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Thin film capacitor
Monday, 04 September 2006

Capacitors are passive elements indispensable to electric and electronic circuits and perform various functions in the circuits in combination with resistor, coil, and IC. A number of main functions in electronic circuits, including filter for removing noises in power circuit, oscillation circuit for stably generating necessary signals, tuning circuit for selecting necessary signals and by-pass circuit for temperature compensation of circuits, largely depend on the characteristics of capacitors. For example, decoupling capacitors provide improved voltage regulation and noise immunity for power distribution. Bypass capacitors are commonly used in electronic circuits in order to bypass noise caused by the operation of active devices. The use of bypass capacitors may allow an electronic system to have a low impedance in the power distribution system over a wide frequency range (i.e. wideband impedance). Decoupling capacitors can be provided in a circuit that is coupled to the power supply to minimize this voltage drop. Furthermore, the electrodes of the capacitor may serve as reference voltage planes in the electronic package for providing excellent power distribution within the package. Capacitors also have wide applications in analog/logic, analog-to-digital, mixed signal, radio frequency circuit operations, and others.

Capacitors are used to decouple the system-level power supply from individual electrical devices of an electronic package. Decoupling of an electronic device from the power supply reduces the overall noise in the power distribution network of the electronic package. Decoupling capacitors in semiconductor circuitry are essential components used to filter much of the noise that may be present between operating supplies such as power and ground. Capacitors used in semiconductor devices may have the structure of a metal oxide semiconductor (MOS) type, PN junction type, polysilicon-insulator-polysilicon (PIP) type, metal-insulator-metal (MIM) type, etc. Capacitors are typically formed to have a metal/insulator/metal structure. A capacitor is comprised of two conductive plates separated by a non-conducting dielectric layer. The dielectric layer is preferably comprised of one or more materials having a very high dielectric constant and low leakage current characteristics. A typical capacitor comprises a pair of electrode layers having dielectric material therebetween. Voltage is applied across the electrode layers and a charge is stored in the capacitor with the amount of charge being storable in the capacitor, e.g. the capacitance, being proportional to the opposing areas of the electrodes and the dielectric constant of the dielectric material. Capacitors are generally fabricated in conventional physical shapes and sizes dictated by the capacitor materials, the manufacturing process, the end use, and the desired electrical properties. Many kinds of capacitors are currently available and are selected for respective uses, for example, paper capacitors, electrolytic capacitors, mica capacitors, ceramic capacitors, plastic film capacitors, and so on. Among these, the plastic film capacitor is most suitable for making a thin film capacitor having minimum size and weight.

In general, capacitors have a structure that a dielectric is sandwiched between a pair of parallel electrodes, and function to store electricity utilizing an electrical polarization caused in the dielectric by application of a voltage between the parallel electrodes. The characteristics of the capacitors largely vary depending on the kinds of dielectrics used. Capacitance has been found to be inversely proportional to the thickness of the dielectric material, thus thin film capacitors are generally seen as a preferable means to achieve high performance. As the electronic circuits are being miniaturized to an increasing degree as a result of advances in the integrated circuit technologies, and the miniaturization of capacitors that are indispensable in the integrated circuits as circuit elements for various kinds of electronic circuits is also becoming especially significant. At present, ceramic capacitors using dielectric ceramics, typically barium titanate, semiconductor ceramic capacitors utilizing semiconductive properties produced by addition of lanthanum oxide to barium titanate, film capacitors using plastic films such as polystyrene or polyethylene terephthalate as dielectrics, and aluminum electrolytic capacitors using as the dielectric layer a porous layer produced by anodic oxidation of the surface of highly pure aluminum foils have been used properly in accordance with purpose in consideration of temperature coefficient of capacitance, insulation resistance, dielectric loss and high-frequency characteristics of capacitors. Electronic circuits are becoming increasingly smaller with the development of integrated circuit techniques. Capacitors having improved high frequency characteristics and large capacity have been in demand to perform an increased number of functions in semiconductor integrated circuits. To achieve high-speed capacitors, the frequency dependency thereof should be decreased. And to achieve large-capacity capacitors, the thickness of an insulating layer interposed between the capacitor electrodes should be reduced. Accordingly, electronic circuit units have been proposed which have thin-film circuit elements, such as resistors, capacitors, and inductors, on an insulative substrate.

