１４０°，０°）のＬｉＴａＯ_３基板上に、様々な厚みのＳｉＯ_２膜及び様々な厚みのタンタルからなるＩＤＴを形成した構造における減衰定数αの変化を示す図。
【図８】オイラー角（０°，θ，０°）のＬｉＴａＯ_３基板上に、様々な厚みのタンタルよりなる電極膜を形成し、さらに規格化膜厚Ｈｓ／λ＝０．１のＳｉＯ_２膜を形成した表面波装置におけるθと、タンタルよりなる電極膜の規格化厚みＨ／λと、減衰定数αとの関係を示す図。
【図９】オイラー角（０°，θ，０°）のＬｉＴａＯ_３基板上に、様々な厚みのタンタルよりなる電極膜を形成し、さらに規格化膜厚Ｈｓ／λ＝０．２のＳｉＯ_２膜を形成した表面波装置におけるθと、タンタルよりなる電極膜の規格化厚みＨ／λと、減衰定数αとの関係を示す図。
【図１０】オイラー角（０°，θ，０°）のＬｉＴａＯ_３基板上に、様々な厚みのタンタルよりなる電極膜を形成し、さらに規格化膜厚Ｈｓ／λ＝０．３のＳｉＯ_２膜を形成した表面波装置におけるθと、タンタルよりなる電極膜の規格化厚みＨ／λと、減衰定数αとの関係を示す図。
【図１１】オイラー角（０°，θ，０°）のＬｉＴａＯ_３基板上に、様々な厚みのタンタルよりなる電極膜を形成し、さらに規格化膜厚Ｈｓ／λ＝０．４のＳｉＯ_２膜を形成した表面波装置におけるθと、タンタルよりなる電極膜の規格化厚みＨ／λと、減衰定数αとの関係を示す図。
【図１２】オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上に、様々な厚みのタンタルまたはアルミニウムよりなる電極を形成した場合の電極の膜厚と電極指の反射係数との関係を示す図。
【図１３】（ａ）は、オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上に、膜厚Ｈ／λ＝０．０８のアルミニウム電極からなるＩＤＴが形成された表面を、（ｂ）は、その上に厚みＨｓ／λ＝０．３のＳｉＯ_２が成膜された表面を、（ｃ）はその断面を示す各走査型電子顕微鏡写真を示す図。
【図１４】（ａ）は、オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上に、厚みＨ／λ＝０．０２５のタンタルからなるＩＤＴが形成された表面を、（ｂ）は、その上に厚みＨｓ／λ＝０．３のＳｉＯ_２が成膜された表面を、（ｃ）はその断面を示す各走査型電子顕微鏡写真を示す図。
【図１５】オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上にタンタルからなるＩＤＴを形成し、その上にＳｉＯ_２膜を形成した構造における、タンタルの規格化膜厚と、ＳｉＯ_２の規格化膜厚と、音速との関係を示す図。
【図１６】オイラー角（０°，１２６°，０°）のＬｉＴａＯ_３基板上にタンタルからなるＩＤＴが形成されており、その上にＳｉＯ_２膜が形成された構造における、タンタルの規格化膜厚と、ＳｉＯ_２の規格化膜厚と、音速との関係を示す図。
【符号の説明】
１…表面波装置
２…圧電基板
３ａ，３ｂ…ＩＤＴ
４…ＳｉＯ_２膜
５ａ，５ｂ…反射器[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a surface acoustic wave device used for, for example, a surface acoustic wave filter, and more particularly, to a surface acoustic wave device using a LiTaO ₃ substrate.
[0002]
[Prior art]
Conventionally, a surface wave device using a 40 ° to 42 ° rotating Y-plate X-propagation LiTaO ₃ substrate has been known as a bandpass filter (for example, the following Non-Patent Document 1). In the band filter of the RF band, an Al film having a thickness H / λ standardized by the wavelength λ of 0.08 to 0.10 is formed on the 40 ° to 42 ° rotation Y plate X-propagation LiTaO ₃ substrate. An IDT was formed.
