Oxide based on pyrochlore and having high dielectric constant and low temperature coefficient

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Field of the present invention]

The present invention relates to a crystalline ceramic material useful for the fabrication of capacitive elements, the ceramic material having a high dielectric constant and a reduced temperature coefficient of the dielectric as compared to the barium titanate strontium ceramic material.

【0002】

Background of the Invention

Efforts to reduce the dimensions of microelectronics and portable devices place considerable constraints on the properties of dielectric materials currently in use. For example, in the case of large-capacity dynamic random access memory (DRAM), it was once considered to use a new type of material with a high dielectric constant, such as barium titanate strontium (BST), for microelectronics capacity (Tay Amori (Tay Amori). T.Eimori) et al., IEDM93 631 (1993)). For macroscopic elements such as those used in microwave communication, Ba with a dielectric constant of 40-90 and a temperature coefficient of dielectric constant of 0-20ppm / ° C₂Ti₉O₂₀And BaLn₂Ti₅O₁₄Bulk ceramics such as are currently being actively developed. (T.Negas, G.Yeager, S.Bell and N.Coats, American Ceramic Society Britain (Am.Cer.Soc) .Bull. 72 80 (1993) and S. Nishigaki, H. Kato, S. Yano and R. Kimura, Ceramic Britain (. Ceramic Bulletin) 661405 (1987).

【0003】

A temperature coefficient with a low dielectric constant is important when temperature stability with high capacitance characteristics is a requirement for specific applications. Such stability is not possible with high dielectric constant materials such as BST due to the high temperature dependence of the dielectric constant K associated with ferroelectricity.

【0004】

Aging the high negative temperature coefficient of a ferroelectric with a Curie temperature (Tc) below room temperature with the high positive temperature coefficient of a ferroelectric having a Tc above room temperature has long been a problem in this area. Is. (NA Andreeva, OAGrushevskaya and VIZhukovskii, Bull.Acad.Sci. USSR, Physics Series ( Phys. Ser.) 24 1281 (1960). Such mixing, near room temperature, depends on relatively large dielectric constants, relatively small temperature dependence and the amount of high TC strong dielectric used in equilibrium. Is expected to have a non-negligible loss coefficient.

【0005】

High temperature ferroelectric perovskite PbNb_2/3Mg_1/3O₃And PbTiO₃And their mixtures, processes and chemical substitutions have been very well studied. (For example, MT Lanagan, N. Yang, DC Dube and SJ. Jang, Journal American Ceramic Society (J.) Am.Ceram.Soc.) 72 481 (1989); J. Chen, HMChan, MP Harmer, Journal American Ceramic Society (J. Am.Ceram> Soc) 72 593 (1989); M.Le Jeune and JP Boilot, American Ceramic Society Bull. 64 679 (1985); and SLSwartz, TRShrout, WASchulze and LECross, Journal-American Ceramic Society ( J.Am.Ceram.Soc.) 67 311 (1984)). Pyrochlore structure Pb₂(Nb, Mg)₂O_{6 + x}Low Tc ferroelectrics have also received considerable attention. (TR Shrout and SLSwartz, Material Research Britain (Mat.Res.Bull.) 18 663 (1983); and JD Siegwarth, W. WN Lawless and AJ Morrow; Journal Applied Phys. 47 3789 (1976)). However, there was generally no practical interest except as an impurity phase in ceramics intended as a perovskite phase material throughout (J. Chen and MP Harmer, Journal American). Ceramic Society (J.Am.Ceram.Soc.73) 68 (1990) and J. Chen, A. Gorton, HMChan and MP Harmer Journal American Ceramic Society (J. Am.Ceram.Soc.69 303 (1986)). Pb₂(Nb, Zn)₂O_{6 + x} Pyrochlor-PbTiO₃ Studies of the perovskite system have found polyphasic ceramics with a K of around 120 and a TCK (temperature coefficient of dielectric constant) of about 200 ppm / ° C near room temperature. (J. Chen, A. Gorton, HMChan and MP Harmer, Journal American Ceramic Society (J. Am. Ceram) .Soc.69 303 (1986)). The same trader found the presence of K in the 70-100 range and low TCK ceramics at +200 to -125ppm / ° C with the lowest TCK in composition with K = 90. , Bi₂O₃-NiO-ZnO-Nb₂O₅Found in the system. (HCLing, MFYan and WWRhodes, Journal Material Research (J.Mat.Res.) 5 1752 (1990)).

