JP2568505C

JP2568505C -

Info

Publication number: JP2568505C
Authority: JP
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Publication date: 1999-06-07

Description

【発明の詳細な説明】産業上の利用分野本発明は焦電型赤外線検出素子、圧電素子、電気光学素子等に用いられる強誘
電体薄膜素子に関する。従来の技術強誘電体のエレクトロニクス分野における応用は、赤外線検出素子、圧電素子
、光変調素子、メモリー素子などさまざまなものがある。近年の半導体技術の進
歩による電子部品の小型化にともない、強誘電体素子も薄膜化が進みつつある。ところで、強誘電体の自発分極Ｐｓの変化を出力として取り出す、例えば焦電
型赤外線検出素子や圧電素子等では、強誘電体材料のＰｓが一方向に揃っている
時、最も大きい出力が得られる。発明が解決しようとする問題点現在、赤外線検出素子や圧電素子等に用いられている強誘電体磁器は多結晶体
であり、結晶軸の配列に方向性は無く、従って自発分極Ｐｓも出たら目に配列し
ている。エピタキシャル強誘電体薄膜、配向性強誘電体薄膜は結晶の分極軸は揃っているが、電気的な自発分極Ｐｓは１８０°ドメインを作り交互に配列してい
る。そこで、これら材料を上述のようなエレクトロニクス素子として用いる場合
、材料に高電界（〜１００ｋＶ／ｃｍ）を印加してＰｓの向きを揃える分極処理
が必要である。また、ＰｂＴｉＯ₃やＰＬＺＴなどの薄膜の作製に関しては多くの報告がある
が、それらの強誘電相の領域の薄膜について、その分極軸であるｃ軸に配向した
薄膜、自発分極までも一方向に配向した薄膜は実現されていない。強誘電体材料に高電界を印加してＰｓを揃える方法では次のような問題点が生
じる。（１）分極処理により絶縁破壊が生ずる場合があり、歩留まりが下がる。（２）高分解能アレイ素子の様に多くの微小素子が高密度に配列しているもので
は、それらを均一に分極することが困難である。（３）半導体デバイス上に強誘電体薄膜を形成した集積化デバイスでは、分極処
理そのものが不可能な場合がある。問題点を解決するための手段基板と、基板上に形成された強誘電体薄膜と、この強誘電体薄膜に付設された
電極薄膜とを備え、前記強誘電体薄膜の膜面積が２０ｍｍ²以下である強誘電体
薄膜素子を用いる。作用上記のような強誘電体薄膜においては、Ｐｓが既に揃った自然分極が均一性よ
く得られ、分極処理をおこなう必要が無く、歩留まり良く、高性能の強誘電体素
子が実現できる。実施例第１図は本発明に従って作製した強誘電体薄膜素子の一実施例の断面図である
。（１００）でへき開し鏡面研摩したＭｇＯ単結晶を基板１とし、下部電極２と
して膜厚０．２μｍのＰｔ薄膜をスパッタリングにより形成した。スパッタガス
はＡｒ−Ｏ₂混合ガスである。ついで、強誘電体薄膜３を膜厚４μｍ、膜面積を
種々変化して成長させた。方法は高周波マグネトロンスパッタ法で、ＡｒとＯ₂
の混合ガスを用い、スパッタリングターゲットは｛（１−Ｙ）Ｐｂ_xＬａ_yＴｉ_zＺｒ_wＯ₃＋ＹＰｂＯ｝ …（１）の粉末である。表１にスパッタリング条件を示す。ついでこの薄膜上に上部電極４としてＮｉ−Ｃｒ電極を蒸着し、強誘電体薄膜
素子を作製した。さらに、強誘電体薄膜３の下部における基板１には開口５を設
けた。第２図に代表的な薄膜のＸ線回折パターンを示す。ペロブスカイト構造の（０
０１）と（１００）反射、及びその高次の反射のみ観察される。また（００１）
反射の強度が（１００）のそれと比べて著しく大きいのでｃ軸配向膜であること
がわかる。ｃ軸配向率αを次の式で定義する。 α＝Ｉ（００１）／｛Ｉ（００１）＋Ｉ（１００）｝ここでＩ（００１）、およびＩ（１００）はそれぞれ（００１）と（１００）
反射の回折強度を表す。得られた薄膜の誘電率と焦電係数の測定を行った。第３図にｃ軸配向率に対するＰｂＴｉＯ₃の焦電係数γの変化、第４図に誘電
率εの変化を示す。よく知られている様に、焦電係数は自発分極Ｐｓの配向に比
例して大きくなる。焦電係数は配向率の増加と共に大きくなり、誘電率は小さく
なる。第３図及び第４図には分極処理（２００℃で１００ｋＶ／ｃｍ²１０分印
加）を行なった場合の結果についても示してある。配向率が小さい場合、分極処
理前後で焦電係数及び誘電率の値は大きく変化する。配向率が７５％になると焦
電係数は１．６×１０^-8Ｃ／ｃｍ²Ｋとなり、この値は２００℃で１００ｋＶ／
ｃｍ印加して分極処理を行ったＰｂＴｉＯ₃セラミクス（γ＝１．８×１０^-8Ｃ／ｃｍ²
Ｋ）とほぼ同等の値である。配向率８０％の場合焦電係数は２．５×１０^-8Ｃ／
ｃｍ²Ｋであり、ＰｂＴｉＯ₃セラミクスの値にくらべかなり大きい。また、分極
処理後の値と比べ殆ど変わらないばかりでなく、配向率が小さい場合の分極後の
値より大きい。誘電率は、配向率７５％の場合、セラミクスの１／２の値で約１
００である。Ｌａを添加したＴｂＴｉＯ₃（ＰＬＴ）の場合でも、同じ結果が得
られた。以上述べたとおりＰｂＴｉＯ₃、及びＰＬＴ薄膜では、薄膜作製時に十分にｃ
軸に配向しておれば分極処理を行わなくても自発分極が揃っており、特に配向率
７５％以上の薄膜でその効果が大きいことが明らかになった。ところで、薄膜のｃ軸の配向率は膜面積により変化することを見い出した。第
５図はｃ軸の配向率と膜面積との関係を表す。ただし膜厚は４μｍときの結果で
ある。図より明らかな様に、膜面積の増大とともにｃ軸の配向率は大きく減少す
る。ＰＬＴ膜の直径が１０ｍｍのとき、ｃ軸の配向率は平均値で７０％以下に下
がってしまう。また、配向率のバラツキは大きくなる。したがって、上記の結果
よりＰｓが既に揃った自然分極が得られるのは、膜面積が約２０ｍｍ²以下が望
ましい。このとき、配向率のバラツキも著しく減少する。下部電極としてＡｕを用いた場合でも全く同じ結果が得られた。本実施例で作製した強誘電体薄膜素子を赤外線センサとして利用する場合、焦
電材料としての性能指数である［焦電係数／誘電率］の値は大きくなる。２００
℃で１０分間１００ｋＶ／ｃｍ印加して分極処理を行ったＰｂＴｉＯ₃セラミク
スの値と比較して、ＰｂＴｉＯ₃薄膜で２．５倍、ＰＬＴ薄膜で３倍の値を示す
。つまり、本発明による強誘電体薄膜を用いると、全く分極処理を行わなくても
優れた特性の赤外線センサが作製されることがわかる。上記の例でも分かるように本発明の強誘電体薄膜を用いた素子では分極処理を
行わなくても大きな出力が取り出せる。これは赤外線センサばかりでなく圧電素
子、光スイッチなど電気光学素子等においても同様である。発明の効果本発明による強誘電体薄膜素子は、分極処理が不要であり、また特性も優れていて、作製も容易であるから、実用的にきわめて有効である。Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric thin film element used for a pyroelectric infrared detecting element, a piezoelectric element, an electro-optical element, and the like. 2. Description of the Related Art There are various applications of ferroelectrics in the field of electronics, such as infrared detection elements, piezoelectric elements, light modulation elements, and memory elements. As electronic components have become smaller due to recent advances in semiconductor technology, ferroelectric elements are also becoming thinner. By the way, the change of the spontaneous polarization Ps of the ferroelectric is taken out as an output. For example, in a pyroelectric infrared detecting element or a piezoelectric element, the largest output is obtained when the Ps of