[0003]
As described above, in the conventional surface acoustic wave device using the 40 ° to 42 ° rotating Y-plate X-propagation LiTaO ₃ substrate, the frequency-temperature characteristic TCF is relatively large at −33 ppm / ° C., so that the temperature characteristic is further improved. Certain specifications could not be fully met. Conventionally, as a method of improving the temperature characteristic TCF of the surface acoustic wave device, there is known a method of forming an SiO ₂ layer after forming an IDT made of Al on a LiTaO ₃ substrate (Patent Document 1 below). .
[0004]
[Non-patent document 1]
Proceedings of the 1997 IEICE General Conference: SA-10-6; 500-501
[Patent Document 1]
JP-A-2-295212
[Problems to be solved by the invention]
However, when forming a resonator or a filter using an IDT made of Al, in order to obtain a large electromechanical coupling coefficient K _saw and a reflection coefficient, as shown in FIGS. H / λ (H is the film thickness, λ is the wavelength of the surface wave) must be considerably thick, 0.08 to 0.10. As described above, since the IDT made of Al is considerably thick, an SiO ₂ film is formed on the portion where the IDT shown in FIG. 13A is formed in order to improve the frequency-temperature characteristics. Once formed, as shown in FIG. 13 (b), (c) , a large step in the SiO ₂ film occurs, there is a crack occurs in the SiO ₂ film. For this reason, the filter characteristics of the surface acoustic wave filter tend to deteriorate due to the occurrence of cracks.
[0006]
In addition, since the thickness of the electrode of the IDT made of Al is large, the effect of covering the irregularities on the electrode surface of the IDT due to the formation of the SiO ₂ film is not sufficient, and the temperature characteristics may not be sufficiently improved. .
[0007]
An object of the present invention is to provide a surface acoustic wave device using a rotating Y-plate X-propagation LiTaO ₃ substrate in view of the above-mentioned state of the art, and not only to improve the frequency-temperature characteristics by forming a SiO ₂ film, By reducing the thickness of the electrode of the IDT, cracks in the SiO ₂ film can be prevented and the attenuation constant can be significantly reduced, so that the intended electrical characteristics such as filter characteristics can be obtained. It is an object of the present invention to provide a surface acoustic wave device and a method of manufacturing the same in which the electromechanical coupling coefficient and the reflection coefficient in the IDT are sufficiently large.
[Means for Solving the Problems]
The surface acoustic wave device according to the present invention is formed on a piezoelectric substrate made of 17 ° to 58 ° rotated Y-plate X-propagation LiTaO ₃ and on the piezoelectric substrate, and has a film thickness of H and a surface wave wavelength of λ. Sometimes, an IDT made of tantalum having a normalized film thickness H / λ of 0.004 to 0.055, and the IDT is formed on the piezoelectric substrate so as to cover the IDT, and is normalized by the wavelength of the surface wave. And a SiO ₂ film having a thickness Hs / λ of 0.10 to 0.40.
[0009]
In a specific aspect of the surface acoustic wave device according to the present invention, the normalized film thickness H / λ of the IDT is in a range of 0.01 to 0.55, more preferably in a range of 0.016 to 0.045. You.
[0010]
In another specific aspect of the surface acoustic wave device according to the present invention, the piezoelectric substrate is formed of a 21 ° -53 ° rotated Y-plate X-propagation LiTaO ₃ substrate.
In the present invention, by using a tantalum electrode, a large electromechanical coupling coefficient and a large reflection coefficient can be obtained with a thin electrode film thickness as shown in FIGS.
