Integrated circuits 261
' arefully cleaned and polished single-crystal siliicon
'
sdrface with an oxidant gas, usually oxygen or steam, slcs oai abeintegrated
jti ternperat1ures ranging from 1500 to 2200'F (800 to
,1,2o00C) in a quartz-walled furnace tube (Fig. 14a).
Theý reaction occurs rapidly and exothermiically on the
S ilicon surface. As the oxide film grows; the rate of
;Jxidatiofl decreases, because the oxidant must trans-
.p~irt to the silicon surface through the growing film.
'Fr large film thicknesses and at high oxidation tem- Fig. 13. Integrated-circuit fabrication sequence form. Fabrication normally proceeds in
pýratures, this transport controls the film growt i
.4, etics, while for thin films the surface reaction is
,dominant. SEE CHEMICAL BOND ING.
!Chemical vapor deposition (CVD) is a gas-phase the impurity has diffused to the proper depth and is
process where a film deposit is obtained by combining in the appropriate concentration in the silicon, the
;tihe appropriate gases in a reactant chamber at ele- process is stopped. Other regions that are not to re-
V~ted temperatures. A typical CVD reaction (silox ceive dopant are masked by using impenetrable films
pr~ocess) is given below. Figure 14b shows a cold- known as diffusion masks. SEE CRYSTAL DEFECTS.
1> As device requirements become more stringent,
1> SiH
+0~ (400-5OO.C) ~ 'very sharp diffusion profiles are needed so that the
Si40 -30F)SiO2 I+21- 2 Tdevice size can be reduced. The solid-state diffusion
(deposits process does not afford sufficient control for the most
as film) advanced device processes. For this purpose, the use
of the direct implantation of impurity ions (electri-
walled, atmospheric-pressure CVD system where the cally charged atoms) into the silicon lattice has been
.silicon slice is heated by rf energy. Figure 14c shows developed.
,alo-rueCDsse hr h lcsadpo Ion implantation is also used when greater precision
',cess gases are heated in a partially evacuated furnace of dopant concentration is required or when a reduced
'-iube. This low-pressure process produces very uni- temperature cycle is advantageous. Ion implantation
-form film thicknesses. SEE VAPOR DEPOSITION, makes use of intense, uniforn beams of high-energy
Evaporation or sputtering of metal coatings is per- ions (typically 10-500 keV) of the desired dopant.
formed in a vacuum, with metal transport being pro- These beams are formed in specialized accelerators
duced either by heat (evaporation) or bombarding ions such as Van de Graaff generators under high vacuum
(§puttering). The vacuum evaporator in Fig. 14d uses conditions (Fig. l4e). The beams can be focused, ac-
flixturing with planetary motion during evaporation. celerated, and purified by using mass spectrometry
This achieves uniform metal thickness over surface techniques such as electrostatic plates and magnetic
topology on the silicon slice. SEE CRYSTAL GROWITH; fields. The desired beam is then made to impinge on
'"SPUTTER!NG. the silicon substrate which has appropriate masking so
Impurity doping. The unique electronic properties of that the dopant beam impinges on the proper area of
semiconductors are produced by substituting selected the silicon surface. The energy is sufficient for the ions
impurities at silicon lattice positions in the silicon in the beam to penetrate the silicon surface, leaving a
crystal, a process called doping. The distortions in the distribution of dopant. The position of the peak of the
Chemical bonding due to the presence of impurities at distribution can be altered by altering the beam energy.
2!ttice positions cause some of the bonding electrons The amount of dopant can be altered by the beam cur-
'lin the crystal to have a higher energy than in a perfect rent and time of exposure. Damage is caused by the
' crysta lattice and therefore be available for electronic collisions of the ions in the beam with the atoms in the
tconduction. Similarly, holes, which are the absence silicon lattice, but much of the damage can be removed
of bonding electrons, are produced by other kinds of by thermal annealing at temperatures of about 1500'F
'ýi.mpurities. Both electrons and holes can carry electric (800-C). Remarkably sharp dopant profiles of precise
Current. In order to construct complex integrated cir- concentration can be achieved by using this technique.
cu1its, the impurities must be placed in adjacent re- SEE ION IMPLANTATION.
.ýgions in the semiconductor surface. The two predom- Lithography. Lithography is necessary to define the
lenant methods of doping semiconductor surfaces are small geometries required in integrated circuits. In li-
:thermal diffusion in high-temperature furnaces (Fig. thography the silicon slice is coated uniformly with a
14a) and ion implantation. thin film of photosensitive material called resist. If the
In the diffusional doping process, the regions of the lithography is to be performed optically, the inte-
.silicon surface to be doped are exposed to a concen- grated-circuit pattern to be transferred to the resist is
Itration of the dopant while maintaining a high tem- first created on a glass plate or "mask." This pattern
'Perature. Boron and phosphorus are dopants which can then be transferred to the resist by a number of
can be introduced by thermal diffusion at tempera- optical techniques. "These techniques range from di-
~Ue
rm1500 to 2200oF (800 to 1200"'C). At these rect contact printing using a collimated source of ul-
tem~iperatures the silicon lattice contains a significant tra-violet light (Fig. 14J), to optical projection of a
Cobrof vacant lattice sites, that is, crystal lattice single integrated-circuit pattern with associated reduc-
Sites with missing silicon atoms. The impurity atoms tion (for example, 10: 1, 1: 1) and a precise x-y motion
~,
can migrate from vacant site to vacant site. The driv- of the silicon slice (direct step-on-wafer; Fig. 14g).
Ing force for this diffusion process is the concentra- Electron-beam direct patterning can be performed,
2'i~gradient of impurity atoms. Near the silicon sur- without a mask, by using a controllable electron beam
fAce there exists a large concentration of dopant, and an electron-sensitive resist. Lithography has also
Wh4ile tn the silicon only a small number of impurities been achieved with x-rays, by their projection through
`,ecit There is a tendency for these concentrations to a special mask in close proximity to the slice. Direct
be equalized, thereby eliminating the gradient. When step-on-wafer photolithography, the most advanced of
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Last modified: September 9, 2013