Chemical Mechanical Planarization (CMP)
is a process that can remove topography
from silicon oxide, metal and polysilicon
surfaces. It is the preferred planarization
step utilized in deep sub-micron IC
manufacturing. More recent scaling of
transistor critical dimension has required
the use of CMP for applications such as
shallow trench isolation (STI) and
trenched metal interconnection (Cu
damascene). CMP has also been utilized
for fabrication and assembly of Micro
Electro-Mechanical System (MEMS).
In principle, CMP is a process of
smoothing and planing surfaces with the
combination of chemical and mechanical
forces. It can, in a way, be thought of as a
hybrid of chemical etching and free
abrasive polishing. Mechanical grinding
alone may theoretically achieve
planarization but the surface damage is
high as compared to CMP. Chemistry
alone, on the other hand, cannot attain
planarization because most chemical
reactions are isotropic. However, the
removal and planarization mechanism is
much more complicated than just
considering chemical and mechanical
effects separately. CMP makes use of the
fact that high points on the wafer would be
subjected to higher pressures from the
pad as compared to lower points, hence,
enhancing the removal rates there and
achieving planarization
CMP is most widely utilized in back-end IC
manufacturing. In these process
technology and steps thin layers of metal
and dielectric materials are used in the
formation of the electrical interconnections
between the active components of a
circuit (e.g. transistors, as formed in the
front-end processing). As shown in figure
1, the interconnect is manufactured by
depositing thin films of materials, and
selectively removing or changing the
properties of these materials in certain
areas. A new level of thin film is deposited
on top of old films and the process is
repeated many times until the interconnect
is complete. The goal of the CMP process
is to planarize step heights caused by the
deposition of thin films over existing nonplanar
features, so that further levels may
be added onto a flat surface.[2]
Damascene process, as well as its
upgraded generation – dual-damascene,
is the critical technology in the transition
from aluminum to copper interconnects in
semiconductor manufacturing.[3] There are
two primary factors driving this transition:
the lower resistivity and the increased
electromigration resistance that copper
offers relative to aluminum. Several new
materials and processes are required in
this change. In the copper interconnect
fabrication process, a simpler dielectric
etching replaces metal-etch as the critical
step that defines the width spacing of the
interconnect lines, while the burden of
planarization shifts to the metal deposition
and CMP steps.