[0011]
In still another specific aspect of the surface acoustic wave device according to the present invention, an adhesion layer is formed between the upper surface of the IDT and the SiO ₂ film, so that peeling of the SiO ₂ film can be controlled. In this case, the adhesion layer may be formed not only on the upper surface of the IDT but also on the interface between the LiTaO ₃ substrate and the SiO ₂ film. The adhesion layer may be formed not only on the upper surface of the IDT but also on almost the entire region of the interface between the IDT and the SiO ₂ film. That is, the adhesion layer may be formed on the side surface of the IDT.
[0012]
In still another specific aspect of the surface acoustic wave device according to the present invention, a plurality of electrodes other than the IDT, including at least a bus bar and an electrode pad for connection to the outside, are further formed on the LiTaO ₃ substrate, A plurality of electrodes may be formed on the base electrode layer made of tantalum and the base electrode layer, and the base metal layer having an upper metal layer made of Al or an Al alloy may be formed in the same step as the IDT. Since the upper metal layer is made of Al or an Al alloy, the adhesion strength of the SiO ₂ film can be increased and the cost of the electrode can be reduced. Furthermore, the wedge bonding property by Al is also improved.
[0013]
In the surface acoustic wave device according to the present invention, preferably, a leaky surface acoustic wave is used as the surface wave, and according to the present invention, it has excellent frequency-temperature characteristics, and has an IDT having a large electromechanical coupling coefficient and a large reflection coefficient. A surface acoustic wave device utilizing a small leaky surface acoustic wave can be provided.
[0014]
In the method of manufacturing a surface acoustic wave device according to the present invention, a step of preparing a LiTaO ₃ substrate of 17 ° to 58 ° rotation Y plate X propagation, and at least one IDT on the LiTaO ₃ substrate, mainly containing tantalum Forming a step using a metal, adjusting the frequency after forming the IDT, and forming an SiO ₂ film on the LiTaO ₃ substrate so as to cover the IDT after the frequency adjustment. It is characterized by having. The metal containing tantalum as a main component includes, in addition to tantalum or an alloy containing tantalum as a main component, a case where a tantalum layer is a main layer having a laminated structure of a tantalum layer and another metal.
[0015]
In a specific aspect of the production method of the present invention, the IDT has a structure in which an electrode made of a metal having a density of 7100 kg / m ³ or more and a tantalum electrode are stacked.
Tantalum has a higher density than Al, and can easily form an IDT having a small electrode film thickness, a large electromechanical coupling coefficient, and a large reflection coefficient, so that cracks in the SiO ₂ film can be prevented. Further, the attenuation constant can be reduced by the SiO ₂ film.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail by describing specific embodiments of the present invention with reference to the drawings.
[0017]
FIG. 1 is a schematic sectional view of a surface acoustic wave device according to an embodiment of the present invention. The surface acoustic wave device 1 is a longitudinally coupled resonator type surface acoustic wave filter, and has a piezoelectric substrate 2 made of a 17 ° to 58 ° rotated Y plate LiTaO ₃ . On the piezoelectric substrate 2, IDTs 3a and 3b made of a tantalum film (Ta film) and reflectors 5a and 5b are formed. The normalized film thickness H / λ (H denotes the thickness of the IDT and λ denotes the wavelength at the center frequency) of the IDTs 3a and 3b is in the range of 0.004 to 0.055. Further, an SiO ₂ film 4 is formed on the piezoelectric substrate 2 so as to cover the IDTs 3a and 3b. The normalized thickness Hs / λ (Hs is the thickness of the SiO ₂ film, λ is the wavelength of the surface wave at the center frequency) of the SiO ₂ film 4 is in the range of 0.10 to 0.40.
[0018]
In this embodiment, as described above, the piezoelectric substrate 2 made of 17 ° to 58 ° rotated Y-plate X-propagation LiTaO ₃ , the IDTs 3a and 3b made of tantalum with H / λ = 0.004 to 0.055, and Hs due to the use of the SiO ₂ film 4 is in the range of /Ramuda=0.10～0.40, temperature coefficient of frequency TCF is small, the electromechanical coupling coefficient ^{K 2} is increased, and the propagation loss is small surface acoustic wave device Can be provided. This will be described based on the following specific experimental examples.
