WO2002008147A1

WO2002008147A1 - Dielectric ceramic barium titanate composition and electronic device

Info

Publication number: WO2002008147A1
Application number: PCT/EP2001/008467
Authority: WO
Inventors: Yukiko Furukawa; Detlev Hennings
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2000-07-26
Filing date: 2001-07-20
Publication date: 2002-01-31

Abstract

The dielectric ceramic composition comprises BaTiO3 as a main component and a grain growth limiter as an additive, while it is free of Bi and Pb and is sinterable at a temperature of less than 1300 °C. Preferably the grain growth limiters are Cr2O3, MnO and Fe2O3. The resulting ceramic has a relative dielectric constant of 900-3000 and a dielectric loss of less than 0.02, preferably less than 0.01. The dielectric ceramic composition can be applied in a capacitor and a capacitive element very suitably.

Description

DIELECTRIC CERAMIC BARIUM TITANATE COMPOSITION AND ELECTRONIC DEVICE

The invention relates to a dielectric ceramic composition comprising BaTiO₃ as a main component, having a sintering temperature of less than 1300 °C and being free of Bi and Pb.

The invention also relates to an electronic device comprising a first and a second electrode and a dielectric layer.

Such a dielectric composition and such an electronic device are known from EP-A-960868. The known dielectric composition comprises (Ba Ca Sr Nd Gd) TiO₃ and a silicatitanate as an additive. This composition has a relative dielectric constant of about 75 and has a temperature dependence of capacity of less than 0.3% over the temperature range of -55 to 125 °C. Due to the low sintering temperature of less than 1300 °C, preferably less than 1200°C, it is sinterable with base metal electrodes such as nickel.

However, for many applications the relative dielectric constant of 75 is low.

It is therefore a first object of the invention to provide a dielectric ceramic composition of the type mentioned in the opening paragraph, which has a relative dielectric constant between 100 and 3,000 and of which the dielectric loss is less than 0.02. It is a second object of the invention to provide an electronic device of the type mentioned in the opening paragraph, of which the first and second electrodes contain base metal and of which the dielectric layer contains the dielectric ceramic composition of the invention. The first object is realized in that the composition comprises means for limiting grain growth. It has been found that the dielectric loss decreases with the grain size. Although the dielectric constant of the BaTiO₃ is kept relatively low by limiting the grain size, it is still high enough. For example, with a grain size of about 30 micrometer after sintering, the relative dielectric constant is more than 8,000. By limiting the grain size to less than 5 micrometer, and especially less than 3 micrometer, the relative dielectric constant is less than 3,000. This different behavior of BaTiO₃ that depends on the grain size is explained through different states. If the grain size is relatively large, for instance larger than 10 microns, the BaTiO₃ will have ferro-electric characteristics. In this state each crystal grain contains a domain where spontaneously polarized foci based on permanent dipole moments are oriented in one direction. If the grain size is very small, the BaTiO₃ will be in a state which is not ferro-electric, but comparable to that in common dielectrics. There is thus a size- effect. So, a dielectric ceramic composition can be provided which fulfills the objects of the invention.

The means for limiting grain growth comprise MnO, Cr₂O₃ andFe₂O₃. However, it cannot be excluded that other salts of Mn, Cr and Fe or other elements are effective as grain growth limiter as well. The inventors explain the behavior of these elements as grain growth limiters, in that these elements cannot be incorporated in the crystal lattice of the grains formed very well. They appear to be present at the surface of a grain primarily and act as a force against aggregation. If such means are present in an amount of more than 5.0% however, the dielectric loss increases and the dielectric ceramic composition will be unusable. In preferred embodiments, the dielectric loss is less than 0.01

