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Fig. 2.16 Successive
stages of the negative
lithographic process (a-f)
then the chip is heated. Alternatively ion implantation by direct action of dopant
ions can be employed. Figure
2.16
illustrates the positive lithographic process in
which a layer of aluminum deposited on the surface of the chip is removed from all
its elements except for the gate area of the field-effect transistor.
Various resists are known which differ in terms of their light sensitivity and
resolution. Often a positive photoresist PMMA (polymethylmethacrylate, Fig.
2.17
)
is used. As a negative resist, COP (a polymer of glycidyl methacrylate and ethyl
acrylate) is used.
Photolithographic technology allows for the creation of complex semiconductor
circuits, but at the same time it represents the limiting factor of the planar semi-
conductor technology. Due to its wave nature light is diffracted by the photomask
elements (Fig.
2.18
). The maximal achievable resolution of the exposed pattern is
of the same order as the wavelength of the light used for illumination. As is known,
the wavelength of visible light is in the range from 0.38
m (violet region of the
μ
spectrum) to 0.76
m (red area). This determines the minimum line width in a
semiconductor structure which can be obtained by optical lithography.
Today's scientific and engineering projects are aimed at improving the key step
in the production of integrated circuits—lithography—which will determine the
physical limits of semiconductor technology in the foreseeable future. Experts note
that because of these restrictions lithography may exhaust its possibilities already in
the beginning of our century.
The development of the lithographic technology since its inception in the early
1970s was directed at reducing the wavelength of the light used. This allowed to
reduce the size of the elements of the integrated circuit. Since the mid-1980s
ultraviolet laser radiation is used in photolithography. In order to apply the pattern
of the circuit to the plate, computer-controlled machines (steppers) are employed.
Configuration of “window” is determined by appropriate masks, and the resulting
μ
