Evolving Capacitors Monolithic Ceramic Capacitors

Part1 Trend
Monolithic Ceramic Capacitors
Indispensable to High-performance Semiconductor Devices

Resistors, capacitors and inductors are passive parts often regarded as somewhat minor components, but in fact, they are indispensable to cutting-edge electronics. In particular, monolithic ceramic capacitors are crucial to leading-edge semiconductor devices. Without them, we would not be able to count on devices to operate properly. Some in the electronics industry predicted that capacitors would be integrated into semiconductor devices sooner or later. In reality, however, the importance of monolithic ceramic capacitors tends to increase as semiconductor devices evolve.

A monolithic ceramic capacitor is smaller than a particle of sugar. Are you aware of the roles this extremely tiny component plays in electronic devices? It has such important roles as supporting the power supply required for semiconductor devices, and eliminating noise that may result in a malfunction or performance degradation. Without monolithic ceramic capacitors, the semiconductor devices manufactured using advanced leading-edge process technology, such as microprocessors, DSPs, microcomputers, and FPGAs, could not be expected to operate properly.

History of Size Reduction and Capacitance Enhancement

Fig. 1: Structure of monolithic ceramic capacitor
Fig. 1: Structure of monolithic ceramic capacitor
The layers of the dielectric material and internal electrodes are laminated on top of each other, thus achieving a greater capacitance.

The size of the monolithic ceramic capacitor market is presently the largest among the markets for various types of capacitors, namely aluminum electrolytic capacitors, tantalum electrolytic capacitors and film capacitors. In 2008, 627.8 billion units of monolithic ceramic capacitors were shipped in Japan and domestic sales reached JPY305.9 billion (according to "Yearbook of Machinery Statistics" published by Japan's Ministry of Economy, Trade and Industry). Aluminum electrolytic capacitors took second place with shipments of 18.2 billion units and sales of JPY174.3 billion. There is a wide gap between first and second place.

Monolithic ceramic capacitors are currently the top-selling product in the capacitor market, but market acceptance was slow to develop when they were first introduced. It was a U.S. company that first came up with the idea of a monolithic ceramic capacitor. In the midst of the Apollo program, which was launched in 1961, a monolithic ceramic capacitor was invented to meet the need for a compact, high-capacitance capacitor. The new capacitor was designed to have the electrodes formed in a number of laminated dielectric layers so that it has a high capacitance in a small size (Fig. 1).

Murata Manufacturing Co., Ltd. introduced this technology ahead of others and put the first product on the market in 1965. It was a 100pF model targeting LC resonant circuits in AM radios and it consisted of 50μm dielectric film layers. The product used titanium oxide (TiO2) as the dielectric material. "When we first launched the product, it did not sell at all," said Kiminori Yamauchi, director of the Components Business Unit of Murata. "But after an ultra-slim radio dubbed 'Paper Radio' was released, the market for monolithic ceramic capacitors grew rapidly because they were smaller than any other type of capacitor."

The history of monolithic ceramic capacitors since then can be described as the "history of size reduction and capacitance enhancement." In general, the capacitance C of a capacitor is expressed as follows:

C = ε S/d

where ε, S and d denote the dielectric constant, the area of the electrodes and the distance between the electrodes (the thickness of the dielectric body), respectively. In short, there are only two ways to increase the capacitance in a certain volume: either to use a material with a higher ε value or to reduce the thickness of the dielectric body.

Murata used titanium oxide as the dielectric material during the initial phase after product release, but introduced barium titanate (BaTiO3) at a relatively early stage. Since then, the relative dielectric constant has been continuously increasing as a result of improvements made to the material. So far, the value has reached approximately 3,000. The relative dielectric constant of titanium oxide is about several dozen at most. In other words, that of BaTiO3 has become two orders of magnitude greater than that of titanium oxide.