Thin film capacitors exist for a wide capacitance range. In general, the dielectric in such film capacitors comprises a plastic film. The electrodes comprise conductive metal areas. These metal areas are either thin conductor films or conductor layers vapor-deposited onto the plastic film. Polyester films have been popularly used as base film of capacitors because they have excellent mechanical, thermal and electrical properties. Such films include a polypropylene film, polyethylene terephthalate film, polyphenylene terephthalate film, and polyethylene naphthalate film. A thin-film capacitor generally has a structure wherein a lower electrode, a dielectric layer, and an upper electrode are deposited on a substrate. The thin-film capacitor is generally formed by depositing, patterning, and etching various layers on a substrate. Typically, a first parallel layer of conductive material is formed by depositing the conductive material over the surface of the substrate. Alternatively, the conductive material may be formed by doping the semiconductor substrate with substantially conductive material. The lower electrode is formed by sputtering chromium or copper on the alumina substrate and etching it into a desired pattern. The dielectric layer is formed by sputtering silicon dioxide or the like onto the lower electrode and etching it into a desired pattern. The upper electrode is formed by sputtering chromium or copper on the dielectric layer and etching it into a desired pattern. A thin film capacitor used in a semiconductor integrated circuit is generally designed to have a multi-layered structure including upper and lower electrodes composed of polysilicon, a silicon dioxide film and a silicon nitride film. Thin-film capacitors are required to have a large relative dielectric constant and Q factor and a temperature coefficient of capacitance of near 0 at a resonant frequency.

The metallized film capacitor has excellent properties of low losses, high withstand voltage, low temperature and frequency dependency. Metallized film capacitors are used extensively in a broad range of electrical and electronic equipment that include motor run and motor start circuits for air conditioners, fluorescent and high intensity light ballasts, power supplies, telecommunication equipment, instrumentation, and medical electronics. These metallized capacitors conserve energy by correcting the power factor of a circuit and in others they are used to perform specific functions, such as timing, filtering, and decoupling. There are generally two major products in metallized-film capacitors; the one uses metallic foil as its electrodes, and the other one uses metal deposited on dielectric film as its electrodes. The metallized-film capacitor using deposited metal as the electrodes can be downsized and is more reliable with respect to dielectric breakdown because of its self-healing performance proper to the deposited electrode, so that it has been widely used. However, a metal-deposited polyester film has a drawback that the adhesion between the base film and the deposited film layer, in particular, the moist-heat resistant adhesion is insufficient. A capacitor structure can be formed by using thin films of electrodes and dielectrics which are deposited on a prefabricated multilayer substrate. Thin film capacitors are typically provided with a multilayer of silicon oxide film and silicone nitride film as the top and bottom polysilicon electrode. The advantages of monolithic multilayer over film foil or metallized film capacitors include lower volume, weight, cost, and higher application temperature.

As capacitance is inversely proportional to the thickness of the dielectric material, thus thin film capacitors are generally seen as a preferable means to achieve high performance. Thin film capacitors are used in a variety of semiconductor integrated circuit devices such as analog circuits, RF circuits, and dynamic random access memories (DRAMs). The integrated high-dielectric thin film capacitor is employed as the decoupling capacitor that suppresses voltage noises or voltage variation caused in the power bus line, the storage capacitor of DRAM or nonvolatile FRAM (NVFRAM), and the dynamic tunable element in applications of the microwave devices. As a technique for forming a thin film is developed, an application field for such a ferroelectric thin film is spreading at present. The ferroelectric nonvolatile memory does not require backup battery because of utilization of the ferroelectric properties (hysteresis effect) of the ferroelectrics. In particular, a thin film capacitor of the MIM type structure has been widely applied to precise analog semiconductor devices since its capacitance variation is typically very small. Thin film capacitors particularly with large capacitive densities and low temperature coefficients of capacitance are highly desired in various areas of microelectronics.


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