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Extra resources for [Magazine] Scientific American. Vol. 274. No 1
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Fine adjustments to the edges of the patterns in the reticle were used to shift the phase of the incoming i-line radiation, permitting crisper edge deÞnitions and therefore smaller featuresÑ in eÝect, circumventing RayleighÕs limit. One of the last adjustments was the simple acceptance of a lower value of the proportionality constant, which is related to the degree of contrast in the image projected onto the wafer during lithography. For i-line operation, manu- SCIENTIFIC AMERICAN January 1996 facturers gritted their teeth and accepted a lower proportionality constant than was previously thought practical.
After being exposed, the photoresist is developed, which delineates the spaces, known as contact windows, where the diÝerent conducting layers interconnect. An etcher then cuts through the oxide Þlm so that electrical contacts to transistors can be made, and the photoresist is removed. More sets of mask layers, based on much the same deposition, lithography Copyright 1995 Scientific American, Inc. and etching steps, create the conducting Þlms of metal or polysilicon needed to link transistors. All told, about 19 mask layers are required to make a chip.
But the components of mitochondriaÑincluding the ATPsynthesizing machinery and the mitochondrial DNA that gives rise to some of that machineryÑare believed to be most vulnerable. Presumably they are at risk in part because they reside at or near the Òground zeroÓ site of free-radical generation and so are constantly bombarded by the oxidizing agents. Moreover, mitochondrial DNA lacks the protein shield that helps to protect nuclear DNA from destructive agents. Consistent with this view is that mitochondrial DNA suÝers much more oxidative damage than does nuclear DNA drawn from the same tissue.