The thickness of the dielectric material was gradually reduced from 50 μm during the initial phase to 0.5 μm at present. Thus, compared with the initial values, the dielectric constant is 100 times higher, while the thickness is 1/100. With a thickness reduced to 1/100, the number of layers can be increased by 100 times. Therefore, in terms of capacitance, it is equivalent to a million-fold increase with the same size. In terms of size, on the other hand, it means that a 1/1,000,000 reduction is possible with the same capacitance.

Decoupling Application Accounts for 70% of the Market

Electronic device Number of monolithic ceramic capacitors used
Notebook computer
730
Mobile phone
230
Digital camcorder
400
Digital TV
1000
Car navigation system
1000

Table 1: Number of monolithic ceramic capacitors used in electronic devices

As mentioned above, monolithic ceramic capacitors are arranged around microprocessors, DSPs, microcomputers, FPGAs or other semiconductor devices to support the proper operation of these devices. The number (total number) of monolithic ceramic capacitors used in an electronic device is enormous. For example, about 730 units are used in a notebook computer and about 230 units can be found in a mobile phone, while digital TVs and car navigation systems both use about 1,000 units (Table 1).

Monolithic ceramic capacitors have two main roles. First, they support the power supply for semiconductor devices. In general, the current required for a semiconductor device varies significantly depending on the operating conditions. In some situations, a large amount of electric power may be suddenly required. The power supply circuit (DC-DC converter, etc.), which is located relatively far from the semiconductor device, will fail to quickly respond to an abrupt load change. To cope with this problem, electric power is stored in a capacitor mounted close to the semiconductor device and supplied from there (Fig. 2).

図2 半導体チップの動作をサポートするデカップリング・コンデンサ
Fig. 2: Decoupling capacitor for supporting operation of semiconductor device
A number of decoupling capacitors are mounted around a semiconductor device. These capacitors play two roles. One is to supply power to the semiconductor device. The other is to shunt the noise component to the power/ground layer. Note that three types of capacitors are mainly used for decoupling. They are tantalum electrolytic capacitors, high-capacitance monolithic ceramic capacitors and monolithic ceramic capacitors with an exceptionally low equivalent series inductance (ESL).(Click to enlarge.)

Another function of monolithic ceramic capacitors is the elimination of the noise component, which can cause electromagnetic interference (EMI). In a sense, it acts as a filter. By utilizing the small high-frequency impedance of a monolithic ceramic capacitor, only the high-frequency noise component can be shunted away to the power/ground layer.

Generally, capacitors featuring the former function are called decoupling capacitors, while those having the latter function are called bypass capacitors. Now, both functions can be fulfilled at the same time by the introduction of high-capacitance monolithic ceramic capacitors.

The second most pervasive application after decoupling and bypassing is smoothing filters arranged at the output of DC-DC converters. Initially aluminum and tantalum electrolytic capacitors have been adopted for this purpose. However, starting in the late 1990s, monolithic ceramic capacitors came into use in order to reduce the size and thickness of electronic devices.

It is largely due to the efforts of manufacturers of power semiconductor devices that monolithic ceramic capacitors are now available for this smoothing filter function. A capacitor serving as a smoothing filter constitutes part of the feedback control loop of a DC-DC converter. Therefore, if the equivalent series resistance (ESR) is too low, the phase margin of the control loop decreases, resulting in the problem where the DC-DC converter will not operate in a stable manner.

Meanwhile, electronics manufacturers have strong demand for smaller and thinner DC-DC converters. To meet this demand, power semiconductor device manufacturers made it possible to use monolithic ceramic capacitors by improving the control circuit in DC-DC converter ICs. Around 2000, power semiconductor device manufacturers began selling the new DC-DC converter IC to electronics manufacturers, making the case that monolithic ceramic capacitors can be used in the IC.

At present, the decoupling and smoothing filter applications alone reportedly account for about 70% of the monolithic ceramic capacitor market. Other typical applications include high-frequency filters, impedance matching and temperature compensation.

* Indicated company and product names are the trademarks or the registered trademarks of each company.

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