[0019]
As surface waves propagating through the LiTaO ₃ substrate, there are leaky surface acoustic waves in addition to Rayleigh waves. Leaky surface acoustic waves have a higher sound speed and a larger electromechanical coupling coefficient than Rayleigh waves. However, leaky surface acoustic waves are waves that propagate while radiating energy into the interior of the substrate. Therefore, the leaky surface acoustic wave has an attenuation constant that causes a propagation loss.
[0020]
FIG. 2 shows the relationship between θ of the Euler angles (0, θ, 0) in the rotating Y plate X-propagation LiTaO ₃ and the attenuation constant (propagation loss) α when the substrate surface is electrically short-circuited. It should be noted that the rotation angle has a relationship of θ-90 degrees.
[0021]
As is clear from FIG. 2, the attenuation constant is small when the Euler angle θ is in the range of 124 ° to 126 °. Outside this range, the damping constant increases.
It is known that when an IDT made of Al having a relatively large film thickness is formed, the attenuation constant decreases when θ = 130 ° to 132 ° (for example, Non-Patent Document 1). Thus, conventionally, the IDT composed of Al, the structure of a combination of a LiTaO ₃ substrate, LiTaO ₃ substrate of rotated Y-plate X-propagation of θ = 130 ° ~132 ° has been used.
[0022]
FIG. 3 shows the relationship between the Euler angle (0, θ, 0) θ and the electromechanical coupling coefficient K ² in the rotating Y-plate X-propagation LiTaO ₃ substrate. Euler angle θ is understood that a large electromechanical coupling coefficient K ² can be obtained in the range of 100 ° to 120 °. However, in the range of θ = 100 ° to 120 °, the attenuation constant is large as is apparent from FIG. Therefore, it is understood that a LiTaO ₃ substrate having such an Euler angle cannot be used.
[0023]
FIG. 4 shows a normalized thickness H / λ of a tantalum film when a tantalum film is formed on a LiTaO ₃ substrate having a 36 ° rotation Y-plate X propagation [Euler angle (0 °, 126 °, 0 °)]. (H is the film thickness, lambda denotes the wavelength at the center frequency of the surface acoustic wave device) and shows the relationship between the electromechanical coupling coefficient K ^2. The range of normalized thickness H / λ = 0.004~0.08, electromechanical coefficient ^{K 2,} 1 of the electromechanical coupling coefficient when the H / λ = 0 (if not deposited). It can be seen that the ratio becomes 3 times or more, when H / λ = 0.01 to 0.055, the ratio becomes 1.5 times or more, and when H / λ = 0.016 to 0.045, the ratio becomes 1.75 times or more.
[0024]
Therefore, by setting the H / λ = 0.004~0.08, it is understood that it is possible to increase the electromechanical coupling coefficient ^{K 2.}
If the normalized thickness of the tantalum film exceeds 0.055, it may be difficult to produce an IDT obtained from tantalum. Therefore, the normalized thickness H / λ of the tantalum film is preferably 0.004 to 0.055, more preferably 0.012 to 0.055, and still more preferably 0.016 to 0.045. Understand.
[0025]
Next, the effect of improving the frequency temperature coefficient TCF when the SiO ₂ film is formed on the LiTaO ₃ substrate will be described. FIG. 5 is a diagram showing changes in the frequency temperature coefficient TCF when an SiO ₂ film is formed on each (0, θ, 0) LiTaO ₃ substrate at θ = 113 °, 126 °, and 129 °.
