Integrated circuits 263
niques, the vertical etch rate can be adjusted to be hundreds of such circuits may be fabricated on a sin-
inuch greater than the lateral etch rate. This enables gle slice of silicon crystal. This feature of planar tech-
.,the etching of fine lines and features without loss ' of nology-simultaneous production of many circuits-
jdefinition, even in films approaching 1 p.Lm in thick- is responsible for the economic advantages and wide
'xiess. use of integrated circuits.
4*1For plasma etching, a plasma is formed .above a The starting material is a slice of single-crystal sil-
fasked surface to be etched by adding large amounts icon, more or less circular, up to 6 in. (150 mm) in
; of energy to a gas at low pressures. This is commonly diameter, and a fraction of an inch (a few millimeters)
accomnplished by electrical discharges in gases at thick. Typically, this material is doped with p-type
about 10-1 atm (10-2 Pa). A plasma contains ions, impurities (Fig. 15a). A film of semiconductor, less
*free radicals, and neutral species, all with high kinetic than 0.001 in. (25 i.Lm) thick, is then grown upon this
,-energies. By adjusting the electrical potential of the substrate in a vapor-phase reaction of a silicon-con-
substrate to be etched, the charged species in the tamning compound. The conditions of this reaction are
Plasma can be directed to impinge on the substrate such that the film maintains the single-crystal nature
And thereby impact the nontmasked regions. The force of the substrate. Such films are called epitaxial (Greek
of the high-energy impact can knock out substrate at- for "arranged upon"). By incorporating n-type im-
orns. This plasma etching process can be made more purities into the gas from which the film is grown, the
*effective with the use of gases in the plasma that are resulting epitaxial film is made n-type (Fig. 15b).
rjactive with the material to be etched. In particular, Next, the silicon slice is placed into an oxygen at-
the reactants should form volatile products that can be mosphere at high temperatures ( 2200*F or 1200'C).
.carried away by the vacuum system. Such gases usu- The silicon and oxygen react, forming a cohesive sil-
ally contain halogens (fluorine, chlorine, and bro- icon dioxide film upon the surface of the slice that is
mine). Reactive ion etching combines the energetic relatively impervious to the electrically active impur-
etching effects of the plasma with the reactivity of the ities (Fig. 15c).
gases and the formation of energetic reactive species To form the particular semiconductor regions re-
:in the plasma. SEE PLASMA flflsics. quired in the fabrication of electronic devices, how-
Bob L. Gregory; Eugene A. Irene ever, p- and n-type impurities must be introduced into
Bipolar process flow. The- principal steps involved certain regions of the semiconductor. In the planar
;in the fabrication of the simple bipolar inverter circuit technology, this is done by opening windows in the
of Fig. 1 are shown schematically in Fig. 15. An in- protective oxide layer by photoengraving techniques,
verter requires only a transistor and resistor, shown in and then exposing the slice to a gas containing the
,,cross section. Complete digital integrated circuits appropriate doping impurity. In the case of an inte-
generally contain tens to hundreds of inverters and grated circuit, the isolation regions-p-type regions
.gates interconnected as counters, arithmetic units, and which, together with the p-type substrate, surround
other building blocks. As indicated by Fig. 15g, the separate pockets of the n-type film-are formed
n-type impurities
Ja) iiontp 4 !4
transistor emitter Al interconnections
(b) y
silicon dioxide film Mf ~.
(c) np-n ransstorFig. 15. Steps In fabrica-
tion of bipolar Inverter cir-
cult. (a) Initial p-type sub-
one ircit ýilion sicestrate. (b) Growth of n-
(g OEM.%~4a type film. (c) Growth of
transistor base p-type impurities Aoxide filim. (d) Opening of
windows In oxide layer
and formation of Isolation
regions, transistor base,
and resistor. (e) Regrowth
and formation of windows
(d) .'In oxide layer, and forma-
~- *<,,'tion of transistor emitter.
.--.- ,
(f) Opening of contacts
~ ~ alminm n-ypesilcon and deposit of metal. (g)
alumnum -typ silcon Numerous circuits Incor-
isolation diffusions resistor silicon dioxide ' p-type silIicon porated on a silicon slice.
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Last modified: September 9, 2013