Semiconductor ceramic capacitors
(1) Surface layer ceramic capacitor The miniaturization of the capacitor means that the capacitor can obtain the largest capacity in the smallest possible volume, which is one of the development trends of the capacitor. For separation capacitor components, there are two basic approaches to miniaturization: ① make the dielectric constant of the dielectric material as high as possible; ② make the thickness of the dielectric layer as thin as possible. In ceramic materials, the dielectric constant of ferroelectric ceramics is very high, but when ferroelectric ceramics are used to make common ferroelectric ceramic capacitors, it is difficult to make the ceramic dielectric thin. The first is that ferroelectric ceramics have low strength and are easy to crack when thinner, which makes it difficult to carry out actual production operations. Secondly, when the ceramic medium is thin, it is easy to cause various structural defects, and the production process is very difficult.
Surface layer ceramic capacitors use a thin insulating layer formed on the surface of a semiconductor ceramic such as BaTiO3 as a dielectric layer, and the semiconductor ceramic itself can be regarded as a series circuit of a dielectric. The thickness of the insulating surface layer of the surface layer ceramic capacitor varies depending on the formation method and conditions, and ranges from 0.01 to 100 μm. In this way, not only the high dielectric constant of the ferroelectric ceramic is used, but also the thickness of the dielectric layer is effectively reduced, which is an effective solution for preparing micro-small ceramic capacitors.
Figure (a) on the right is the general structure of a surface-layer ceramic capacitor, and (b) is its equivalent circuit. Shape on semiconductor ceramic surface
There are many methods for forming a surface dielectric layer, which are only briefly introduced here. The silver electrode is fired on two parallel planes of the BaTiO3 conductor ceramic, and the contact interface between the silver electrode and the semiconductor ceramic will form an extremely thin barrier layer. Since Ag is a metal with a large electron work function, under the action of an electric field, a barrier layer lacking electrons will appear on the contact interface between the BaTiO3 conductor ceramic and the Ag electrode, and the barrier layer itself has space charge polarization. Interface polarization. In this way, the barrier layer between the semiconductor ceramic and the Ag electrode constitutes an actual dielectric layer.
This capacitor porcelain is first sintered in the atmospheric atmosphere, and then forcibly reduced semiconducting in a reducing atmosphere, and then the surface layer is reoxidized into an insulating dielectric layer in an oxidizing atmosphere. The thickness of the reoxidation layer should be controlled appropriately. If the oxide film is too thin, the rectifying characteristics of the pn junction can still be present between the electrode and the ceramic, and the insulation resistance and electric strength cannot be improved. As the thickness gradually increases, the rectifying characteristics of the pn junction disappear, the insulation resistance increases, and the dependence on the DC bias decreases. However, the re-oxidation time should not be too long, otherwise it may cause re-oxidation inside the ceramic and reduce the capacity of the capacitor. The temperature of the reduction treatment is 800 to 1200 ° C, and the temperature of the reoxidation treatment is 500 to 900 ° C. After the reduction treatment of ceramic materials, the insulation resistivity can be reduced to 10-103Ω · cm, and the resistivity of the surface layer is lower than that of the inner ceramic body. The resistivity of thin ceramics is generally thicker than that of thicker ceramics with the same processing conditions. The volume resistivity is lower. Since the thickness of the surface insulating dielectric layer formed by the re-oxidation treatment is relatively thin, although the dielectric constant is not necessarily high, the unit capacity of the surface-layer semiconductor ceramic capacitor after the reduction and re-oxidation treatment can still reach 0.05 to 0.06 μF / cm2.
(2) The surface of BaTiO3 semiconductor ceramics with relatively well-developed grains in the grain boundary layer ceramic capacitors is coated with an appropriate metal oxide (such as CuO or Cu2O, MnO2, Bi2O3, Tl2O3, etc.) at an appropriate temperature and under oxidation conditions. After the heat treatment is carried out, the coated oxide will form a eutectic solution phase with BaTiO3, and rapidly diffuse and penetrate into the ceramic along the open pores and grain boundaries, forming a thin solid solution insulation layer on the grain boundaries. The resistivity of this thin solid solution insulation layer is very high (up to 1012 ～ 1013Ω · cm). Although the crystal grains of the ceramic are still semiconductors, the entire ceramic body shows a significant dielectric constant as high as 2 × 104 to 8 × 104 insulator dielectric. Capacitors made with this porcelain are called boundarg layer ceramic capacitors, or BL capacitors for short. [1] 

High-voltage ceramic capacitors
(I. Overview
With the rapid development of the electronics industry, it is urgent to develop high-voltage ceramic capacitors with high breakdown voltage, small loss, small size, and high reliability. In the past 20 years, high-voltage ceramic capacitors successfully developed at home and abroad have been widely used in power systems, laser power supplies, video tape recorders, color TVs, electronic microscopes, photocopiers, office automation equipment, aerospace, missiles, and navigation.