[0026]
As apparent from FIG. 5, theta is 113 °, in either case of 126 ° and 129 °, the film thickness of the SiO ₂ normalized thickness Hs / λ (Hs is SiO ₂ film, lambda surface waves It can be seen that the TCF falls within the range of −24 to +17 ppm / ° C. when the wavelength at the center frequency of the device is 0.10 to 0.45. However, since the formation of the SiO ₂ film it takes time, it is desirable that the thickness Hs / lambda of the SiO ₂ film is 0.40 or less. Therefore, preferably, the thickness Hs / λ of the SiO ₂ film is in the range of 0.10 to 0.40, whereby the film can be formed in a short time, and the TCF is in the range of −20 to +17 ppm / ° C. can do.
[0027]
Heretofore, there have been some reports that TCF such as Rayleigh wave is improved by forming an SiO ₂ film in a structure in which an IDT made of Al is formed on a LiTaO ₃ substrate (for example, the above patent document) 1 etc.). However, there is no report that an experiment was conducted in a laminated structure of an LiTaO ₃ substrate, an electrode composed of tantalum and an SiO ₂ film in consideration of the thickness of the electrode and the attenuation constant of leaky surface acoustic waves.
[0028]
6 and 7 show an Euler angle (0 °, 120 °, 0 °), an IDT made of tantalum of various thicknesses on each LiTaO ₃ substrate at (0 °, 140 °, 0 °), FIG. 3 is a diagram showing attenuation constants when SiO ₂ films having various thicknesses are formed.
[0029]
As is clear from FIG. 6, when θ = 120 °, the thickness Hs / λ of SiO ₂ is 0.1 to 0.40 and the normalized thickness H / λ of the electrode made of tantalum is 0.0 to 0. It can be seen that the attenuation constant is small in the range of 10. On the other hand, as is clear from FIG. 7, when θ = 140 °, the normalized thickness H / λ of the electrode made of tantalum is in the range of 0.0 to 0.06 regardless of the thickness of the SiO ₂ film. It can be seen that the damping constant is large.
[0030]
That is, in order to reduce the absolute value of TCF, obtain a large electromechanical coupling coefficient, and reduce the damping constant, the _three angles of the cut angle of the LiTaO ₃ substrate, the thickness of the SiO ₂ film, and the thickness of the electrode made of tantalum are used. It turns out that conditions must be considered.
[0031]
8 to 11 show the relationship between θ and the damping constant when the normalized thickness Hs / λ of the SiO ₂ film and the normalized thickness H / λ of the electrode film made of tantalum are changed.
As is clear from FIGS. 8 to 11, when the normalized thickness H / λ of the electrode made of tantalum is 0.01 to 0.055 and 0.016 to 0.045, the thickness of the SiO ₂ film is optimized. The relationship with θ is as shown in Tables 1 and 2 below. The optimum θ may vary from about −2 ° to + 4 ° due to variations in the electrode finger width of the tantalum electrode and variations in the single crystal substrate.
[0032]
[Table 1]

[0033]
[Table 2]

[0034]
That is, as is clear from Tables 1 and 2, when the thickness H / λ of the electrode made of tantalum is 0.01 to 0.055, in order to improve the temperature characteristics, the normalized film of the SiO ₂ film is used. When the thickness is in the range of 0.1 to 0.4, θ at the Euler angle of LiTaO ₃ is in the range of 107 ° to 148 °, that is, in the range of 17 ° to 58 ° in rotation angle, more preferably, SiO 2. _It can be seen that the Euler angles shown in Table 1 should be selected according to the film thickness of No. ₂ .
[0035]
Similarly, as is clear from Table 2, the normalized thickness of the electrode made of the tantalum film is 0.016 to 0.045, and the thickness of the SiO ₂ film is set to 0.1 to improve the frequency temperature characteristics. In the case where the Euler angle is in the range of 1 to 0.4, the Euler angle of the LiTaO ₃ substrate may be in the range of 109 ° to 144 °. More preferably, the Euler angle in Table 2 is set according to the thickness of the SiO ₂ film. It turns out that you have to choose.
