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TITLE: Chemical Mechanical Polishing
Designers should be aware of the layout requirements imposed by
Chemical Mechanical Polishing or CMP. The move to deep sub-micron
processes has placed conflicting requirements on the
photolithographic systems employed by wafer vendors. The feature
size of the process is directly proportional to the numerical
aperture of the lenses used for printing while the depth-of-
field of the lenses is inversely proportional. Hence, this
situation has forced wafer manufacturers to implement severe
planarity requirements on their deep sub-micron processes which
can only be achieved by using CMP.
CMP removes material from uneven topography on a wafer surface
until a flat (planarized) surface is created, allowing subsequent
photolithography to take place with greater accuracy and
enabling film layers to be built up with minimal height
variations. CMP combines the chemical removal effect of an acidic
or basic fluid solution with the "mechanical" effect provided by
polishing with an abrasive material. The CMP system usually
has a polishing "head" that presses the rotating wafer against a
flexible pad. A wet chemical slurry containing a micro-abrasive
is placed between the wafer and pad. It is the designer's
responsibility to achieve a minimum density on various layers to
provide the necessary structural support for this process. If
the design does not intrinsically contain sufficient density,
then additional "fill" material should be used. However, the
designer must be careful not to use this material in such a way
that the primary circuit is affected.
CMP is performed primarily in the interconnect structure of the
chip, where it is used multiple times. CMP is especially critical
for the fabrication of copper wiring structures. This advanced
planarization capability enables chipmakers to continue shrinking
circuits and extends the performance of lithographic tools.
For additional information, access: CMP
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