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1、1.7| DEVICE FABRICATION TECHNIQUES: OXIDATIONObjective: Describe the formation of an oxide on silicon. The integrated circuit is a direct result of the development of various processing techniques needed to fabricate the transistor and interconnect lines on the single chip. The total collection of t
2、hese processes for making an IC is called technology. Introductions to basic fabrication processes are given throughout the text where appropriate. Here, we discuss one such processthermal oxidation.A major reason for the success of silicon ICs is the fact that an excellent native oxide, silicon dio
3、xide (SiO2), can be formed on the surface of silicon. This oxide is used as a gate insulator in the metal-oxide-semiconductor field-effect transistor (MOSFET). As we will see in Chapter 6, the oxide is an integral part of this electronic device. Most other semiconductors do not form native oxides th
4、at are of sufficient quality to be used in device fabrication.Silicon dioxide is an important material in the fabrication process for devices, as we will see throughout the text. In addition, the oxide is used as an insulator, known as the field oxide, between devices. Metal interconnect lines that
5、electrically connect various electronic devices on the chip can be placed on top of the field oxide. The thermal oxidation of silicon in an atmosphere of oxygen proceeds according to the reactionSi(solid) + O2(gas) Si02(solid)This process is called a dry oxidation since oxygen is present without any
6、 water vapor. After an oxide layer forms on the surface of the silicon, oxygen molecules must diffuse through the existing oxide to reach the silicon surface where the reaction occurs. This process is shown in Figure l.25. The probability of Si diffusing through the Figure 1.25 Schematic of the oxid
7、ation process indicating the diffusion of oxygen through the existing silicon dioxide.1 . 6 . 2 Epitaxial GrowthA common and versatile growth technique that is used extensively in device and integrated circuit fabrication is epitaxial growth. Epitaxial growth is a process whereby a thin, single-crys
8、tal layer of material is grown on the surface of a single- crystal substrate. In the epitaxial process, the single-crystal substrate acts as the seed, although the process takes place far below the melting temperature. When an epitaxial layer is grown on a substrate of the same material, the process
9、 is termed homoepitaxy. Growing silicon on a silicon substrate is one example of a homoepitaxy process. At present, a great deal of work is being done with heteroepitaxy. In a heteroepitaxy process, although the substrate and epitaxial materials are not the same, the two crystal structures should be
10、 very similar if single-crystal growth is to be obtained and if a large number of defects are to be avoided at the epitaxial-substrate interface. Growing epitaxial layers of the ternary alloy AlGaAs on a GaAs substrate is one example of a heteroepitaxy process.Chemical Vapor-Phase Deposition One epi
11、taxial growth technique that has been used extensively is called chemical vapor-phase deposition (CVD). Silicon epitaxial layers, for example, are grown on silicon substrates by the controlled deposition of silicon atoms onto the surface from a chemical vapor containing silicon. In one method, silic
12、on tetrachloride reacts with hydrogen at the surface of a heated substrate. The silicon atoms are released in the reaction and can be deposited onto the substrate, while the other chemical reactant, HCl, is in gaseous form and is swept out of the reactor. A sharp demarcation between the impurity dop
13、ing in the substrate and in the epitaxial layer can be achieved using the CVD process. This technique allows great flexibility in the fabrication of semiconductor devices.Liquid-Phase Epitaxy Liquid-phase epitaxy is another epitaxial growth technique. A compound of the semiconductor with another ele
14、ment may have a melting temperature lower than that of the semiconductor itself. The semiconductor substrate is held in the liquid compound and, since the temperature of the melt is lower than the melting temperature of the substrate, the substrate does not melt. As the solution is slowly cooled, a
15、single-crystal semiconductor layer grows on the seed crystal. This technique, which occurs at a lower temperature than the Czochralski method, is useful in growing group III-V compound semiconductors.Moleculal Beam Epitaxy A versatile technique for growing epitaxial layers is the molecular beam epit
16、axy (MBE) process. A substrate is held in vacuum at a temperature normally in the range of 400 to 800 0C，a relatively low temperature compared with many semiconductor-processing steps. Semiconductor and dopant atoms are then evaporated onto the surface of the substrate. In this technique, the doping can be precisely controlled, resulting in very complex doping profiles. Complex ternary compounds, such as SlGaAs , can b