[0036]
The range of the Euler angle of LiTaO ₃ is a range in which the attenuation constant is 0.05 or less. Further, a more preferable range of the Euler angle of LiTaO ₃ in Tables 1 and 2 is one in which the attenuation constant is specified to be 0.025 or less. When the normalized thickness of the electrode film made of tantalum is 0.012, 0.015, 0.042, and 0.053, the relationship between the thickness of the SiO ₂ film and the Euler angle is shown in FIGS. The values of the film thickness and the Euler angle of the SiO ₂ films in Tables 1 and 2 are obtained by converting from the normalized film thickness of the electrode film made of tantalum shown in FIG.
[0037]
FIGS. 14A, 14B and 14C are scanning electron micrographs of the surface of the surface acoustic wave filter of the above embodiment. Here, the results before and after the SiO ₂ film having the normalized thickness Hs / λ = 0.3 is formed on the IDT made of tantalum having the normalized thickness H / λ = 0.025 are shown. Have been. As is clear from the photograph after the film formation in FIG. 14B, no crack is observed on the surface of the SiO ₂ film, and therefore, it is understood that deterioration of the characteristics due to the crack hardly occurs. Compared with the Al electrode, the tantalum electrode has a large electromechanical coupling coefficient and a large reflection coefficient with a thin film thickness. Therefore, even if SiO ₂ is formed on the thin tantalum electrode, there is an advantage that large steps and cracks do not occur in SiO ₂ as shown in FIGS. 14B and 14C.
[0038]
In manufacturing the surface acoustic wave device according to the present invention, an IDT made of a metal containing tantalum as a main component is formed on a rotating Y-plate X-propagation LiTaO ₃ substrate, the frequency is adjusted in that state, and then the attenuation constant is set. It is desirable to form a SiO ₂ film having a thickness in a range where α can be reduced. This will be described with reference to FIGS. FIG. 15 shows a normalized thickness H / of tantalum when an IDT and SiO ₂ films made of tantalum are formed on a rotating Y plate X-propagation LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °). The relationship between λ, the normalized thickness Hs / λ of the SiO ₂ film, and the sound velocity of the leaky surface acoustic wave is shown. FIG. 16 shows leakage when the IDT made of tantalum of various thicknesses is formed on a LiTaO ₃ substrate having the same Euler angle, and the normalized thickness of the SiO ₂ film formed thereon is changed. 3 shows a change in the speed of sound of a surface acoustic wave. As is apparent from a comparison between FIG. 15 and FIG. 16, the change in the sound speed of the surface wave is much larger when the thickness of the tantalum is changed than when the thickness of the SiO ₂ film is changed. . Therefore, it is desirable to adjust the frequency before forming the SiO ₂ film. For example, it is preferable to adjust the frequency after forming an IDT made of tantalum by laser etching or ion etching.
[0039]
Note that, as described above, the present invention provides a piezoelectric substrate made of 17 ° to 58 ° rotated Y plate X-propagation LiTaO ₃ , an IDT made of tantalum with H / λ = 0.004 to 0.055, and Hs / λ. = which is characterized by having a SiO ₂ film is 0.10 to 0.40, thus, there is no particular limitation on such IDT of number and structure. That is, the present invention can be applied not only to the surface acoustic wave device shown in FIG. 1 but also to various surface acoustic wave resonators and surface acoustic wave filters as long as the above conditions are satisfied.
[0040]
When tantalum is used as a main component, when tantalum and another electrode are stacked, the weight ratio multiplied by the density and the thickness is not less than half, not the thickness ratio. It should be noted that even if laminated at the same ratio as an electrode made of a metal such as W, Au, Pt, Cu, Ag, or Cr having a density of 7100 kg / m ³ or more above or below tantalum, it is the same as a tantalum electrode single layer. Needless to say, it has an effect.
