Watermark Elimination via Surface Tension Gradient Dryer

Introduction

Wet chemical cleaning continues to be the process of choice for the semiconductor, LED, MEMS, disk drive and other electronic manufacturing industries. Almost 30% of the processing steps in semiconductor manufacturing involve wet cleaning, either in single wafer or batch mode. The last step of every wet cleaning processing sequence before the substrate moves on to the next layer is drying, and hence the most critical step as well.

In the hard disk industry the final step prior to sputtering the magnetic films on hard disk drive media is the critical drying step. Improved aerial density and lower costs are drivers for improved wet processing and drying methods. In the case of semiconductors, smaller feature dimensions for 90nm nodes and beyond, higher aspect ratios up to 50:1 and new substrate materials such as high-k gates place ever-increasing demands on cleaning and drying. Particles of less than 0.1µm and smaller are monitored for the most advanced applications. The most challenging issue with wet processing is the occurrence of watermarks on the substrate after the final drying step.

Watermark Formation and Elimination

Watermarks are well known to be a major source of yield loss. They are most detrimental on bare silicon and can cause electrical failure due to the formation of thicker local oxide and preventing adequate film adhesion1. Watermarks also impact yield by causing increased leakage currents, CD variations and killer defects2. Due to their catastrophic nature on small features, ITRS roadmap has prescribed that even a single watermark cannot be tolerated below 90nm node3 to prevent impact on device yield.

Water marks form when dissolved, non-volatile material (often silica) is left behind as water droplets begin to evaporate3. Typically this occurs during the transfer of substrates between cleaning and drying cycle or during the drying cycle itself. Watermarks are especially detrimental on bare silicon as it gets oxidized in the presence of oxygen and water to form silicic acid or hydrated silica4. During conventional drying of hydrophobic films (water resistant, high contact angle), water films break apart into droplets which precipitate material onto the substrate surface as defects. It is relatively easier to dry hydrophilic films (water attracting, low contact angle). Due to the low contact angle water sheets off the film completely but the film is also easily re-contaminated during the drying process. The most difficult films to dry in FEOL layers are those with mixed hydrophobic and hydrophilic surfaces such as metal/low k or Si/Oxide. This type of surface condition became more prevalent as the industry moved to dilute HF (DHF)-last cleaning in the early 1990s. HF-last cleaning causes some regions on the substrate to form hydrogen-terminated hydrophobic surfaces due to some etching by HF1. This dramatically increases the chances of forming watermarks. In BEOL layers and for some FEOL memory layers the challenge is to dry high aspect ratio structures to prevent wicking or watermark formation from water trapped in the deep trenches. Spin-rinse dryers have generally been effective in drying substrates but leave a residual film of the order of 0.1-0.2um after drying which determines the size of residual material on the surface5. They are also ineffective in preventing watermarks especially on hydrophobic films like low k materials. Spin-rinse dryers induce high stresses on larger substrates at higher speeds that could be detrimental to the device features6.

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 50C4. IPA vapor dryers use heated baths of IPA to create vapors that are then condensed on the substrate to remove the water. Although IPA dryers do a good job of drying the substrates, their downsides are the high consumption of IPA poses a fire risk and they add organic contaminants to the water6. Since traditional drying techniques such as spin-rinse and IPA dryers discussed above have proven inadequate to meet the drying needs of future device technology nodes, the industry has moved to the surface tension gradient style substrate drying technique.

Surface Tension Gradient Drying 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 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 gradient5. 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. Several authors have reported that combining rinse and dry steps is most effective for contamination and watermark control. Organic compounds, such as IPA, with low vapor pressure at room temperature, that are water-soluble and produce a large reduction in surface tension when dissolved are ideal for this application. (Please see Appendix for IPA properties.)

Fig. 1. IPA concentration gradient induces surface tension gradient drying the wafer without watermarks.

