NL8303448A - MULTI-LAYER CERAMIC CAPACITOR. - Google Patents

Description

* i .. t. · EHN 10.795 1 N.V. Philips1 Incandescent light factories in Eindhoven "Multilayer ceramic capacitor"

The invention relates to a downward capacitor composed of alternating layers of a dielectric oxidic ceramic material and an electrode material, forming a compact unit by virtue of the application of temperature and pressure.

5 More layer capacitors of the type described above (so-called monolithic ceramic capacitors) have hitherto been qp industrial scale i.h.a. manufactured in the following manner.

A slurry of finely ground ceramic dielectric powder mixed with a binder is deposited in thin layers which are dried into films and then electrodes provided by screen printing a metal paste on it. These films are stacked and compressed and divided into separate capacitor bodies. These condensate bodies are sintered at temperatures between 1200 and 1400 ° C depending on the composition of the ceramic dielectric material. During sintering, the ceramic dielectric powder shrinks compactly and compacts into a dense polycrystalline structure. At the same time, the powder particles from the metal paste sinter to form electrode layers of metal which form a coherent whole with the dielectric layers.

In practice, however, it is difficult to make a stack of flawless thin layers. One has to do with the occurrence of short circuits at low voltages (these are attributed to too high porosity, or to the occurrence of cracks and delaminations = non-adhesion of layers in the final product).

The porosity could be limited by having the sintering take place under simultaneous application of pressure (so-called pressure sintering). If this pressure is not applied on all sides (isostatic), but in a direction transverse to the plane of the layers (uniaxial), the occurrence of delamination could also occur for greaves.

However, tests which have led to the present invention have shown that a new problem arises when using a uniaxial pressure sintering process to manufacture downward capacitors: the end products do not meet the requirements because the metal Λ * - n * 7 ' * '' '' ............. ~ ....... ...... '—......

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of the electrode layers zidi Mixes to a greater or lesser extent with the oxidic ceramic material of the dielectric layers.

This is attributed to the fact that due to the high pressures exerted in the uniaxial pressure interprocess, the yield strength of the metal of the electrode layers is passed.

The relatively soft metal under these conditions is then easily pressed into the ceramic material, which is not yet fully closed at the start of the sintering. This pressing problem is more serious the thinner the ceramic layers.

The invention solves this problem in that it provides a multilayer capacitor of the type described in the opening paragraph, which is characterized by layers of electrode material consisting of a mixture of 7 to 60 volume percent metal with a high electrical conductivity and of 40 to 93 volume percent ceramic particles. 15 of a material having a temperature above the sintering temperature of the oxidic ceramic of the dielectric layers.

The addition of a certain amount of ceramic particles, which do not participate in the sintering process, to the metal 20 of the electrode layers leaves space for the metal during the uniaxial pressure interprocess between the dielectric ceramic layers. The ceramic particles act as pillars which ensure that the metal cannot be pressed into the ceramic layers.

The "pillars" have a relieving effect, as a result of which the pressure is not put directly on the metal. For this function to be effectively performed, the proportion of ceramic particles in the electrode layer should preferably not be less than 40% by volume. With a proportion of more than 90% by volume of ceramic particles, the conductivity of the electrode layers may become too low for certain applications. Depending on, among other things, the size of the particles, a proportion of 7 is just full. % of metal in the electrode layer still has a reasonable conductivity.

Representative materials for the metal of the electrode layers are e.g. Pd (melting point 1552 ° C), Pt (melting point 1774 ° C), 35 Ni (melting point 1445 ° C), Ag (melting point 961 ° C), Cu (melting point 1083 ° C) and alloys thereof.

Suitable materials for feeding the ceramic particles into the electrode layers are, for example, ZrO, and A ^ O ^. Also barium titanate "i" ~ * Λ

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2 2 2 ΕΉΝ 10,795 3 is useful if it has such a sinter reactivity different from the sinter reactivity of the barium titanate of the dielectric that it does not sinter well with it. Representative oxidic ceramic materials for the dielectric are titanates, zirconates, titanate zirconates, and tungstates. Most of these are ferroelectric at room temperature. The exception is Sr-titanate, which is non-ferroelectric at room temperature. This and other non-ferroelectric materials can also be used as a dielectric in capacitors according to the invention.

