is the process of increasing the flatness or planarity of the surface of
a semiconductor wafer through various methods known as planarization
raw wafers for semiconductor device fabrication are ideally flat or
planar. However, as the wafer goes through the various steps of
device fabrication, layers of different materials, shapes, and depths
are deposited over the wafer surface through different growth and
deposition techniques. Also, portions of materials already
deposited over the wafer need to be removed from time to time.
This series of material growth, deposition, and removal steps decreases
the flatness or planarity of the wafer surface.
trends towards device fabrication that involve four or more layers of
metallization has aggravated the problem of wafer non-planarity. The
shift to narrower and narrower metal lines also prompted the emergence
of thicker metal lines in order to meet the current requirements of the
device. Modern fabrication techniques that increase the number of metal
layers on the wafer while decreasing the width of the metal lines will
continue to make the problem of wafer non-planarity worse.
A decrease in
the flatness of the wafer's surface introduces at least two problems to
device fabrication. First, ensuring ample step coverage of very
fine lines so that no breaks in the continuity of the lines arise
becomes more difficult as the wafer becomes less flat. Second,
progressive loss of planarity eventually makes the imaging of
fine-featured patterns on the wafer challenging, if not impossible.
There are several planarization techniques used in wafer fabrication today.
There are two categories for planarization techniques, namely, local
planarization and global planarization.
to smoothing techniques that increase planarity over short distances.
Global planarization, on the other hand, consist of techniques that
decrease long-range variations in wafer surface topology, especially
those that occur over the entire image field of the stepper.
include: 1) oxidation; 2) chemical etching; 3)
taper control by ion implant damage; 4) deposition of films of
low-melting point glass; 5) resputtering of deposited films to smooth
them out; 6) use of polyimide films; 7) use of new resins and
low-viscosity liquid epoxies; 8) use of spin-on glass (SOG) materials;
9) sacrificial etch-back; and of course, 10) mechanical-chemical
polishing of the wafer.
Incoming Wafers; Epitaxy;
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