line - The University of Texas at Austin

The speed of a microprocessor is to a large extent a function of the number
of transistors on the chip, and reducing the size of the transistor allows this increase
in chip density. A benchmark for the ability to increase chip density has been the
minimum line width, or critical dimension (CD), of the gate in a MOSFET. The
current technology used to pattern these small features is known as
photolithography. If this trend in increasing microprocessor density is to continue,
photolithography must continue to be a cost-effective solution.
1.3 PHOTOLITHOGRAPHY
A typical photolithography process is shown in Figure 1.3. A Si wafer is
coated with a photosensitive compound known as “photoresist”, or “resist” for
short, which changes in chemical nature upon irradiation to become more or less
soluble in a given solvent. Negative tone resists undergo changes upon exposure
that reduce the solubility in the exposed regions. Positive tone resists change in
chemical nature making them more soluble in developer. The polymer image
remaining after removing the more soluble material is then used as an etch mask in
a reactive ion etch (RIE) process, which transfers the physical image into the
underlying material.
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