Cause of Watermarks

Watermarks are well known to be a major source of yield loss. Water marks form when dissolved, non-volatile material (often silica) is left behind as water droplets begin to evaporate. Typically this occurs during the transfer of substrates between cleaning and drying cycles or during the drying cycle itself. Watermarks are especially detrimental on bare silicon as it becomes oxidized in the presence of oxygen and water forms silicic acid or hydrated silica. Spin-rinse dryers have generally been effective in drying substrates but leave a residual film in the order of 0.1-0.2um after drying which determines the size of residual material on the surface. They are also ineffective in preventing watermarks especially on hydrophobic films like low k materials.

If the surface water film can be completely displaced thus also removing oxygen from the process, formation of watermarks can be effectively prevented. Water can be displaced by a liquid with lower surface tension, for example IPA (isopropyl alcohol) has a surface tension of 22 dynes/cm compared to water at 67 dynes/cm at 50C.

Accudry Technology

Drying based on surface tension gradient forces is an ultra-clean drying process. In this technique a volatile organic compound with lower surface tension than water is introduced in the vicinity of a substrate in the form of vapor as it is slowly withdrawn from the water. As the small quantity of alcohol vapor comes into contact with the continuously refreshed water meniscus, it absorbs in the water creating a surface tension gradient. The gradient causes the meniscus to partially contract and assume an apparent finite angle via a flow. This causes the thin water film to flow off the substrate leaving it dry (Fig 1). This flow also removes non-volatile contaminants and entrained particles.

Besides the elimination of watermarks on hydrophilic, hydrophobic and combination films, IPA vapor drying provides various other benefits. Drying does not require placing any mechanical stresses on the substrate. The technique works well on practically any flat substrate and no surfactants are necessary to change the substrate properties to enhance drying performance. Compared to traditional vapor dryers, the accudry consumes very little IPA because of its patented technology. When integrated with cleaning and rinsing, the Accudry can provide a one-step process in various applications such as fabrication and cleaning of ICs, solar cells, fuel cells, MEMS, etc.

Process Sequence

Step 1: Tank Fill

After the substrates are introduced the tank is filled with water to completely immerse them. The water temperature is maintained at ambient or slightly cooler and controlled by fab facilities. Depending on the flow rate of the water this step could take up to 30 seconds.

Step 2: Cascade water overflow

Substrates can be cleaned in dilute chemicals and subsequently rinsed in the tank to remove chemical impurities. The water is allowed to overflow into the overflow tank during the rinse cycle. The duration of this step depends on the amount of rinsing required.

Fig. 2 Cross section of the Accudry process chamber.

Step 3: IPA/N2 Flow

IPA liquid is vaporized using heated N2 and deposited on the water as a fog through nozzles situated directly above the water for about 60 seconds. IPA liquid is kept constantly circulating in the loop to make it readily available at the nozzle. The N2 temperature must be controlled and maintained between 80 and 100ºC to regulate the rate of IPA vaporization. The flow rates of both IPA and N2 are controlled independently. IPA flow typically lasts for 60 seconds.

Step 4: Slow Drain

Water is drained slowly from the tank through an outlet at the bottom thus ensuring a stable, repeatable, downward moving meniscus. The meniscus is independent of the surface contact angle or the pattern on the substrate thus ensuring a much broader process window for a wide variety of films. The Nitrogen and IPA are focused on the interface formed between the water and the substrate as the substrate emerges from water. The IPA assists in drying the wafer by the surface tension gradient effect (Fig 1). IPA is readily absorbed at the tip of the meniscus, where it lowers surface tension. The resulting surface tension gradient pulls water away from the substrate as the water continues to drain. The smooth rounded bath cavity helps prevent water from remaining in the chamber.

This is the most important process step in the sequence and can range from 120 to 300 seconds. The three main parameters that control drying efficiency are nitrogen flow, IPA concentration and water drain speed. The amount of IPA injected and flow rate needs to be controlled carefully and adequate to keep the thin layer of IPA on the surface independent of surface features. If less IPA vapor is used it will not produce enough surface tension reduction at the interface to remove residual water from the substrate surface. However, excess IPA vapor results in extra fluid on the substrate surface that cannot be evaporated within the process time to maintain the throughput. Higher IPA consumption also makes effluent management more expensive. Nitrogen flow, typically maintained at 50sccm, should be enough to carry IPA to the meniscus without breaking the film. Higher N2 flow results in quicker evaporation of the IPA reducing the surface tension gradient resulting in incomplete drying. A faster drain speed has a similar effect of exposing new substrate surface too quickly resulting in watermark formation and incomplete drying of high aspect ratio structures.

Step 5: Heated Gas Flow

In the final step of the process heated N2 gas is flowed to remove the remaining water and IPA film on the substrate surface.

Conclusion

In the past decade there have been many advances in wafer drying techniques to achieve watermark-free clean substrates. The surface tension gradient dryers have emerged as the dryer of choice to achieve watermark free performance on practically any type of substrate, be it hydrophobic, hydrophilic or a combination of both. Although single-wafer drying provides the benefit of replicating process conditions wafer-to-wafer, integrated batch dryers are still more efficient, have higher wafer outputs and consume less IPA. Imtec has introduced the Accudry, a batch surface tension gradient dryer that will dry a variety of substrates including III-V Semiconductors, MEMS, solar Cells, fuel cells as well as ICs.