JPS6148245B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention particularly relates to a method for forming grain boundary layers in semiconductor ceramics, in which metal oxide is thermally diffused into the grain boundaries of strontium titanate semiconductor ceramics. It is conventionally known that a grain boundary layer type semiconductor ceramic capacitor can be obtained by insulating the grain boundaries of strontium titanate-based semiconductor ceramics. This strontium titanate-based semiconductor porcelain is obtained by adding a trace amount of a pentavalent metal compound such as niobium or tantalum to strontium titanate, molding it, and firing it at a high temperature of about 1300 to 1450°C in a reducing atmosphere. That is, it can be obtained by methods such as atomic control and forced reduction. Next, for grain boundary insulation, there is a method of heat-treating in the atmosphere and re-oxidizing the grain boundaries, but since the insulation resistance and dielectric breakdown voltage are said to be low, recently, valence compensation elements, e.g. A method of obtaining a grain boundary layer with good electrical insulation by thermally diffusing copper, bismuth, manganese, etc. is becoming popular. The conventional method for manufacturing grain boundary layer type semiconductor ceramic capacitors has been described above, but a feature of grain boundary layer semiconductor ceramic capacitors is that they have a structure in which a dielectric layer (insulating layer) is interposed between semiconductor particles. Although the dielectric constant of the body layer itself is small, the apparent dielectric constant becomes extremely large. That is, the apparent dielectric constant is approximately determined by the ratio between the average particle diameter of the semiconductor particles and the average thickness of the dielectric layer.
Therefore, for semiconductor ceramics with a substantially constant average grain size, by diffusing a certain amount of valence control element, a grain boundary layer type semiconductor ceramic capacitor with stable quality should be obtained. however,
Conventionally, when forming a grain boundary layer, it has been usual to define the formation of the grain boundary layer by the diffusion species, the amount of adhesion, and the heat treatment temperature. In a method based on such regulations,
It was unclear to what extent the diffused components actually existed inside the sintered body after diffusion. In fact, by attaching a certain amount of diffused species to a certain sintered body (semiconductor porcelain),
It has been confirmed that the amount of diffused species in the sintered body always reproduces a constant value only when heat-treated in the air at a constant temperature in a certain arrangement on a certain pod, and the electrical performance is always the same. is obtained.
However, it is clearly confirmed that when the type of pod is different or when the sintered body is arranged differently, the amount of diffused species present is different. This is due to dispersal species being taken away by contact with the pods or evaporation into the atmosphere. Therefore, if the above-mentioned conditions are stipulated, it is necessary to stipulate the material and arrangement of the pods, and also stipulate that the atmosphere must be static or in a constant airflow. The present invention takes into consideration the shortcomings of the conventional regulations regarding grain boundary layer formation as described above, and by confining semiconductor porcelain to which diffused species are attached in a semi-closed container and thermally diffusing it, the amount of attached diffused species and the amount after heat treatment are determined. This made it possible to almost match the amount of diffused species in the sintered body. With this method, there is no need to consider how the sintered bodies are arranged, and a uniform diffusion effect can be obtained even if the sintered bodies are packed in bulk, making it an extremely mass-producible method. Hereinafter, the present invention will be described in detail based on examples, while also describing conventional examples to make the explanation easier to understand. Titanium oxide (TiO ₂ ), strontium carbonate (SrCO ₂ ), and niobium pentoxide (Nb ₂ O ₅ ) were mixed at a molar ratio of 1:1:0.002, and then molded at a pressure of 300 Kg/cm ² Then, it was calcined at 1200℃. Thereafter, the powder was pulverized so that 90% or more of the particles had a particle size of 3 μm or less, and the pulverized powder was mixed with a 5% aqueous solution of polyvinyl alcohol at a weight ratio of 10:1.
This is granulated through a 30 mesh sieve,
It was molded to a size of 11.5 mmφ x 0.48 mmt under a pressure of 1 ton/cm ² . After removing the binder from this molded body at 1000℃ for 2 hours, it was heated at 1390℃ in a mixed gas flow of hydrogen and nitrogen.
After firing for 4 hours, a sintered body of 9.8 mmφ×0.4 mmt was obtained. The average grain size of the crystal grains of the sintered body thus obtained was 20 μm, and the average grain size was almost uniform for all the sintered bodies. Furthermore, the electrical resistivity of this sintered body was 0.4Ω·cm, which was sufficient to make it a semiconductor. A predetermined amount of a mixture of Cu ₂ O and Bi ₂ O ₃ was deposited on the surface of the semiconductor porcelain obtained in this manner using an ethyl cellulose carbinol acetate solution as a binder, and the binder was removed at 500°C for 30 minutes after deposition. . Table 1 shows the composition of the diffused species, the amount of adhesion, the composition after diffusion, the status of the remaining amount, and the representative results for each when diffused at 1200℃ for 2 hours using the various pod packing methods shown in Figures 1 to 3. It exhibits good electrical characteristics. However, in Table 1, ε is the apparent dielectric constant, and from the capacitance C measured under an AC voltage of 1V, 1KHz, the element thickness t, and the electrode area S, ε=
This is a value determined by the formula Ct/ε ₀ S. Here, ε ₀ is the dielectric constant of vacuum. Furthermore, tan δ is a value similarly measured under an AC voltage of 1 V and 1 KHz. CR is the above capacitance C and DC voltage 50V.
CR = the product of the insulation resistance value R immediately after applying the voltage for seconds
There is a relationship: εε ₀ r. Here r is specific resistance and r=
Rt/S. BDV is the breakdown voltage, which is created by boosting the DC voltage with a variable voltage transformer and applying it between the electrodes of the element body.
This is the voltage just before a current of 1mA or more flows. Further, the composition and residual amount after diffusion were determined from quantitative elemental analysis values by optical emission spectrometry. In FIGS. 1 to 3, 1 is a sintered body to which a diffusion species is attached, 2 is an alumina brick, 3 is a high-purity alumina plate, and 4 is a platinum wire.