Barium titanate is preferably used for the dielectric layers to achieve 10 cm multilayer scan results that meet certain high quality requirements. For various purposes, this may still contain small but specific amounts of Co, Ni, Mn, Mg, Ta, Bi and / or Nb. In order for the barium titanate to obtain the desired dielectric properties, it must be fired in an oxidizing atmosphere (high partial oxygen pressure). When manufacturing multi-day capacitors by means of a conventional sintering process has hitherto mostly used Pd as the electrode material.

The invention makes possible the use of a uniaxial pressure sintering process, whereby the top temperature can be about 200 ° C lower than in the conventional sintering process. This means that the electrode material can consist of an Ag-Pd alloy. The lower the busy inter-temperature, the higher the Ag content (and the lower the price) can be. If the Ag content increases, especially above 30 at%, 25 experiments have shown that without the addition of particles of ceramic material not participating in the sintering process which the invention prescribes, the probability that the electrode layers are pressed into the ceramic material becomes increasingly greater.

In the context of the conventional sintering process of multilayer scanning capacitors, it has been proposed to add ceramic particles to the material of the electrode layers. However, this had a very different purpose than that which is pursued by the invention, namely to absorb the difference in expansion coefficient between electrode material and ceramic or better to sinter electrode material and ceramic together. In both cases, the ceramic material to be added to the electrode metal had essentially the same composition as the ceramic material of the dielectric layers. Since no pressure is exerted on the depositing structures in the conventional sintering process, the problem for which the invention provides a solution does not arise.

The invention will be further elucidated on the basis of an exemplary embodiment.

Figure 1 shows a busy inter system;

Figure 2 shows, partly in view and partly in section, a multilayer ceramic capacitor;

Figure 3 shows the phase diagram of Ag-Pd.

For the manufacture of multilayer ceramic capacitors 10 according to the invention, a pressurized inter system as shown in Figure 1 can be used. This is in principle a pressure system consisting of a hydraulic press (1) fitted with a cylinder (2) with a water-cooled head. The pressures applied depend on the top temperature applied during the pressure sintering process. The interesting pressure range is between 200 bar and 5 kbar. The pressure can be read on a manometer (3). The pressure is transferred via a (top) stem (4) of refractory (= high temperature resistant) material such as Al2 ° 3 SiC.

A package (5) to be compressed is placed on another stripe 20 (6), also of refractory material. Care is taken to ensure that the star faces that apply pressure to the package are as parallel as possible. The outriggers (4,6) are arranged in a tube furnace (7) provided with a heating coil (8). The hydraulic system (1,2) offers the possibility of controlling the pressure build-up, while the closed casing offers the possibility of operating under different gas conditions.

For example, to make the dielectric ceramic films, e.g. use a low-doped barium titanate, preferably with a particle size such that after hot pressing, the grain size is less than 1 µm.

With a barium titanate powder containing 1.2 wt.% Nb 2 O <5 031 0.3 wt.% Co 2 O 3, a dielectric constant £ * r of 3000 can be realized and a capacitor having a terreperature coefficient of the capacity TC can be obtained =. 100% which is smaller ^ 25 35 than 15% in the temperature range-from -55¾ to 125¾. This complies with the so-called X7R specification. (For comparison, in order to meet the Z5U specification, the temperature coefficient of capacitance must be between + 22% and -56% in the temperature range “” ** ^ * * f * · «* PHN 10.795 5 +10 ° C to 85 ° C) The powder is mixed with an organic binder (eg polyvinyl alcohol). Sheets are drawn with a thickness of several tens of microns, in particular from the casting mass thus obtained. 20-50micron. After drying, films are cut from this to the correct size 5. Patterns of electrode material consisting of a mixture of oxidic-ceramic powder and metallic powder in a binder are applied to the films by means of screen printing.