【0006】

[Abstract of the present invention]

The present invention is high Tc perovskite Pb (Nb, Mg, Ti) O₃ Low Tc pyrowl phase Pb with a small amount of₂(Nb, Mg, Ti)₂O_{6 + x} Equilibrate the temperature dependence of.

【0007】

The resulting material has a dielectric constant of approximately 200, which varies by ± 100 ppm over a temperature range of 80 ° C centered at 20 ° C. The Q value is approximately 200 due to the presence of perovskite. Given their properties, these new dielectric materials are considered to be an intermediate type between BSTs considered for microelectronics and BLTs considered for bulk.

【0008】

In the present invention, Pb₂(Nb, Mg, Ti)₂O_{6.5 + x} Pycroll and Pb (Nb, Mg, Ti) O₃ Dielectric materials made by properly mixing perovskite have been found to have a dielectric constant of approximately 200 and a temperature dependence of less than a percentage of K near room temperature. Given that the chemistry of these materials is very similar to well-studied ferroelectric perovskite such as PZT, they have already been used and are well known in the fabrication of electronic devices using these materials. The same bulk formation process and thin film deposition technique should be directly transferable. By combining a relatively large dielectric constant with a low temperature coefficient of dielectric constant, these materials may meet the requirements for electronic applications that require small volume, small area and good thermal stability capacitive elements. is there.

【0009】

Description: TECHNICAL FIELD [Detailed description of the present invention]

In the present invention, the combination of a high dielectric constant and its small temperature dependence is a planar quaternary system PbO-MgO-Nb.₂O₅-TiO₂I found that it was possible. In this case, perovskite and pyrochlor are in equilibrium, and for example, 1PbO: Σ (Mg + Ti + Nb) = 1 is preferable. The most suitable and preferred material is Pb₂Mg_1/3Nb_2/3O_0.65-PbTiO₃-PbMg_1/3Nb_2/3O₃It's in a triangle.

【0010】

By powder X-ray diffraction evaluation of these materials, Nb, Mg and Ti form a wide range of solid solutions in both pycrole and perovskite phases, have dielectric constants in both single-phase and multi-phase materials, and are multi-phase. The material was found to change significantly as a function of the transition metal ratio. A preferred mixture, which is a properly compensated material, is 90-95% Pb (estimated from diffraction measurements).₂(Nb, Mg, Ti)₂O_{6 + x}Pychlore and 5-10% Pb (Nb, Mg, Ti) O₃ It was a mixture of perovskite. The relatively small amount of perovskite phase required to compensate for the TCK of pyrochlor interferes with the Q value of the synthetic material and is dominated by the loss of the ferroelectric perovskite, resulting in degradation or extremely low values.

【0011】

The following examples are intended to show examples and are not intended to limit the field of view of the present invention in any case.

【0012】

[Example]

Starting materials for ceramic synthesis are PbO, Nb₂O₅, TiO₂And Mg (OH)₂Met. The powder was mixed in an appropriate molar ratio and mechanically ground with a stirring motor and pestle. The powder is densely coated Al₂O₃Burned in the atmosphere in a crucible. The first treatment was at about 800 ° C for about 16 hours, followed by about 850 ° C for 16 hours. After re-grinding, the powder burned at about 925 ° C for about 4 hours and burned at about 1000 ° C for about 3 hours. The powder is then ground a third time and pressed into 0.5 inch diameter pellets in a covered crucible 20 mm in diameter and 4 mm in height for about 4 hours, at about 1100 ° C, then for about 3 hours. Burned at about 1200 ° C. The cover and short reaction time were a precaution against the volatility of PbO, and the surface was ground before testing the dielectric properties of the pellets. No loss of PbO was shown.

【0013】

A contact was made with a 1: 1 Ga: In alloy applied to the pellet surface. The dielectric constant and dielectric loss tangent (loss tangent) were measured at 100 kHz and 1 MHz with an HP4192A impedance analyzer. The temperature was changed at -20 to 60 ° C in the RANSCO variable temperature vessel.