[Table] As is clear from Table 1, the amount and composition of the remaining diffusion species differ depending on the method of packing the sintered body into the pod. A silver electrode was provided on each element after diffusion, and the electrical characteristics were examined. As a result, it was confirmed that the electrical characteristics were clearly influenced by the residual state of the diffused species. This indicates that the dispersed species also diffuse into the pods used, and that they are affected to a large extent by evaporation and scattering into the atmosphere. Therefore, as mentioned above, it is thought that it is greatly influenced by the material of the pod and whether or not the atmosphere is flowing. Table 2 shows the pod packing method according to the present invention shown in Figure 4 A and B and Figure 5 A and B, and the data in Table 1.
These are the results of an experiment attempting to reproduce the residual amount and composition after diffusion shown in No. 1. Figures 4A and 4B show an embodiment in which the sintered body 1 with diffusion species attached is packed in a semi-closed container formed by a high-purity alumina plate 5 and a high-purity alumina crucible 6 with a capacity of, for example, 30 c.c. be. In addition, in FIGS. 5A and 5B, a high purity alumina plate 8 is installed on the inner wall of the alumina pod 7, a sintered body 1 with diffusion species attached is placed therein, and a high purity alumina plate 8' and an alumina lid are placed. This is an example in which 9 is covered and packed in a pod. Note that the measurement conditions for the numerical values shown in Table 2 are the same as those in Table 1.

[Table] From this result, according to the method of the present invention, the amount of adhered species and the amount of remaining diffused species are almost the same, and almost no change in composition is observed. Therefore, it is clear that the amount of diffusion can be controlled by the amount of adhering species. In the above table, the composition and residual amount after diffusion were determined by emission spectrometry. As described above in the examples, according to the method of the present invention, diffused species are distributed at the grain boundaries of semiconductor ceramics in a desired composition.
It is also possible to diffuse the desired amount without waste, and there is no need to consider how the sintered bodies are arranged, making it particularly meaningful for application to the production of grain boundary layer semiconductor ceramic capacitors. You can say that. It goes without saying that this method is effective not only for grain boundary layer semiconductor ceramic capacitors but also for products in which grain boundary layers are formed using a diffusion method.

[Brief explanation of the drawing]

Figures 1 to 3 are perspective views showing each embodiment of the pod packing method in the conventional method, and Figures 4A and 4B and 5A and 5B respectively show representative examples of the pod packing method in the method of the present invention. They are a perspective view and a sectional view.

Claims (1)

[Claims] 1. After coating strontium titanate-based semiconductor porcelain with a metal compound that is oxidized by heat treatment, the semiconductor porcelain is packaged in a semi-sealed container and heat treated to oxidize the metal compound. A method for forming grain boundary layers in semiconductor ceramics characterized by simultaneously diffusing into grain boundaries. 2. The method for forming grain boundary layers in semiconductor ceramics according to claim 1, characterized in that a mixture containing at least one of cuprous oxide and bismuth oxide is used as the metal compound.