OPTICAL LITHOGRAPHY
The dramatic
increase in performance and cost reduction in the electronics
industry are attributable to innovations in the integrated circuit
and packaging fabrication processes. ICs are made using Optical
Lithography. The speed and performance of the chips, their associated
packages, and, hence, the computer systems are dictated by the
lithographic minimum printable size. Lithography, which replicates
a pattern rapidly from chip to chip, wafer to wafer, or substrate
to substrate, also determines the throughput and the cost of electronic
systems. From the late 1960s, when integrated circuits had linewidths
of 5 µm, to 1997, when minimum linewidths have reached 0.35
µm in 64Mb DRAM circuits, optical lithography has been used
ubiquitously for manufacturing. This dominance of optical lithography
in production is the result of a worldwide effort to improve optical
exposure tools and resists.
A lithographic
system includes exposure tool, mask, resist, and all of the processing
steps to accomplish pattern transfer from a mask to a resist and
then to devices. Light from a source is collected by a set of
mirrors and light pipes, called an illuminator, which also shapes
the light. Shaping of light gives it a desired spatial coherence
and intensity over a set range of angles of incidence as it falls
on a mask. The mask is a quartz plate onto which a pattern of
chrome has been deposited.
It contains
the pattern to be created on the wafer. The light patterns that
pass through the mask are reduced by a factor of four by a focusing
lens and projected onto the wafer which is made by coating a silicon
wafer with a layer of silicon nitride followed by a layer of silicon
dioxide and finally a layer of photo-resist. The photo resist
that is exposed to the light becomes soluble and is rinsed away,
leaving a miniature image of the mask pattern at each chip location.
Regions unprotected
by photo resist are etched by gases, removing the silicon dioxide
and the silicon nitride and exposing the silicon. Impurities are
added to the etched areas, changing the electrical properties
of the silicon as needed to form the transistors.
As early
as the 1980s, experts were already predicting the demise of optical
lithography as the wavelength of the light used to project the
circuit image onto the silicon wafer was too large to resolve
the ever-shrinking details of each new generation of ICs. Shorter
wavelengths are simply absorbed by the quartz lenses that direct
the light onto the wafer.
Although
lithography system costs (which are typically more than one third
the costs of processing a wafer to completion) increase as minimum
feature size on a semiconductor chip decreases, optical lithography
remains attractive because of its high wafer throughput.
RESOLUTION
LIMITS FOR OPTICAL LITHOGRAPHY
The minimum
feature that may be printed with an optical lithography system
is determined by the
Rayleigh equation:
W=k1?
NA
where, k1 is the resolution factor, ? is the wavelength of the
exposing radiation and NA is the numerical aperture.