Page 187 - Sami Franssila Introduction to Microfabrication

166 Introduction to Microfabrication

(a) (b) (c)
Figure 16.1 Applications of polishing: (a) smoothing; (b) planarization and (c) damascene

Down easier than polishing stacks of materials, or structures
force with different materials present simultaneously. The
mechanical properties of the wafer itself must also be
considered: if it is bowed, the pressure will be different
at the centre and the edges, leading to non-uniform
Spindle
polishing. Pressure can be applied through the chuck
Slurry to the wafer backside: this will equalize centre–edge
Chuck dispense differences and compensate for wafer bow.
Wafer The pad should be rigid so that it uniformly polishes
Pad
the wafer. However, such a rigid pad will have to
be aligned and kept in alignment with the wafer
surface at all times. Therefore, real pads are often
Platen
stacks of soft and hard materials that conform to wafer
topography to some extent. Pads are porous polymeric
materials (with 30–50 µm pore size) that are consumed
in the process and must be reconditioned regularly.
Polyurethane is commonly used for pads. Pads are very
much proprietary, and people usually refer to pads by
Figure 16.2 Schematic structure of a rotary CMP equip-
ment their trade names, rather than by chemical or other
unambiguous properties.
Slurries incorporate both mechanical elements via
Wafer Metal lines abrasive particle size and hardness, and chemical effects
via reactivity and pH of the fluid. Typical slurry
CVD oxide
Slurry materials are silica (SiO 2 ) and alumina (Al 2 O 3 ), with
some experiments being carried out on cerium oxide
Pad Asperities (CeO 2 ). Abrasive particle-size distribution is related
to smoothness: monodisperse slurry leads to smoother
Figure 16.3 Close-up of CMP set-up: wafer, upside surfaces. Copper can be polished in ammonia-based
down, is pressed against the pad with slurry in between.
slurry with 2% NH 4 OH and abrasive particles of Al 2 O 3
Pad asperities make contact with the wafer
at 2.5%wt concentration. Slurries are a cause of concern
for post-CMP: particles must be cleaned away after
Structure height obviously affects CMP, but pattern polishing. Like pads, slurries are often proprietary,
density is also important because it determines effective and the information given is often restricted to pH
contact area: denser patterns are polished at a lower rate value, base liquid (for instance, NH 4 OH-based) and
due to lower pressure. Polishing of a single material is abrasive particle size. Slurries can be buffered against