The respective sheets are stacked to form a package (5) in which, with each final capacitor body, the electrode 10 cartridges (9, 10) pass alternately to one side and to the opposite side, so that each two consecutive electrode cartridges partially overlap (fig. 2). To obtain good adhesion of the layers to each other, the package (5) is first laminated by subjecting it to a pressure of about 15 kbar at a temperature of about 75 ° C. This is followed by the sticking out of the binder from the films. This can be done in a separate oven. However, in the use of a uniaxial pressure interprocess, the firing can be done in a very practical manner by causing it to take place in the pressure sintering equipment during the heating of the complete packages to the top temperature.

The package 5 to be subjected to a printing interprocess is placed between the stamps 4 and 6 of the printing system 1.

The furnace 7 with the package 5 placed therein to the pressure sintering temperature takes place in about 90 minutes. Depending on the composition of the oxidic ceramic, dielectric material, the pressure temperature can be between about 900 and 1200 ° C. The pressure sintering temperatures are about 20 ° C lower than the sintering temperature and in the conventional sintering process. When the pressure temperature is reached, the pressure is applied. In the present experiments, the pressure intertemperature was about 1080 ° C and the pressure about .500 bar. Depending on the set temperature and pressure, a sintering time of up to about 120 minutes may be required. The pressure is then released and the whole cooled to room temperature.

The compact package obtained after hot pressing and cooling to room temperature is cut into pieces to obtain individual monolithic ceramic capacitor chips. For each such body, the sides (11, 12) in which the electrodes r> T “~’ ', Λ i ;. -i, -. - EHN 10.795 6 \ mouths to form metallized head contacts (fig. 2). The resulting capacitors showed the following properties:

Dimension N n d C tg <5. V

5 (nF) (1 ° J (104mH) (v) 1210 34 50 17 230 157 2 630 1812 70 50 17 900 160 1 580 10

Herein is: N: number of dielectric layers d: thickness of dielectric layers n: number of samples C: capacity 15 tg ^: dissipation factor (loss tangent) R. Λ: insulation resistance

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VQ: voltage breakdown

These results show that a very high capacity per volume unit is achievable. If dielectric layers thinner than 17 µm are realized, then one is even present in the ^ uF region.

Figure 3 shows the phase diagram of the Ag-Pd system. Depending on the Ag content, the melting point of Ag-Pd is between 961 ° C and 1552 ° C. The invention allows the use of Ag-Pd with a substantial Ag content as the electrode metal.

In summary, it can be stated that the invention enables the manufacture of multi-layer ceramic capacitors with dielectric layers with a thickness of at most 20 µm, which are practically pore-free (density> 99%). 30 35 G '': 4 3

Claims (6)

A multilayer capacitor composed of alternating layers of a dielectric ceramic ceramic and an electrode material, forming a compact unit on the basis of the application of temperature and pressure, characterized in that the capacitor forms a unit on the basis of a heat treatment while simultaneously applying pressure in a direction transverse (¾) to the plane of the layers, and layers of electrode material have a mixture of 7 to 60% by volume metal with high conductivity and from 40 to 93% by volume particles of a ceramic material with a sintering temperature 10 which is above the inter temperature of the axydic ceramic material of the dielectric layers. Multilayer capacitor according to claim 1, characterized in that the ceramic material of the electrode layers consists of ZrC> 2. Multilayer capacitor according to claim 2, characterized in that the axydic ceramic material of the dielectric layers consists of barium titanate. Multilayer capacitor according to claim 1, 2 or 3, characterized in that the metal of the electrode layers consists of a 20 Ag-Pd alloy. Multilayer capacitor according to claim 4, characterized in that the Ag-Pd alloy contains at least 30 at% Ag. Multilayer capacitor according to any one of the preceding claims, characterized in that the layers of dielectric, oxidic, ceramic material have a thickness of at most 20 µm and a density of at least 99%. 30 35 -ï ~ «* ·" J