In the production of microelectronics, substrate drying to obtain 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 liquid 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 doctoral 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compound with a lower surface tension than water that is introduced in the vicinity of the substrate as it is slowly withdrawn from the water.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.
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 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).
Table 1.
Figure 1: IPA concentration gradient induces surface tension gradient drying without watermarks. 
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.
Additionally, because the low-surface tension IPA does not react with HF, these chemicals can be added in low concentration to the rinse water to affect the final properties of the substrate. For examples, the HF will keep the silicon surface hydrophobic and facilitate drying, and also prevent watermark formation.
Applications
There are several applications of surface tension gradient drying that uniquely take advantage of the benefits of this drying technique. Below is a quick summary of three.
1. Very thin substrates
Stiction from the rinsing process can pull the thin wafers together, causing the wafers to stick to one another as they are pulled from rinsing baths and placed into a drying chamber. This is especially a concern when rinsing hydrophilic wafers were the capillary action of the water pulls the wafers together. Once between the wafers, the water can increase the chance of hydrogen bonding of the water to the wafer surfaces, making it very difficult to debond the wafers from one another. To prevent this occurrence, the Marangoni dryer has an overflow rinse before the drying step; this prevents the wafers from being pulled out of the rinsing bath. The rinse water is used as the higher surface tension liquid of the Marangoni dryer. The IPA layer on top of the water bath displaces the water (Figure 2), lowering the surface tension, and thus preventing the stiction from occurring. Wafers as thin as 80 µm can be dried. MEMS, solar, and other thin wafers will benefit from this drying technique.
Figure 2: Cross-sectional drawing of the surface tension gradient drying process.
2. Substrates with significant topology
High aspect ratio feature drying can be facilitated by using a low surface tension gradient liquid. Isopropyl alcohol vapor will effectively dry the deep contact holes or vias of these structures, but at the expense of leaving residue at the bottom of the trough, because the IPA does not dissolve the residue. By drying with the Marangoni method, not only are the deep features dried from the displacement of the water, but when the water is displaced, it retains the contaminants from the bottom of the feature, whether it be silicates or post-etch reside in the form of “particles.”
Additionally, pattern collapse of high aspect ratio features such as lines, cantilever structures, or freestanding capacitors used for DRAM fabrication can succumb to stiction. The use of a low surface tension fluid in the drying process can help mitigate the stiction in the same manner as thin substrates. In this case, less stress is placed on the features, and they do not collapse when being removed from the rinsing tank.
3. Combination hydrophobic and hydrophilic surfaces
Almost all device types have some layers to be dried that are a combination of hydrophobic and hydrophilic surfaces, for example after the delineation of shallow trench isolation or prior to gate oxide formation. When this combination of surface energies exist, there is a high probability of watermark formation on the hydrophobic surface, especially at the interface of the two surface energies, where droplets of water are hard to remove. When these droplets evaporate, they leave a watermark. When using a spin-drying technique, this is very pronounced. By using a Marangoni dryer, there are no remaining droplets, thus no remaining watermarks. The low surface tension IPA wets both the hydrophobic area and the hydrophilic area and can prevent the nucleation of the water droplet, providing a watermark-free surface.
Conclusion
When integrated with cleaning and rinsing of the substrate, surface tension gradient drying can provide a one-step process of molecular level drying for such applications as the fabrication and cleaning of ICs, solar cells, fuel cells, MEMS, and disk drives.
Tom Vukosav has been president of MicroTech, also known as MT Systems, since its inception. A semiconductor industry veteran, he has held management positions for several wafer processing equipment companies over the past 30 years. MicroTech has licensed the patent rights of Marangoni drying from current owners to develop and manufacture a surface tension gradient dryer, called The Gradient Dryer. www.microtechprocess.com