Besides the elimination of watermarks on hydrophilic, hydrophobic and combination films, surface tension 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, surface tension gradient dryers consume very little IPA. When integrated with cleaning and rinsing, this drying can provide a one-step process in various applications such as fabrication and cleaning of ICs, solar cells, fuel cells, MEMS, etc

Gradient Technology, Inc, has developed The Gradient Dryer™, a new batch vapor surface tension gradient dryer that can be integrated on the Gradient Technology, Inc, modular wet process stations. The Gradient Dryer ™ dryer can process substrates of various sizes for diverse applications. Gradient Technology, Inc, has a 25-year history in wafer cleaning and drying technology. Their automatic wet processing station platform is used for multiple cleaning processes, including SPM photoresist stripping, RCA critical cleaning, wafer reclaim, and wafer manufacturing cleaning. The tool is a fully automatic, cassette-to-cassette system with flexible architecture. The modules that can be integrated on the platform include temperature-controlled re-circulating and filtered chemical tanks, quick-dump-rinse tanks, and The Gradient Dryer™. Semi-automatic and manual platforms are also available and custom configurations can be ordered. Process and recipe development are strengths of the company and can be included with each application.

Product Overview

The Gradient Dryer™ system is an integrated clean-rinse-dry system. In addition to the "no watermarks" benefit of the dryer, it also delivers very low particle counts, produces no substrate breakage, has no moving parts, and leaves no feature damage. It is designed to handle substrates of various sizes ranging from 4" to 12" diameter and is excellent for high aspect ratio structures. A batch of substrates enters the rinse tank and emerges vertically as the water drains while an IPA fog is deposited on the surface as shown in Fig 2. The complete process sequence is shown in Table 1 with typical process times and ranges. The total process time for a batch of 25 substrates is under 12 minutes. The individual process steps are described in more detail next.

Fig 2. Cross section of The Gradient Dryer™

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.

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: Surface Tension Gradient IPA Drying

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.

Table 1. Drying process sequence with POR (Process of Record) parameters

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. Gradient Technology, Inc, has introduced The Gradient Dryer™, 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. The Gradient Dryer™ module is an option on the industry-proven Gradient Technology, Inc, wet process stations that serves multiple cleaning applications.

References

1. "Water Spots: The Scourge of Wafer Dryers", Laura Peters, Semiconductor International, Aug 1998.

2. "Surface Preparation Technology Requirements, Challenges, and Proposed Solutions for Future Semiconductor Manufacturing", Jagdish Prasad and M.Rao Yalamanchili, Semiconductor FabTech, 14th Edition, February 2005.

3. ITRS Roadmap, 2006 Update.

4. "Substrate Cleaning and Drying for Semiconductor Manufacturing", D. Martin Knotter and Jagdis Prasad, Semiconductor FabTech, 29th Edition, May 2006.

5. "Physical Principles of Marangoni Drying", J. Marra and J.A.M. Huethorst, Langmuir 1991, 7, 2748-2755.

6. IC Knowledge, Chapter 5.

7. "Quick Drying Enables Single-wafer Cleans", Maria Lester, Semiconductor International, October 2000.

APPENDIX

Chemical Properties

• Isopropyl alcohol
• IUPAC nomenclature: 2-propanol
• CAS No: 67-63-0
• Chemical formula: C3H8O
• Molecular weight:: 60.09 g/mole
• Vapor pressure: 33 mm at 20°C, 44 mm at 25°C
• Specific gravity: 0.79 g/ml
• Surface tension: ~30 mN/m (~72 mN/m for H2O)
• Critical temperatures
• Boiling point 82-106°C
• Flash point: 12-35°C (depending on reference)
• Melting point -89°C
• MSDS
• Health: 1; Flammability: 3; Reactivity: 0
• Low toxicity
• Skin irritant
• Highly flammable

Chemical Usage

• Typical IPA usage
• POR 5 ml/min
• Range 5 ml/min to 15 ml/min
• Tank with vapor pressure ~5 to 7 ml depending on cassettes
• POR 60 sec
• Range 60 sec to 180 sec
• Specification for disposal: San Jose
• Limits on toxic organics <10µg/L
• Limit on disposal of pure organics with flash points <60°C
• No spec found on allowable IPA in water disposal limits (request for information in progress)
• Typical N2 usage
• POR flow rate at 50 sccm