Раздел 3 Первое занятие

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Problems in Microelectronic Circuit Technology

1. The manufacture of silicon microcircuits consists of a number of carefully controlled processes, all of which have to be performed to well-defined specifications.

Processing a “wafer” of silicon, a substrate on which the micro­electronic circuits are made, is not a simple technological process.

In order to understand how transistors and other circuit elements can be made from silicon, it is necessary to consider the physical na­ture of semiconductor materials.

In a conductor current is known to be carried by electrons that are free to flow through the lattice¹ of the substance².

In an insulator all the electrons are tightly³ bound to atoms or mol­ecules and hence⁴ none are available to serve as a carrier of electric charge.

The situation in a semiconductor is intermediate⁵ between the two: free charge carriers are not ordinarily present, but they can be generat­ed with a modest expenditure⁶ of energy.

Semiconductors are similar to insulators in that they have their lower bands completely filled⁷. The semiconductor will conduct if more than a certain voltage is applied. At voltages in excess of this critical voltage, the electrons are raised from the top⁸ on the band 1 (the valence band) to the bottom⁹ of band 2 (the conducting band). Below¹⁰ this critical voltage, the semiconductor material acts as an insulator. Semiconduc­tors such as that described above are called intrinsic (природный) semi­conductors — they are pure materials (for example, silicon or germani­um). It should be noted that a crystal of pure silicon is a poor¹¹ conductor of electricity Thus¹², conductivity poses¹³ a problem.

Several other requirements are imposed on materials. The basic demand appears to be conductivity because it can substantially im­prove¹⁴ the resistance and delay times for VLSI. The improvement of conductivity has been made in several ways. Most semiconductor de­uces are known to be made by introducing controlled numbers of im­purity atoms into a crystal, the process called doping.

Two independent lines of development are considered to lead to l^croscopic technique that produced the present integrated circuits. One Evolves the semiconductor technology; the other is a film technology.

Микроэлектроника настоящее и будущее

Let us consider the former¹⁵ one first. To improve the semicon­ductor crystal the impurities known as dopants are added to the silicon to produce a special type of conductivity characterized by either posi­tive (p-type) charge carriers or negative (л-type) ones. The dopants are diffused¹⁶ into semiconductor crystals at high temperatures. In the furnace (печь) the crystals are surrounded by vapour containing at­oms of the desired dopant. These atoms enter the crystal by substitut­ing¹⁷ for the semiconductor atoms at regular sites¹⁸ in the crystal lat­tice and move into the interior¹⁹ of the crystal.byjumping from one site to an adjacent²⁰ vacancy²¹.

Silicon crystals may be doped with different elements. Suppose sil icon is doped with boron. Each atom inserted²² in the silicon lattice cre­ates a deficiency²³ of the electron, a state that is called a hole. A hole also remains associated with an impurity atom under ordinary circumstanc­es²⁴ but can become mobile in response to an applied voltage. The hole is not a real particle, of course, but merely²⁵ the absence of an electron at a position where one would be found in a pure lattice of silicon atoms. Nevertheless²⁶, the hole has a positive electric charge and can carry elec­tric current. The hole moves through the lattice in much the same way that the bubble²⁷ moves through a liquid medium. An adjacent atom trans­fers²⁸ an electron to the impurity atom, “filling” the hole there but cre­ating a new one in its own cloud of electrons; the process is then repeat­ed, so that the hole is passed²⁹ along from atom to atom.

Silicon doped with phosphorus or another pentavalent element is called an л-type semiconductor. Doping with boron or another triva- lent element gives rise to a /?-type semiconductor.

Impurities may be introduced by the diffusion process. At each dif fusion step³⁰ in which л-type or/?-type regions are to be created in cer­tain areas, the adjacent areas are protected³¹ by surface layer of silicon dioxide, which effectively blocks the passage of impurity atoms. This protective layer is created very simply by exposing³² the silicon wafer at high temperatures to an oxidizing atmosphere. The silicon dioxide is then etched³³ away in conformity (в соответствии) with a sequence³⁴ of masks that accurately delineates³⁵ multiplicity³⁶ of л-type and p-type regions.

To define the microscopic regions that are exposed to diffusion in various stages³⁷ of the process, extremely precise³⁸ photographic pro­cedures³⁹ have been developed. The surface of the silicon dioxide is