[0041]
【The invention's effect】
In the surface acoustic wave device according to the present invention, the normalized film thickness H / λ is 0.004 to 0.055 and is made of tantalum on the piezoelectric substrate made of 17 ° to 58 ° rotated Y plate X-propagation LiTaO _3. An IDT is formed, and an SiO ₂ film of Hs / λ = 0.10 to 0.40 is formed so as to cover the IDT. The frequency temperature coefficient TCF is improved by the SiO ₂ film, and the IDT is formed of a tantalum film. Since the film thickness H / λ of the IDT is in the above specific range, the electromechanical coupling coefficient and the reflection coefficient are large, and the rotation angle of the LiTaO ₃ substrate is in the above specific range, so that the attenuation constant is small. Is done. Therefore, it is possible to provide a surface acoustic wave device having excellent frequency-temperature characteristics, a large electromechanical coupling coefficient, and a small propagation loss.
[0042]
In particular, when the thickness H / λ of the IDT is in the range of 0.10 to 0.55, more preferably in the range of 0.016 to 0.045, the electromechanical coupling coefficient can be effectively increased. .
[0043]
When the rotation angle of the piezoelectric substrate made of LiTaO ₃ is in the range of 21 ° to 53 °, the damping constant can be further reduced.
Further, since the thin tantalum electrodes, deterioration of the characteristics of this for SiO ₂ can not be a large step or cracks SiO ₂ be deposited tantalum electrodes IDT on, insertion loss or the like due to their occurring when the Al electrode Nor.
[Brief description of the drawings]
FIG. 1 is a schematic plan view showing a surface acoustic wave device according to one embodiment of the present invention.
FIG. 2 is a diagram showing a relationship between θ of a LiTaO ₃ substrate having an Euler angle (0, θ, 0) and an attenuation constant α.
[3] Euler angles (0, θ, 0) shows the relationship between the LiTaO ₃ and the theta in the substrate electromechanical coefficient ^{K 2} of.
FIG. 4 shows a normalized thickness H / λ of the electrode film and an electromechanical coupling coefficient K ² in a structure in which an electrode film made of tantalum is formed on a LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °). FIG.
FIG. 5 shows a SiO ₂ film on each LiTaO ₃ substrate having Euler angles (0 °, 113 °, 0 °), (0 °, 126 °, 0 °) and (0 °, 129 ° 0, 0 °). shows the normalized thickness Hs / lambda of the SiO ₂ film in the case of forming, the relation between the temperature coefficient of frequency TCF.
FIG. 6 shows a change in attenuation constant α in a structure in which an IDT made of various thicknesses of SiO ₂ films and various thicknesses of tantalum is formed on a LiTaO ₃ substrate having Euler angles (0 °, 120 °, 0 °). FIG.
FIG. 7 shows a change in attenuation constant α in a structure in which an IDT made of various thicknesses of SiO ₂ films and various thicknesses of tantalum is formed on a LiTaO ₃ substrate having Euler angles (0 °, 140 °, 0 °). FIG.
FIG. 8 shows an example in which electrode films made of tantalum having various thicknesses are formed on a LiTaO ₃ substrate having Euler angles (0 °, θ, 0 °), and SiO _{2 having a} normalized film thickness Hs / λ = 0.1. FIG. 6 is a diagram showing a relationship between θ in a surface acoustic wave device having a film formed thereon, a normalized thickness H / λ of an electrode film made of tantalum, and an attenuation constant α.
FIG. 9 shows an example in which electrode films made of tantalum of various thicknesses are formed on a LiTaO ₃ substrate having Euler angles (0 °, θ, 0 °), and SiO _{2 having a} normalized film thickness Hs / λ = 0.2. FIG. 6 is a diagram showing a relationship between θ in a surface acoustic wave device having a film formed thereon, a normalized thickness H / λ of an electrode film made of tantalum, and an attenuation constant α.
FIG. 10 shows an example in which electrode films made of tantalum of various thicknesses are formed on a LiTaO ₃ substrate having Euler angles (0 °, θ, 0 °), and SiO _{2 having a} normalized thickness Hs / λ = 0.3. FIG. 6 is a diagram showing a relationship between θ in a surface acoustic wave device having a film formed thereon, a normalized thickness H / λ of an electrode film made of tantalum, and an attenuation constant α.