the ferroelectric material is aligned in one direction. . Problems to be Solved by the Invention At present, ferroelectric porcelain used for infrared detecting elements, piezoelectric elements, and the like is a polycrystalline body, and there is no directionality in the arrangement of crystal axes. They are arranged in the eyes. In the epitaxial ferroelectric thin film and the oriented ferroelectric thin film, the polarization axes of the crystals are aligned, but the electric spontaneous polarization Ps forms a 180 ° domain and is alternately arranged. Therefore, when these materials are used as the above-mentioned electronic elements, a polarization treatment for applying a high electric field (〜100 kV / cm) to the materials and aligning the direction of Ps is necessary. Although there are many reports on the preparation of thin films such as PbTiO ₃ and PLZT, thin films in the region of the ferroelectric phase are oriented in the c-axis, which is the polarization axis, and spontaneous polarization in one direction. No oriented thin film has been realized. The method of applying a high electric field to the ferroelectric material to make Ps uniform has the following problems. (1) The dielectric breakdown may occur due to the polarization treatment, and the yield is reduced. (2) If many microelements are arranged at a high density like a high-resolution array element, it is difficult to uniformly polarize them. (3) In an integrated device in which a ferroelectric thin film is formed on a semiconductor device, polarization processing itself may not be possible. Means for Solving the Problems A substrate, a ferroelectric thin film formed on the substrate, and an electrode thin film attached to the ferroelectric thin film, wherein the film area of the ferroelectric thin film is 20 mm ² or less Is used. Effect In the ferroelectric thin film as described above, natural polarization in which Ps is already obtained can be obtained with good uniformity, and there is no need to perform polarization processing, and a high-yield, high-performance ferroelectric element can be realized. Embodiment FIG. 1 is a cross-sectional view of one embodiment of a ferroelectric thin film device manufactured according to the present invention. A MgO single crystal cleaved at (100) and mirror-polished was used as a substrate 1, and a Pt thin film having a thickness of 0.2 μm was formed as a lower electrode 2 by sputtering. Sputtering gas is Ar-O ₂ mixed gas. Next, a ferroelectric thin film 3 was grown with a film thickness of 4 μm and various film areas. The method is a high-frequency magnetron sputtering method, in which Ar and O ₂
A mixed gas of the sputtering target is a powder of _{{(1-Y) Pb x} La y Ti z Zr w O 3 + YPbO} ... (1). Table 1 shows the sputtering conditions. Next, a Ni—Cr electrode was deposited as an upper electrode 4 on the thin film to produce a ferroelectric thin film element. Further, an opening 5 was provided in the substrate 1 below the ferroelectric thin film 3. FIG. 2 shows an X-ray diffraction pattern of a typical thin film. Perovskite structure (0
Only the 01) and (100) reflections and their higher-order reflections are observed. Also (001)
Since the reflection intensity is significantly higher than that of (100), it can be seen that the film is a c-axis oriented film. The c-axis orientation ratio α is defined by the following equation. α = I (001) / {I (001) + I (100)} where I (001) and I (100) are (001) and (100), respectively.