The ceramic materials of high-voltage ceramic capacitors are mainly two types: barium titanate-based and strontium titanate-based.
Barium titanate-based ceramic materials have the advantages of high dielectric constant and good AC withstand voltage characteristics, but also have disadvantages such as the capacitance change rate increases with the medium temperature and the insulation resistance decreases.
The Curie temperature of strontium titanate crystals is -250 ° C, and it has a cubic perovskite structure at normal temperature. It is a paraelectric body, there is no spontaneous polarization phenomenon, and the dielectric constant of strontium titanate-based ceramic materials under high voltage Small changes, small tgδ and small capacitance change rate, these advantages make it very advantageous as a high voltage capacitor dielectric.
(Two) the main points of the manufacturing process
(1) Raw materials must be selected
Factors affecting the quality of high-voltage ceramic capacitors, in addition to the composition of the ceramic material, it is very important to optimize the process manufacturing and strict process conditions. Therefore, we must consider both the cost and purity of the raw materials. When selecting industrial pure materials, we must pay attention to the applicability of the raw materials.
(2) Preparation of frit
The quality of the frit preparation has a great impact on the ball grinding fineness and firing of the porcelain. If the frit synthesis temperature is low, the synthesis is insufficient. Detrimental to subsequent processes. If residual Ca2 + remains in the composite, it will hinder the rolling process: if the synthesis temperature is too high, the frit will be too hard, which will affect the ball milling efficiency: the introduction of impurities in the grinding medium will reduce the activity of the powder and cause the firing temperature of the porcelain to increase .
(3) Molding process
When molding, it is necessary to prevent uneven pressure in the thickness direction, and there are too many pores in the closed body. If there are large pores or layer cracks, it will affect the electrical strength of the porcelain.
(4) firing process
The firing system should be strictly controlled, and temperature control equipment with good performance and kiln furniture with good thermal conductivity should be adopted.
(5) Encapsulation
The selection of the encapsulant, the control of the encapsulation process, and the cleaning of the surface of the porcelain have a great impact on the characteristics of the capacitor. For this reason, it is necessary to choose an encapsulant with good moisture resistance, close combination with the surface of the porcelain body, and high electrical strength. At present, most of them choose epoxy resin, and a few products also use phenolic resin for encapsulation. There is also the method of first coating with insulating paint and then encapsulating with phenolic resin, which has a certain significance for reducing costs. Large-scale production lines often use powder encapsulation technology.
In order to improve the breakdown voltage of ceramic capacitors, coating a layer of glass glaze around the edge of the interface between the electrode and the dielectric surface can effectively improve the withstand voltage and high temperature load performance of ceramic capacitors used in high-voltage circuits such as televisions. Lead borosilicate glass glaze can make the capacitor under a DC electric field; the breakdown voltage is increased by 1.4 times; the breakdown voltage under the AC electric field is increased by 1.3 times. [2] 

Multilayer ceramic capacitor
Multilayer Ceramic Capacitor (MLCC) is the most widely used type of chip component. It is an internal electrode material and ceramic body stacked in multiple layers alternately in parallel and co-fired into a whole. Chip monolithic capacitors have the characteristics of small size, high specific capacity, and high precision. They can be mounted on printed circuit board (PCB) and hybrid integrated circuit (HIC) substrates, effectively reducing electronic information terminal products (especially Portable products) to increase product reliability. In line with the development trend of miniaturization, light weight, high performance and multi-function of the IT industry, the country's 2010 vision outline clearly proposes that new components such as surface mount components are the focus of the development of the electronics industry. It not only has simple packaging and good sealing performance, but also can effectively isolate the opposite electrode. MLCC can play the role of storing electric charge, blocking DC, filtering, combining, distinguishing different frequencies and tuning circuits in electronic circuits. It can partially replace organic film capacitors and electrolytic capacitors in high-frequency switching power supplies, computer network power supplies, and mobile communication equipment, and greatly improve the filtering performance and anti-interference performance of high-frequency switching power supplies.
