Page 316 - A Practical Guide from Design Planning to Manufacturing

286 Chapter Nine

The next likely incremental improvement is immersion lithography.
Liquids have a higher index of refraction than air or vacuum, which allows
light waves to be bent more sharply. Immersion lithography improves res-
olution by placing a drop of liquid between the lithography lens and the
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wafer enabling sharper focus. This and other evolutionary changes will
continue to be developed to try and prevent Moore’s law from being slowed
by lithography.

Etch
Etching, the removal of unwanted material layers, is the processing step
typically performed after lithography. This removes the material uncov-
ered by developing the photoresist while leaving behind the material still
covered. Because deposition steps in general cover the entire wafer, etch-
ing is crucial to creating the desired pattern of material. Different etch
processes are chosen based on etch rate, selectivity, and isotropy.
Etch rate is a measure of how quickly a given thickness of material
can be removed. Time is money, and etch times of many hours would
make high-volume manufacturing expensive. On the other hand, it
wouldn’t be possible to provide good control for an etch of only a few sec-
onds. The etch process must be designed to give etch rates that are fast
enough to not limit volume production while still being slow enough to
give reproducible results.
Etch selectivity is the ratio of etch rates between the material to be
removed and the material beneath it. Ideally, an etch process will be
highly selective so that it will quickly etch through the unwanted mate-
rial but only very slowly affect the layer underneath. Isotropy describes
how directional an etch is.
At the left in Fig. 9-17 is a purely isotropic etch. This type of etch pro-
ceeds at the same rate in all directions. In the center of the figure is the
opposite, a pure anisotropic etch. This type of etch cuts only vertically.
At the right is shown a preferential etch that proceeds both horizontally
and vertically but cuts more quickly in the vertical direction. Etch rate,
selectivity, and isotropy must all be considered in developing a suitable etch
for each layer. Etches are divided into two general categories: wet etches
that use liquid solutions and dry etches that use gasses or plasmas.
Wet etches involve immersing the wafers in a liquid bath usually con-
taining acids mixed with water. A wet etch removes material through a
purely chemical process, so the key is finding the right chemical reaction.
For example, hydrofluoric acid (HF) will readily dissolve SiO but not sil-
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icon. This makes it possible to create a fast highly selective wet etch of

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Stix, “Shrinking Circuits with Water.”