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CHEMICAL MECHANICAL POLISHING

17.6 WAFER PROCESSING

film (i.e., Rodel DF200). The shape of the plate and film at the back of the wafer determines the
wafer curvature during CMP. Hence the term, back-referencing carrier. For example, a backing plate
with a center bow outward of about 2 µm over 200 mm will result in a center fast polishing process.
Temperature changes during CMP can influence back-referencing carrier profiles and subsequently
the polishing nonuniformity by thermally changing backing plate shape.
In addition to the backing plate curvature, the position of the CMP retaining ring on the back-
referencing carrier can influence edge-polishing rates. These consumable wear rings are typically
made out of polymers or ceramics. By increasing the wear ring pressure to the pad, polishing at the
edge of the wafer can be reduced. For some systems this is completed pneumatically, and with oth-
ers it is determined by the distance the wafer protrudes from the carrier wear ring. For typical back-
referencing carriers using a fixed wear ring, this wafer protrusion above the wear ring should be set
from 75 to 150 µm. This protrusion is normally set via shims and needs to be completed with a dry
carrier film. Care must be taken to keep the wafer protrusion even around the carrier wear ring. A
total range of less than 50 µm is suggested for optimum polishing uniformity.
Tuning polishing nonuniformity on newer CMP systems requires an understanding of front-
referencing carriers. These carriers are typically pneumatic diaphragms made out of industrial rub-
bers. The wear ring position relative to the wafer surface is less important for tuning the polishing
process. These systems typically have independent wear ring and polish diagram pressure settings.
These carriers are called front-referencing carriers since the flexible diaphragm allows the wafer to
fully conform to the polishing pad surface during processing. Basically, the ideal wafer shape for
optimum polishing uniformity is that of the polishing pad profile. One can reduce the polish rates at
the wafer edge by increasing the wear ring pressure relative to the bladder pressure. Care must be
taken not to reduce wear ring pressure to less than 1 psi than the polishing pressure to prevent wafer
loss during CMP. More advanced carriers may have up to seven or eight unique pressure zones
behind the diagram to tune polish nonuniformity. For these advanced carriers, one will need to use
software supplied by the CMP tool manufacturer to optimize polishing nonuniformity. In general,
pneumatic carriers use polishing pressures from 2 to 10 psi for CMP processes.

17.3.3 Process Consumables
Process engineers can ensure that the CMP process remains consistent by properly managing both
the slurry and the polishing pad. Slurry management is often left to the facilities people in the wafer
fab to maintain and control. Using proper slurry mixing, filtering, titrating, and fluid system rinsing
processes can mitigate slurry-related issues. Slight changes in the solid contents of slurries, pH,
and/or particle shape and size will change material removal rates. In addition, defective polishing
often results from slurry housekeeping-related issues. Dried slurry is very difficult to remove from
wafer surfaces and will scratch silica films if entrained in a slurry distribution system from dirty
CMP tool components.

17.3.4 Pad Conditioning
In general, slurry-related CMP process issues are less common than pad-related issues. The polish-
ing pad is the most critical process consumable in CMP. This is because the CMP polishing pad is
important for both transporting fresh chemicals to the wafer surface and removing process by-products.
In addition, the polishing pad has a dramatic influence on both within wafer and within die polishing
nonuniformity. For the best process control, the pad must be properly managed. A dried pad will
scratch wafers, while an old pad will planarize differently than a new pad.
To get the best results for one’s CMP process, one must properly take care of the pad. CMP pads
for ILD processes are stacked or two-layer pads. A typical stacked ILD polishing pad is a Rodel
IC1000 pad backed with a SUBA IV subpad of similar thickness. A good ILD pad stack is a 0.080-in
thick IC1000 with a 0.050-in thick SUBA IV subpad. Metal CMP processes typically use fibrous sin-
gle-layer pads like those supplied by Thomas West Inc. Overall, pad management of oxide and metal
CMP pads are similar, and slightly process dependent. In general, polishing pad management for
optimum process stability includes, at a minimum, the following:

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