In an advantageous embodiment before sintering an average grain size of at most 0.3 microns. Due to the increasing surfacial area with decreasing average grain size sintering will take place more easily. Hence, it is possible to sinter the ceramic composition at a lower temperature. Next to the fact that the electrodes may not withstand high sintering temperatures, it was found that the dielectric loss increases with increasing sintering temperature. In another embodiment (Ca Sr Ti Zr)O₃- based ceramics is present in the dielectric composition. For example this is (Cao.₈Sro.₂)(Ti_1-xZr_x)O₃, with x between 0 and 1.0. The composition according to this embodiment has a relative dielectric constant of 200. When x = 0.85 or higher the temperature dependency of capacity is less than 120 ppm/°C and fulfills the NPO-standard; i.e. has a temperature dependency of dielectric constant of less than 0.5% over the temperature range of -55 to 125 °C. For smaller values of x, the temperature dependency of this kind of ceramics is negative. On the contrary, the BaTiO₃ with grain growth limiters has a relative dielectric constant of 1,000-3,000 and a thermal dependency of the dielectric constant of +2,000 ρpm/°C. By taking the BaTiO₃ with grain growth limiters and the (Ca Sr Ti Zr)O₃-based ceramics together, the resulting dielectric ceramic composition will have a relative dielectric constant between 200 and 1000 and a very small thermal dependency of the dielectric constant.

In order to prevent any chemical reaction between the phase of the (Ca Sr Ti Zr)O₃ on the one hand and the phase of BaTiO₃ with grain growth limiters on the other hand, it is preferred to use so-called A-site rich BaTiO₃, small amounts of additives with relatively high melting points or eutectic points and a low sintering temperature. In order to obtain a low sintering temperature, a Si-containing additive can be used, such as ZnSiTiOs, CaSiTiO₅, ZnSi₂TiO₇, SiO₂, LiMgSiO_y.

The second object of the invention is realized in an electronic device having a first and a second electrodes and a dielectric layer, wherein that the dielectric layer comprises the dielectric composition of any of the previous Claims. The electrodes are preferably capacitor electrodes. The electronic device of the invention can be a discrete ceramic multilayer capacitor, a low-temperature cofired ceramic substrate, an array functioning as a ceramic filter. Due to the relatively high dielectric constant these electronic devices with the dielectric composition of the invention are suitable for high-voltage applications.

The electronic device can also be a multilayer laminate, wherein the dielectric composition is integrated in the polymeric layers as a powder. Due to the powdery form of the ceramics in a polymeric layer with a low dielectric constant, a high dielectric constant of the powder is very important to integrate capacitors into such a multilayer laminate. Thus, the dielectric composition with its relatively high dielectric constant and its low dielectric loss is suitable for that.

In another embodiment, the electronic dielectric layer is a thin film device, such as a passive network or a integrated circuit on a device. The dielectric layer can be part of a thin-film capacitor. It can also be a high-K dielectric layer in use as gate dielectrics in a transistor. It is known that on application of a BaTiO₃- or a (Ca Sr Ti Zr)O₃ based ceramics or any similar ceramics as a thin film, the dielectric constant decreases strongly in comparison to the dielectric constant in bulk. Due to the relatively high dielectric constand and its low dielectric loss the dielectric composition of the invention is suitable for these embodiments of the device of the invention.

These and other aspects of the dielectric ceramic composition and the electronic device of the invention will be further elaborated in the following embodiments.

Embodiment 1

The dielectric ceramic composition according to this invention was prepared as follows: To BaTiO₃ was added an additive such as Cr₂O₃, Fe₂O₃ and MnO, the addition amount being adjusted as appropriate, and the mixture was subjected to wet mixing with water as a solvent for 15 hours in the presence of balls made of yttria-stabilized zirconia, and dried. To the resulting powdery mixture polyvinylalcohol was added to serve as an organic binder, to turn the powder into a mass of particles. The resulting particulate mixture was subjected to uni-directional pressure molding under a press molder exerting a surface pressure of 3 ton/cm², to give disc-shaped pieces each 16.5 mm in diameter and 0.7 mm in thickness. The disc-shaped samples were sintered at 1,150 to 1,300 °C for two hours in a reductive atmosphere consisting of 1% H₂ plus 99% N₂. After sintering , each sintered ceramic body had its both surfaces coated with silver paste, and fired at 750 °C in the air to have external electrodes formed thereupon.

For a single plate type capacitor prepared from the sintered body, its relative dielectric constant (ε_r), and dielectric loss (tan δ) were determined at 1 MHz and 1 Vrms using an automated bridge-based meter. Determination of the temperature dependency of the electrostatic capacity of a test capacitor (TC)(ppm/°C) consisted of measuring the capacity at +25°C to use the measurement as a standard, and determining the temperature dependency of the electrostatic capacity between -55 and +125°C. Table 1 lists the electric properties of single plate type capacitors prepared from sintered bodies obtained as above.