1. miniaturization
For compact electronic products such as camcorders and mobile phones, more compact MLCC products are needed. On the other hand, due to advances in precision printed electrodes and lamination processes, ultra-small MLCC products have also gradually appeared and obtained applications. Taking the development of the rectangular MLCC in Japan as an example, the external dimensions have been reduced from 3216 in the early 1980s to 0603 today. The mainstream products of MLCC produced by domestic enterprises are 0603 type, which has broken through the technical difficulties of large-scale production of 0402 MLCC. 0201-type MLCC has been developed samples, industrialization technology and domestic market demand are in the stage of development and maturity, the smallest 0201-type MLCC is currently less than 500 μm long side. [3]
2. Cost reduction-base metal inner electrode MLCC
Because the traditional MLCC uses expensive palladium electrode or palladium-silver alloy electrode, 70% of its manufacturing cost is occupied by the electrode material. New-generation MLCCs, including high-voltage MLCCs, use cheap base metal materials nickel and copper as electrodes, which greatly reduces the cost of MLCCs. However, the base metal internal electrode MLCC needs to be sintered at a lower oxygen partial pressure to ensure the conductivity of the electrode material, and too low an oxygen partial pressure will bring the semiconducting tendency of the dielectric ceramic, which is not conducive to the insulation and reliability. Murata has developed several anti-reduction ceramics, which are sintered in a reducing atmosphere. The reliability of the capacitors is comparable to that of capacitors using noble metal electrodes. Such capacitors soon entered the market. At present, the sales of base metallized Y5V capacitors have accounted for about half of the MLCCs in this group. In addition, they are seeking to expand the application of base metal electrodes to capacitors in other groups.
China has also made significant progress in this regard. Tsinghua University cooperates with component manufacturers to chemically prepare high-purity barium titanate nano-powders (20-100 nm). The acceptor doping and double rare earth doping are used to build a "core-shell" structure to improve the material's high temperature resistance to reduction and Achieve temperature stability characteristics, developed a series of temperature stable high performance nano / sub-micron crystal anti-reduction barium titanate ceramics with independent intellectual property rights. The developed material formula composition and preparation method are original, and the comprehensive material performance ranks internationally. Leading level. Among them, the high-performance X7R (0302) base metal MLCC porcelain has a relative dielectric constant of up to 3,000 at room temperature, a ceramic grain size of less than 300 nm, a capacitance temperature change rate of less than ± 12%, and a dielectric loss of less than 2.5 × 10-2. The insulation resistivity is about 1013 Ω · cm. The MLCC breakdown field is stronger than 70 MV / m. MLCC products with ultra-thin base metal internal electrodes have been prepared. The thickness of the single layer of ceramic dielectric is about 3 μm.
3． Large capacity and high frequency
On the one hand, with the low-voltage driving and low power consumption of semiconductor devices, the operating voltage of integrated circuits has been reduced from 5 V to 3 V and 1.5 V; on the other hand, the miniaturization of power supplies requires small, large-capacity products to replace large aluminum Electrolytic capacitors. In order to meet the development and application of such low-voltage and large-capacity MLCCs, in terms of materials, relaxation-type high-dielectric materials with a relative dielectric constant that is 1 to 2 times higher than BaTiO3 have been developed. In the process of developing new products, three key technologies have been developed at the same time, namely ultra-thin green sheet powder dispersion technology, improved green film formation technology, and internal electrode and ceramic green sheet shrinkage matching technology. Recently, Japan's Matsushita Electronic Components Co., Ltd. successfully developed a large-capacity MLCC with a maximum capacitance of 100 μF and a maximum withstand voltage of 25 V. This product can be used for liquid crystal display (LCD) power lines.
The rapid development of the communications industry requires higher and higher frequency requirements for components. Cer-F series MLCC launched by American Vishay Company has high-frequency characteristics comparable to film capacitors and can replace film capacitors in certain applications in high frequency bands. However, there is still a certain gap between China's high-frequency and ultra-high-frequency MLCC products and foreign countries, mainly due to the lack of research and development of basic raw materials and their formulas. With the continuous updating of technology, products with low distortion and low impact noise, high-frequency, wide-temperature, long-life products, high-security products, and high-reliability and low-cost products have emerged. [3]