Represents the diffraction intensity of the reflection. The dielectric constant and pyroelectric coefficient of the obtained thin film were measured. FIG. 3 shows a change in the pyroelectric coefficient γ of PbTiO ₃ with respect to the c-axis orientation ratio, and FIG. 4 shows a change in the dielectric constant ε. As is well known, the pyroelectric coefficient increases in proportion to the orientation of the spontaneous polarization Ps. The pyroelectric coefficient increases as the orientation ratio increases, and the dielectric constant decreases. FIG. 3 and FIG. 4 also show the results obtained when the polarization treatment (applying 100 kV / cm ^{2 for} 10 minutes at 200 ° C.) is performed. When the orientation ratio is small, the values of the pyroelectric coefficient and the dielectric constant largely change before and after the polarization treatment. When the orientation ratio becomes 75%, the pyroelectric coefficient becomes 1.6 × 10 ⁻⁸ C / cm ² K, which is 100 kV / 200 ° C.
PbTiO ₃ ceramics (γ = 1.8 × 10 ^-8 C / cm ²⁾
This is almost the same value as K). When the orientation ratio is 80%, the pyroelectric coefficient is 2.5 × 10 ⁻⁸ C /
cm ² K, which is considerably larger than the value of PbTiO ₃ ceramics. In addition, the value is not substantially different from the value after the polarization treatment, and is larger than the value after the polarization when the orientation ratio is small. The dielectric constant is about 1 at a value of の of the ceramics when the orientation ratio is 75%.
00. The same result was obtained in the case of TbTiO ₃ (PLT) to which La was added. As described above, with PbTiO ₃ and PLT thin films, c
It has been clarified that spontaneous polarization is uniform even if no polarization treatment is performed if the film is oriented in the axis, and that the effect is particularly large in a thin film having an orientation ratio of 75% or more. By the way, it has been found that the c-axis orientation ratio of the thin film changes depending on the film area. FIG. 5 shows the relationship between the c-axis orientation ratio and the film area. However, the results are when the film thickness is 4 μm. As is clear from the figure, the c-axis orientation rate greatly decreases as the film area increases. When the diameter of the PLT film is 10 mm, the c-axis orientation ratio drops to 70% or less on average. Also, the variation in the orientation ratio becomes large. Therefore, it is desirable that the film area is about 20 mm ² or less for obtaining the natural polarization in which Ps is already obtained from the above results. At this time, the variation in the orientation ratio is significantly reduced. Exactly the same results were obtained when Au was used as the lower electrode. When the ferroelectric thin film element manufactured in this embodiment is used as an infrared sensor, the value of [pyroelectric coefficient / dielectric constant], which is a figure of merit as a pyroelectric material, increases. 200
Compared to the value of PbTiO ₃ ceramics subjected to polarization treatment by applying 100 kV / cm at 10 ° C. for 10 minutes, the value of PbTiO ₃ thin film is 2.5 times and that of PLT thin film is 3 times. That is, it can be seen that the use of the ferroelectric thin film according to the present invention makes it possible to produce an infrared sensor having excellent characteristics without performing any polarization treatment. As can be seen from the above example, in the device using the ferroelectric thin film of the present invention, a large output can be obtained without performing the polarization treatment. This applies not only to infrared sensors but also to electro-optical elements such as piezoelectric elements and optical switches. Effect of the Invention The ferroelectric thin film element according to the present invention does not require a polarization treatment, has excellent characteristics, and is easy to manufacture, and is therefore extremely effective in practice.