【0014】

In the present invention, the study of Pb-Nb-Mg-Ti-O phase diagram is (Nb₂O₅+ MgO + TiO₂) With a ratio of PbO equal to 1: 1 ternary plane, eg PbTiO₃Surface containing and PbMg_1/3Nb_2/3O₃Perovskite, Pb₂Nb₂O₇And Pb₂Mg_1/3Nb_2/3O_6.5Focused on pyrocrawls. The dielectric constants at 20 ° C and their temperature dependence were measured with various compositions.

【0015】

It was found that one particular region of the phase diagram shown in FIG. 1 shows a good combination of high dielectric constant (K) and low temperature dependence (TCK) of the dielectric constant. Figure 1 shows the contours taken at intervals of ΔK = 50 at 100-400K in this small part of the 1PbO: (xNb + yMg + zTi) O5 / 2x + y + 2z quaternary plane. Here, x + y + z = 1. Near the contour of K = 200, the shaded region of FIG. 1 shows a set of compositions in which the TCK changes sign from negative at the low dielectric constant to positive at the high dielectric constant. As the positive TCK increases, the dielectric constant increases and Pb (Nb, Mg, Ti) O₃The proportion of perovskite phase increases. Two parts of this shaded composition area were selected for further examination. The goal is to find the most balanced composition of high K and low TCK near room temperature between around 0.15Ti and 0.20-0.25Ti content.

【0016】

Dielectric data for the best materials examined in detail are shown in Table 1. The data shown here are concentrated in the area near the intersection with 0.25Ti in the shaded area in Fig. 1. Table 1, FIG. 2 and FIG. 3 show the data at 100 kHz, and the data at 1 MHz are significantly different.

[table 1]

<img file="JPH09118564A_D0001.tif" />【0017】

The dielectric constant for a balanced composition is in the range 180-240 and the value of D (= tan δ) is in the range 0.005-0.007. The general behavior of the material is that it is relatively constant for small changes in composition, i.e., for a K near 200, the material is well balanced, but the detailed temperature dependence of K near room temperature is It is quite sensitive to slight changes in composition. This is, for example, in Table 1, Pb₁±_0.03Nb₀_.65Mg_0.105Ti_0.245O_xIt is clear as a function of Pb excess and deficiency for the properties of. In addition, the T vs. K details have a sensitivity of ± 1-2% with respect to the Nb, Mg and Ti contents.

【0018】

The temperature dependence of the dielectric constants for some typical materials is shown in FIGS. 2 and 3. For example, the T vs. K minimum near 20 ° C shown in Figure 2 is the result of a good equilibrium between the positive and negative temperature coefficients of K for the perovskite and pyrochlor phases. Figure 3 shows the sensitivity of T to K leading to lead excess or deficiency, with the best balanced composition near the "stoichiometric" PbO: (Nb + Ti + Mg) = 1: 1. In Fig. 2 and Table 1, the best well-balanced material with high K centered around 20 ° C is Pb._1.01Nb_0.65Mg_0.10Ti_0.25O_x It is shown that the dielectric constant has a very shallow minimum of T vs. K at about 180 and 30 ° C. Also, the material PbNb, which has a fairly high dielectric constant of about 230._0.645Mg_0.105Ti_0.25O_x Note that at a temperature of 20-60 ° C, which is expected to be the normal operating temperature for electronic elements, it equilibrates very well and the magnitude of K varies by 0.2% over that temperature range. is important.

[Simple explanation of drawings]

[Figure 1]

PbO-Nb in the plane of Pb: Σ (Nb + Mg + Ti) = 1: 1₂O₅-MgO-TiO₂In a part of the phase diagram, the contour shows the value of the dielectric constant at 20 ° C, and the shaded region shows the composition region where the slope of T vs. K changes from negative at low K to positive at high K.

[Figure 2]

The figure which shows the temperature change of K around 20 ° C for some of the best compensated materials.

[Fig. 3]

The figure which shows the temperature change of K around 20 ° C with respect to the concrete Nb: Mg: Ti ratio as a function of Pb excess and deficiency.