Silicon dioxide circuits

An integrated circuit (IC) is a monolithic assembly of electrically isolated circuit elements. What this means is that each circuit element is formed on top of, or beneath, other circuit elements to form a compact assembly. Each conductive layer is separated by a non-conducting layer, usually composed of an oxide such as silicon dioxide, Si02- The assembly includes... 

Silicon dioxide is also used to insulate regions of the integrated circuit. Here silicon dioxide is grown on the silicon surface by heating it to about 1000°C in the presence of oxygen. 

Metal oxide semiconductor field-effect transistors (MOSFETs) are field effect transistors with a thin film of silicon dioxide between the gate electrode and the semiconductor. The charge on the silicon dioxide controls the size of the depletion zone in the polype semiconductor. MOSFETs are easier to mass produce and are used in integrated circuits and microprocessors for computers and in amplifiers for cassette players. Traditionally, transistors have been silicon based but a recent development is field-effect transistors based on organic materials. 

In the present chapter, we will turn our attention to films deposited by thermal CVD that are either dielectrics or semiconductors. There are, as one would expect, many films that can be deposited by this technique. In addition, there are many gaseous reactants that one can use to create each film, the choice depending on the film characteristics desired. Rather then attempt to catalogue all of the possible films and reactants, we will choose instead to focus on silicon dioxide, silicon nitride, polysilicon, and epitaxial silicon as the films of interest. At the same time, we will only look at those reactant gases that have been used for integrated circuit manufacture. An excellent survey of the film types that can be deposited by CVD and the many reactants that have been used to obtain them has been given by Kern.1... 

Silicon dioxide films have been an essential factor in the manufacture of integrated circuits from the earliest days of the industry. They have been used as a final passivation film to protect against scratches and to getter mobile ion impurities (when doped with phosphorus). Another application has been as an interlayer dielectric between the gate polysilicon and the aluminum metal-ization. Initially, most such films were deposited in atmospheric pressure systems. In recent years, low pressure processes have assumed greater importance. We will begin by examining the atmospheric process. 

There are two aspects of tungsten CVD for integrated circuits that have taken on commercial importance. One is the blanket deposition and subsequent patterning, so it can be used as a conductor to replace high-resistivity doped poly. The second area of interest is the "selective" CVD of tungsten, where deposition occurs on silicon but not on silicon dioxide. Here one can selectively fill via holes to either provide a thin barrier metal or to deposit a thicker layer to help planarize the circuit. Both applications involve only one processing step, and are attractive for this reason. 

PECVD of silicon nitride has been of commercial importance since 1976.1 The original motivation was to find a final passivation layer for an integrated circuit that would replace the doped silicon dioxide films then in use. The latter were not reliable enough to permit packaging of integrated circuits in plastic. Silicon nitride was recognized as a better final passivation film, but the only available technique for its deposition was the high-temperature thermal process. Since it had to cover an aluminum final metallization layer that would melt at 600°C, this clearly could not work. The solution was to use PECVD at 350° to 400°C. 

Transparent, conducting, tin oxide coatings are used in applications where light must pass through the substrate in order to strike the active element such as a photoconductive or photoelectric material. Chemically deposited films of silicon dioxide serve as masks on semiconductor materials for selective doping in the preparation of integrated circuits that can later be removed by chemical etching. 

Silicon dioxide is the most widely used ceramic material in the semiconductor industry, and the majority of Si02 deposits in microelectronic circuits are formed by CVD. Silica layers are used as diffusion sources, intermetallic dielectrics, and dopant and etch barriers in the microelectronics industry. CVD of Si02 is also commonly used in manufacturing energy-efficient glass windows, surface coatings for fiber optics, and micromechanical applications. ... 

Conventional field effect transistors are the building blocks of binary electronics. Worldwide production is 1017 transistors per year, and per capita use in the United States is 107 transistors per year. The cost per transistor is one micro dollar, and the yield of fabrication is greater than 99 percent. The metalization on the interconnects of the chips was previously aluminum, but it is starting to be switched over to copper. Likewise, the solid-state circuit feature size was 0.1 pm but is now becoming smaller. Silicone dioxide comprised the top layer of the device. 

Perhaps the best studied gate dielectric treatment is the application of silanes (e.g. octadecyltrichlorosilane or OTS) to thermally grown silicon dioxide. While this structure is limited in its utility in integrated circuits, it provides a well controlled system in which to study structure/function relationships that can then be extrapolated to more practical, but less controlled, integrated structures (e.g. PECVD or PVD deposited oxides). 

The main difference between the JFET and the MOSFET is that the gate on the MOSEET, which is the input to the FET, is insulated by a thin dielectric material (silicon dioxide, referred to as gate oxide) from the other two electrodes of the transistor. In contrast, the gate of the JFET forms a physical p-n junction with the other electrodes of the transistor. JFETs are used extensively in GaAs integrated circuits. ... 

A printed circuit has many n-p-n junction transistors. Fig. 10.33 illustrates the formation of one transistor area. The chip begins as a thin wafer of silicon that has been doped with an n-type impurity. A protective layer of silicon dioxide is then produced on the wafer by exposing it in a furnace to an oxidizing atmosphere. The next step is to produce a p-type semiconductor. To do this, the surface of the oxide is covered by a polymeric photoresist, as shown in Fig. 10.33(a). A template that only allows light to shine... 

The etching process results in removal of the silicon dioxide in the selected areas defined, for example, by the clear or opaque areas in the mask outlining the circuit pattern, so that the silicon is exposed. 

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