As the geometries of circuitry shrink, the drying of the substrate becomes a more critical step.
In the production of microelectronics, substrate drying to btain a clean, contamination-free surface often centers on the term “Marangoni drying.”
The Marangoni effect is the mass transfer along an interface
between two fluids due to surface tension gradient. Since a liq-
uid with a high surface tension has higher cohesive forces than
one with a low surface tension, the presence of a gradient in
surface tension will naturally cause the liquid to flow away from
regions of low surface tension. The general effect is named after
Italian physicist Carlo Marangoni, who studied it for his doc-
toral dissertation and published his results in 1865 and was first
documented as a drying method by University of California,
Berkeley scientists in the early 1960s.
The Marangoni drying technique uses a volatile organic
compound with a lower surface tension than water that is
introduced in the vicinity of the substrate as it is slowly withdrawn from the water. As the small quantity of alcohol vapor
comes into contact with the refreshed water meniscus, it dissolves into the water creating a surface tension gradient. The
gradient causes the meniscus to partially contract and assume
an apparent finite angle via a Marangoni flow. This causes the
thin water film to flow off the substrate, leaving it dry. The
Marangoni flow also removes contaminants and particles.
Aqueous cleaning in microelectronics has been increasingly
used due to stricter environmental regulation of organic cleaning agents, the widespread use of aqueous processing in the
semiconductor, flat panel display, and optics industries, plus
the increasing need for cleanliness brought about by decreases
in pattern geometries and the push for increased yields. These
ever-increasing demands dictate the need for an ultra-clean
drying process that removes residual water and contaminants
and mitigates watermark formation from critical surfaces.
This drying method is fundamentally cleaner and more
efficient than those using heat, forced air, or high-speed rotation of the substrate, like the spin rinser dryer used for so
many years in semiconductor fabs around the world.
ratio structures also dictate that surfaces are clean and contamination-free. As geometries shrink below the sub 100-nm
level, both metal and particulate contamination becomes more
critical. Also, high speed rotation drying can chip the wafer
edge, resulting in particle contamination. All of these processing demands have increasingly escalated the use of surface tension gradient drying in semiconductor manufacturing, as well
as MEMS and solar devices.
Advantages of surface tension gradient drying
Since this method of drying completely eliminates water
from the surface of a substrate, no watermarks (a type of
contamination in themselves) are left on the surface (Figure
1). The newer, more evolved dryers have low particle counts
and only trace levels of organics compared to both spin rinser
dryers and IPA vapor dryers. Drawing the water away from
the surface of the substrate with gravity results in no feature
damage, no edge chipping, and no substrate breakage. Because
there are no high-speed spinning steps in the drying process,
ESD (electrostatic discharge) damage to the wafer is virtually
eliminated; and the dryer produces very low metallic contamination levels (Table 1).
Industry needs evolve
Semiconductor device manufacturing tends to be at the leading edge of microelectronic development with the continued
advancement of Moore’s Law and constantly shrinking wafer
geometries. New drying methods for these smaller geometries
have evolved for several reasons.
As geometries continue to decrease, the patterned structures become more fragile. Fragile structures can be easily
damaged by high-speed drying methods like the spin rinser
dryer. Dimensions with deeper trenches and higher aspect
Element Spec Limit Dryer Results
Al 50 63
Mg 5 2. 9
Ca 40 18. 7
K 5 0.01
Na 20 14. 9
Cu 2 1.7
Fe 5 1.4
Ni 3 0.6
Zn 3 0.7
Cr 2 0.05
Table 1: Selected Metal Contamination
VPD-ICP-MS Analysis —1 x E9 atoms/cm2
Besides the elimination of watermarks on hydrophilic,
hydrophobic, and combination films, surface tension drying
provides other benefits. This drying method does not place
any mechanical stresses on the substrate. The technique works
well on practically any flat substrate. No surfactants are necessary to change the substrate properties to enhance drying performance. Compared to traditional vapor dryers, gradient dryers consume very little IPA, reducing chemical consumption.