Embodiment 2 The dielectric ceramic composition of the invention was used in a ceramic multilayer capacitor. In the same manner as described above, BaTiO₃ as the main component, and the additive were subjected to wet mixing with water as a solvent in the presence of balls made of yttri-stabilized zirconia for 20 hours. To the resulting powdery mixture polyvinylalcohol is added as an organic binder, and the yield was subjected to wet mixing to give a ceramic slip. The ceramic slip was molded by the doctor blade method into sheets, and rectangular sheets with a thickness of 21 μm were obtained. Next, Ni-based electroconductive paste was printed on the ceramic green sheet, to form an internal electrode thereupon. A plurality of green sheets were placed one over another so that their ends in contact with the electroconductive paste layer appeared alternately, to give a laminated body. This laminated body was sintered in a reductive atmosphere of 1% H₂ plus 99% N₂ at 1150 °C for two hours. After sintering, the lateral surfaces of the ceramic sintered body were coated with silver paste. The body was fired in the air at 750 °C to form external electrodes connected electrically with internal electrodes. The ceramic multilayer capacitor obtained as above has an external size of 3.2 mm in width, 1.6 mm in length and 0.5 mm in thickness. Each dielectric ceramic layer between adjacent internal electrodes has a thickness of 10 μm, and the total number of effective dielectric ceramic layers was five. The feature characteristic with the capacitor based on the dielectric ceramic composition obtained as above includes the following: a relative dielectric constant of 900 to 3000, a dielectric loss of 0.003 to 0.01.

Embodiment 3

A 2 mol% Cr₂O₃ is added to BaTiO₃ to give a BaTiO₃-based ceramic. This ceramic has a relative dielectric constant of 800, a thermal dependence of dielectric constant of + 1500 ppm/°C and a dielectric loss (tan δ) of 0.01. A (Cao.₂₉₇Sro.₆₉₃)(Tio.₉₇Zro.o₃)O₃ based ceramic has a relative dielectric constant of 220, a thermal dependency of dielectric constant of - 1820 ppm/°C and a dielectric loss of 0.0001. These components are subject to a preliminary sintering at a temperature of 1300 °C, while care is being taken to prevent the two substances from reacting with each other. The two components are briefly subjected to wet mixing with water as a solvent in the presence of balls of yttria-stabilized zirconia, and dried. To the resulting powdery mixture is added polyvinylalcohol to serve as an organic binder. The resulting particulate mixture is subjected to uni-directional pressure molding under a press molder extering a surface pressure of 3 ton/cm , to give disc shaped pieces. These pieces are subjected to liquid-phase sintering at a temperature of 1150 °C for two hours in a reductive atmosphere of 1%H₂ and 99% N₂, to give ceramic sintered bodies.

Claims

CLAIMS:

1. A dielectric ceramic composition comprising BaTiO₃ as a main component, having a sintering temperature of less than 1300 °C and being free of Bi and Pb, characterized in that the composition comprises means for limiting grain growth.

2. A dielectric ceramic composition as claimed in Claim 1, characterized in that said means are chosen from the group of MnO, Fe₂O₃ and Cr₂O₃.

3. A dielectric ceramic composition as claimed in Claim 1 or 2, characterized in that the composition contains grains which have an average grain size of at most 0.3 microns before sintering.

4. A dielectric ceramic composition as claimed in Claim 2, characterized in that Cr₂O₃ is present in an amount of 0.1-3 mol%.

5. A dielectric ceramic composition as claimed in Claim 2, characterized in that

Fe₂O₃ is present in an amount of 0.1-5 mol%.

6. A dielectric ceramic composition as claimed in Claim 2, characterized in that MnO is present in an amount of 0.1-5 mol%.

7. A dielectric ceramic composition as claimed in Claim 1 or 2, characterized that (Ca Sr Ti Zr)O₃- based ceramics is present.

8. A dielectric ceramic composition as claimed in Claim 7, characterized in that the molar ratio of the BaTiO₃ and the (Ca Sr Ti Zr)O₃ is in the range of 0.2 to 5.

9. An electronic device comprising a first and a second electrode and a dielectric layer, characterized in that the dielectric layer comprises the dielectric composition of any of the previous Claims.