ADT’s
Model 8000
NextStep
system removes only non-silicon elements from the streets leaving the
silicon wafer bare and virtually unaffected. The wafer can then be diced
using a standard mechanical dicing process.
With
ever-increasing transistor density, shrinking metal lines and gate
dimensions, producers are forced to use metals of higher conductivity
such as copper, "low-k" isolation materials such as "black diamond" and
more test pads.
These new manufacturing standards set a different threshold for wafer
singulation whereby traditional step cut dicing no longer meets the
quality requirements and cost targets for manufacturing.
ADT meets the market challenge with a new, revolutionary type of laser
for wafer scribing
WHITE PAPER
NEXT
GENERATION LASER SCRIBING FOR DICING COMPLEX SILICON WAFERS
The
well- known law named after Gordon Moore predicts the doubling of
integrated circuit (IC) performance every 18 to 24 months. The
semiconductor industry owes part of the uninterrupted continuation of
this trend to two significant changes it has recently adopted. The first
is the transition of metal interconnects from aluminum to copper, due to
the higher electrical conductance of the latter. The second is the
conversion from the use of traditional silicon dioxide dielectrics to a
new class of materials with lower dielectric constants, appropriately
termed “low-k” materials. Both these changes, although extremely
beneficial to IC performance, have been found to result in undesired
effects on the dicing process, in which the silicon wafer is separated
into individual die.
Copper is considerably more ductile than aluminum resulting in premature
clogging of the dicing blade, which in turn leads to increased loading
and excessive chipping that deteriorates the cut quality. The porous
organic and other materials that make up the group of low- k materials
are more fragile than silicon dioxide and are hence much more
susceptible to chipping and cracking during dicing. Not only has the
nature of the dielectric material changed, but also the thickness of the
active layers has increased up to a thickness of 10 microns. Additional
developments in the fabrication of integrated circuits have resulted in
further challenges to the dicing process. The number of metallic layers
has steadily increased (with eight layers not uncommon) and the
incorporation of multiple functions on a single chip has led to larger
die that call for polyimide coatings to mediate thermo- mechanical
effects between die and package. Both the additional metal and the
presence of polyimide lead to premature blade clogging and an unstable
dicing process.
The
current solution to the problem of dicing complex silicon wafers is
based upon a two- step process. In the first step, a very shallow cut is
made into the wafer in order to remove only the non- silicon components.
This step is usually performed at very low feed rates (less than 10
mm/s) in order to attempt to overcome the difficulties presented above.
In spite of the low feed rate and the attempts of blade manufacturers to
offer specific products for low-k materials, this first cut is often
accompanied by severe chipping, metallic smearing and layer peeling. In
a second dicing step, the remaining wafer is cut all the way through at
standard dicing feed rates.
Clearly, the main obstacle to lowering cost of ownership in wafer
singulation is the problematic first step in the process. Accordingly,
the industry is looking at alternatives to this step, most often in the
form of laser scribing.
Out
of several laser- based solutions presented for singulating low-k
wafers, the one offered by Advanced Dicing Technologies (ADT) stands out
in its simplicity, ingenuity and cost- effectiveness. The patented
technology introduced by ADT makes use of a low- power CO2 laser to
ablate only the non- silicon components in the wafer streets. The
silicon itself is transparent to the specific wavelength used and
remains virtually unaffected by the radiation. Not only does the
radiation have no direct effect on the silicon, but unlike other laser-
based solutions that have a detrimental effect on die strength, also
secondary thermal effects in the ADT system are negligible due to the
very limited heat- affected zone of the CO2 laser.
After
laser-scribing with the ADT system, the clean- street wafer can be
traditionally diced on a dicing saw. Due to the unprecedented feed- rate
of up to 600 mm/s at one pass, the ADT system can provide scribed wafers
for two to three twin dicing systems thus providing a truly cost-
effective solution to the industry. With the high throughput of the
process, its excellent cut quality and the minimal capital expense
required, ADT have presented the industry with a truly winning
combination.
