Silicon capacitor

A macroporous silicon substrate with pores of about a micrometer and a pore depth of a few tenths of a millimeter offers a surface area enhancement of about two or three orders of magnitude compared to an unetched silicon surface. An example of such a macroporous substrate used for fabrication of a silicon capacitor (SIKO) is shown in Fig. 10.20 b. 

The test structures used were iridium - silicon dioxide-silicon capacitors with rather large area. Available commercial hydrogen and ammonia sensors consist, e.g., of a sensing element in the form of a transistor, a heater (a diffused resistor) and a temperature sensors (a pn-junction) on the same chip with dimensions smaller than about 1x1 mm. Research and development work is, however, in several cases most easily done on metal-insulator semiconductor capacitors mounted on a thermostatted sample holder. The description of the test structures, their fabrication and physics, given below is, however, very short. More details can be found in several of the references, e.g., ref. [1 — 3,8]. 

An attempt to forecast the further shrinkage of integrated circuits has been made by Gleason (2000). He starts out with some up-to-date statistics during the past 25 years, the number of transistors per unit area of silicon has increased by a factor of 250, and the density of circuits is now such that 20,000 cells (each with a transistor and capacitor) would fit within the cross-section of a human hair. This kind of relentless shrinkage of circuits, following an exponential time law, is known as Moore s law (Moore was one of the early captains of this industry). The question is whether the operation of Moore s Law will continue for some years yet Gleason says that attempts to forecast an end to the validity of Moore s Law have failed dismally it has continued to hold well beyond expectations . The problems at... 

The silicon diode (photodiode) detector consists of a strip of p-type silicon on the surface of a silicon chip (n-type silicon). By application of a biasing potential with the silicon chip connected to the positive pole of the biasing source, electrons and holes are caused to move away from the p-n junction. This creates a depletion region in the neighbourhood of the junction which in effect becomes a capacitor. When light strikes the surface of the chip, free... 

Capacitors, which store an electric charge and consist of two conductors usually made of tantalum, titanium, nichrome, platinum, or gold, separated by a dielectric, usually made of silica or silicon nitride. 

Integrated circuits (IC s) are circuits in which bipolar transistors, field-effect transistors (FET), resistors, capacitors, and their required connections are combined on a single chip of semiconductor material which is usually made of single-crystal silicon. 

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

Diode array detectors consist of silicon integrated circuit (IC) chips incorporating up to one or two hundred pairs of photodiodes and capacitors. Each photodiode measures about 0.05 x 0.5 mm and is sensitive to... 

The measurement of changes of the surface potential Vo at the interface between an insulator and a solution is made possible by incorporating a thin film of that insulator in an electrolyte/insulator/silicon (EIS) structure. The surface potential of the silicon can be determined either by measuring the capacitance of the structure, or by fabricating a field effect transistor to measure the lateral current flow. In the latter case, the device is called an ion-sensitive field effect transistor (ISFET). Figure 1 shows a schematic representation of an ISFET structure. The first authors to suggest the application of ISFETs or EIS capacitors as a measurement tool to determine the surface potential of insulators were Schenck (15) and Cichos and Geidel (16). 

Vfb The flat-band voltage of an electrolyte/insulator/silicon (EIS) capacitor ... 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

Anodic oxidation is a very common process in the electrochemical industry, used for example in the manufacture of aluminum and tantalum capacitors. The anodic oxidation of silicon is not of comparable importance, because the electrical properties of anodic oxides are inferior to those of thermal oxides. 

The material properties of PS offer new ways of making electronic devices. For the manufacture of cold cathodes, for example, oxidized microporous polysilicon has been found to be a promising material. The application of basic semiconductor processing steps such as doping, oxidation and CVD to a macroporous material enable us to fabricate silicon-based capacitors of high specific capacitance. Both devices will be discussed below. 

Lower right First macroporous silicon-based chip capacitor (100 nF, 10 V) on a match for size comparison. 

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

Tan, J., et ah, Metal-Oxide-Semiconductor Capacitors Formed by Oxidation of Polycrystalline Silicon on SiC, Appl. Phys. Lett., Vol. 70, 1997, p. 2280. 

Surface layers of silicon oxide are important in semiconductor device fabrication as interlayer dielectrics for capacitors, isolation of conducting layers, or as masking materials. However, anodic oxides, due to their relatively poor electrical properties, breakdown voltage, and leakage current, have not yet found much use in device technology, and cannot compete with thermal oxides obtained at high temperatures of 700 to 900 °C. 

Sodium contamination and drift effects have traditionally been measured using static bias-temperature stress on metal-oxide-silicon (MOS) capacitors (7). This technique depends upon the perfection of the oxidized silicon interface to permit its use as a sensitive detector of charges induced in the silicon surface as a result of the density and distribution of mobile ions in the oxide above it. To measure the sodium ion barrier properties of another insulator by an analogous procedure, oxidized silicon samples would be coated with the film in question, a measured amount of sodium contamination would be placed on the surface, and a top electrode would be affixed to attempt to drift the sodium through the film with an applied dc bias voltage. Resulting inward motion of the sodium would be sensed by shifts in the MOS capacitance-voltage characteristic. 

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