FIG. 11 shows that an electrode film made of tantalum of various thicknesses is formed on a LiTaO ₃ substrate having Euler angles (0 °, θ, 0 °), and SiO _{2 having a} normalized film thickness Hs / λ = 0.4. FIG. 6 is a diagram showing a relationship between θ in a surface acoustic wave device having a film formed thereon, a normalized thickness H / λ of an electrode film made of tantalum, and an attenuation constant α.
FIG. 12 is a graph showing the relationship between the electrode film thickness and the reflection coefficient of an electrode finger when electrodes made of tantalum or aluminum having various thicknesses are formed on a LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °). The figure which shows a relationship.
FIG. 13A shows a surface on which an IDT made of an aluminum electrode having a film thickness of H / λ = 0.08 is formed on a LiTaO ₃ substrate having an Euler angle (0 °, 126 °, 0 °). (B) is a view showing a scanning electron microscope photograph showing a surface on which a SiO ₂ film having a thickness of Hs / λ = 0.3 is formed, and (c) is a cross-sectional view thereof.
FIG. 14 (a) shows the surface of a LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °) on which an IDT made of tantalum having a thickness of H / λ = 0.025 is formed, as shown in FIG. () Shows a scanning electron microscope photograph showing a surface on which a SiO ₂ film having a thickness of Hs / λ = 0.3 is formed, and (c) shows a cross section thereof.
FIG. 15 shows a normalized film thickness of tantalum in a structure in which an IDT made of tantalum is formed on a LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °) and a SiO ₂ film is formed thereon; shows the normalized thickness of the SiO _2, the relationship between the speed of sound.
FIG. 16 shows a standardized film of tantalum in a structure in which an IDT made of tantalum is formed on a LiTaO ₃ substrate having Euler angles (0 °, 126 °, 0 °) and a SiO ₂ film is formed thereon. shows the thickness, and the normalized thickness of the SiO _2, the relationship between the speed of sound.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Surface wave device 2 ... Piezoelectric substrates 3a, 3b ... IDT
4. SiO ₂ films 5a, 5b reflector

Claims (6)

A piezoelectric substrate made of 17 ° to 58 ° rotated Y plate X-propagation LiTaO ₃ ,
An IDT made of tantalum formed on the piezoelectric substrate and having a normalized thickness H / λ of 0.004 to 0.055 when the thickness is H and the wavelength of the surface wave is λ;
A surface acoustic wave device comprising: a SiO ₂ film formed on the piezoelectric substrate so as to cover the IDT and having a thickness Hs / λ normalized to the wavelength of the surface wave of 0.10 to 0.40. . The surface acoustic wave device according to claim 1, wherein the normalized film thickness H / λ of the IDT is in a range of 0.01 to 0.055. The surface acoustic wave device according to claim 2, wherein the normalized thickness H / λ of the IDT is in a range of 0.016 to 0.045. The surface acoustic wave device according to claim 1, wherein the piezoelectric substrate is a 21 ° -53 ° rotated Y-plate X-propagation LiTaO ₃ substrate. Preparing a LiTaO ₃ substrate of 17 ° to 58 ° rotation Y plate X propagation, at least one IDT in the LiTaO ₃ substrate, forming a metal composed mainly of tantalum, the IDT 5. The method according to claim 1, further comprising: performing a frequency adjustment after the formation, and forming a SiO ₂ film on the LiTaO ₃ substrate so as to cover the IDT after the frequency adjustment. A method for manufacturing the surface acoustic wave device according to any one of the above. The IDT method of producing a surface acoustic wave device according to claim 5 having an electrode density of from 7100kg / m ³ or more metals, a structure in which a tantalum electrodes are stacked.

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