【図面の簡単な説明】第１図は本発明の一実施例における強誘電体薄膜素子の断面図、第２図は本発
明の一実施例に於ける強誘電体薄膜のＸ線回折パターンを示す図、第３図はｃ軸
配向率と焦電係数の関係を示す図、第４図はｃ軸配向率と誘電率の関係を示す図
、第５図はｃ軸配向率と膜面積との関係を示す図である。１……基板、２……下部電極、３……強誘電体薄膜、４……上部電極、５……
開口部。BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a ferroelectric thin film element according to one embodiment of the present invention, and FIG. 2 is an X-ray diffraction pattern of the ferroelectric thin film according to one embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the c-axis orientation ratio and the pyroelectric coefficient, FIG. 4 is a diagram showing the relationship between the c-axis orientation ratio and the dielectric constant, and FIG. 5 is a diagram showing the relationship between the c-axis orientation ratio and the film area. FIG. DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Lower electrode, 3 ... Ferroelectric thin film, 4 ... Upper electrode, 5 ...
Aperture.

Claims

Claims: (1) A substrate and a lead titanate / lead zirconate titanate formed on the substrate
A ferroelectric thin film, and an electrode thin film attached to the ferroelectric thin film, and a film area at the time of production at a substrate temperature set such that the ferroelectric thin film has a crystal structure exhibiting ferroelectricity is obtained. A ferroelectric thin film element having a thickness of 20 mm ² or less. (2) One or more lower electrodes formed on the substrate, a ferroelectric thin film formed on the lower electrode, and one or more upper electrodes provided on the ferroelectric thin film 2. The ferroelectric thin film element according to claim 1, wherein said lower electrode is a thin film exhibiting crystal orientation. ( 3 ) The ferroelectric thin film element according to claim 1, wherein the ferroelectric thin film is formed by a sputtering method.