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CHEMICAL MECHANICAL POLISHING

17.8 WAFER PROCESSING

wafer polishing uniformity is poor, one will significantly over polish regions of the wafer to clear the
entire film. This issue is especially critical for metal CMP processes, since residual metal films on a
device wafer result in short circuits. Overpolishing of metal wafer features reduces vertical line
dimensions and subsequently increases local current densities in circuits.
Optical endpoint systems are used as an alternative to friction-based endpoint systems. In general,
optical-based systems are far more complicated and expensive than friction-based systems; however,
they provide better process control. Optical endpoint systems can use monochromatic or white light
depending on the detection system. They can also use interference or reflectivity to determine the
film condition at the wafer surface. Ideal optical endpoint systems should view as much of the wafer
surface as possible, and be immune to the optical impact slurry and polishing debris. Most optical
endpoint systems require placing sensors or holes in the polishing pad (Rodel). As the wafer moves
across the polishing pad, these optical sensors look for changes in material reflectivity, color, or
spectral interference patterns. For multipressure zone polishing heads, optical endpoint systems can
be used to reduce local pressures in regions where the endpoint is achieved to reduce overpolishing.
More advanced CMP tools such as the Novellus Momentum platform can use the systems to opti-
mize polishing uniformity and hold consistent stock material removal rates on a run-to-run basis.

17.3.6 In Situ Metrology
The ultimate in process control metrology for oxide CMP is an embedded metrology tool like a Nova
Inc. or Filmetrics Inc. system. These metrology tools are embedded into a CMP platform to measure
within die features and film stack thickness in critical areas immediately after polishing. These tools
often require that the wafer be placed die side down over the measuring head. The measurements are
performed under deionized (DI) water to prevent slurry particles from drying to the wafer surface.
In general, these embedded metrology tools are slow to take measurements and are relatively expen-
sive. As optical endpoint systems mature, within wafer nonuniformity polishing improvements
should allow for improved process tuning to reduce overpolishing and provide a planar wafer with-
out extensive in situ metrology verification.

17.4 POST-CMP WAFER CLEANING

Originally, CMP tools were used just to polish wafers. After the polishing process was complete,
wafers were delivered to a submerged wafer cassette. After a process lot was completed, wafers were
placed in a spin rinse dryer (SRD) and continued on their way through manufacturing. These older
CMP tools are know as “dry in, wet out” (DIWO) tools. Today’s advanced CMP tools regularly
deliver dry wafers to the production line. These “dry in, dry out” (DIDO) CMP tools eliminate the
need for a local spin rinse drier for the tool. The following section discusses the advantages of dry
in, dry out CMP tools with regard to wafer cleaning and defect levels.

17.4.1 Dry in-Wet out CMP Tools

DIWO CMP tools were state-of-the-art for many years. Older single and dual platen CMP tools often
fed wafers into cassettes via water tracks and slides. The main issues with these tools in the wafer
production line are wet wafers. When CMP wafers are allowed to dry, slurry particles can chemisorb
onto the wafer surface. They become nearly impossible to remove after the wafer dries off. In addi-
tion, a wafer fab environment is typically dry; therefore the logistics of managing with wet wafers
complicates the manufacturing process flow. Older CMP tools are often flanked by a spin rinse-drier
system used to dry wafers immediately after polishing. This local drying system is used to reduce
slurry-related surface defects. As these DIWO tools evolved, a second polishing platen or buffing
system was added to post scrub the wafer after polishing. The buffing process removes larger, say
more than 0.5 µm, slurry particles quite effectively. These systems typically use textured polytex pads
(Rodel) at high linear velocities using very low polishing downforces (less than 2 psi). DI water or a

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