Wet Substrate Surface Cleaning

Wet chemical cleaning and conditioning of wafer surfaces is a critical process step in most, if not all, semiconductor device fabrication schemes. The acidic cleans listed above have a long history of use in semiconductor processing; forecasts call for their continued use into the foreseeable future. The chemical composition, actions and process applications of these wet cleaning methods are covered in Wafer Surface Cleaning.

This section will briefly describe some of the equipment used for wet cleaning silicon substrates in modern semiconductor fabrication plants. Such equipment can range from very simple "wet bench" cleaning stations in older technologies to sophisticated automated single wafer cleaning tools employed in advanced technologies.

Reinhardt and Kern state that, "in the past, wafer cleaning and surface preparation were considered more of an art than a science". This was especially true in the era when "wet benches" for simple batch immersion and rinsing dominated substrate cleaning. Scientific studies over the past three decades have revealed the importance of chemical concentration control over all points on a substrate surface. This is needed in order to guarantee the required material properties of the layers and interfaces in the often severe topographies of advanced devices. Batch processing can limit the degree to which such control can be achieved in high aspect ratio geometries, and single wafer approaches have been developed to solve such issues. Indeed, back end of line (BEOL) post-patterning cleaning, especially for cleaning Cu interconnect lines and high-k interlayer dielectrics, has had to move entirely away from batch wet bench processing to more controlled single wafer wet cleaning approaches. Furthermore, batch methods, while very convenient for wafer diameters up to 150 mm and which can be made workable for 200 mm substrates, become unwieldy when applied to substrates with diameters beyond 200 mm. This too has contributed to a shift to single wafer wet cleaning technologies. Finally, batch methods are not as easily automated as are single wafer approaches. However, conventional wet benches will often be found in semiconductor fabs, either in less critical front end of line (FEOL) application areas or in R&D and process development applications.

A modern wet bench cleaning station is shown in Figure 1. Modern benches are configured with a FOUP (front opening universal pod) interface and wafer handling robotics to ensure low particulate substrate transfer into and out of the wet bench. Within the wet bench are liquid tanks in which a cassette containing substrates can be immersed in the desired cleaning solution. Cleaning solutions are typically pre- or in situ mixed within the wet bench station using ultra-high purity semiconductor grade reagents and ultrapure water (UPW). Once the substrates have been immersed in the cleaning solution for a prescribed time, the chemical reaction is rapidly stopped and any chemical residues removed by a combination of high volume UPW spray rinsing and rapid overflow dump rinsing. In this latter step, the wafers are subjected to repeated cycles of a very fast hot or cold UPW immersion and draining with spray rinsing using ultrapure water. Once a cassette of substrates has been rinsed, it is quickly dried, typically using some kind of high speed spin drying. Megasonic agitation and/or isopropyl alcohol (IPA) may be employed during the rinse/dry cycle to enhance the rinsing and drying abilities of the system. Depending on the tool design, these steps may require multiple tanks and cassette spinners or all functions may be integrated within a single tank.

As noted above, advanced device designs and larger substrates cannot be cleaned using batch wet bench approaches. Over the past decade, single wafer cleaning systems have been developed that have leveraged improvements in economical spray as well as new and improved cleaning chemistries. Highly automated, single wafer treatment systems have been designed that employ multiple cleaning stations, each cleaning one wafer at a time, to maintain production-scale throughputs. These new single wafer systems incorporate significantly more precise control over localized surface chemistries than can be achieved using batch processing. Additionally, particle contamination is inherently more controllable in single wafer handling and processing systems; this is a critical aspect for low defect processing in modern equipment. Figure 2 shows a schematic of a typical automated single wafer cleaning arrangement in a commercial single wafer cleaning tool.

MKS supplies equipment for producing certain chemicals used in cleaning solutions, specifically our line of ozonated water delivery systems. DI water/ozone solutions (DIO₃) have been found to provide a clean, safe and highly effective replacement for Piranha and RCA SC-1 and SC-2 cleans in many aspects of surface cleaning.

Organics Removal and Photoresist Strip

As a strong oxidizer, ozone, O₃, rapidly reacts with most organic chemical compounds. O₃ reacts directly with hydrocarbons and also generates oxygen radicals that react even more vigorously with organics. Thus DIO3 can be used as an effective clean that removes ambient organic molecules adsorbed on the wafer surface. More importantly, when coupled with megasonic agitation, DIO₃ is very effective in removing photoresist residues from a wafer surface.

SCROD Cleaning

While pure DIO₃ is not suitable for cleaning metals and particles from a wafer surface, cleaning protocols that combine the use of O3 with HF and/or hydrochloric acid are highly effective for this purpose. Single-wafer spin cleaning with repetitive, alternating use of DIO₃ and dilute HF (DHF) is known as the SCROD cleaning method. SCROD cleaning oxidizes the substrate (through the action of the O₃) creating a self-limited oxide layer of about 1 nm thickness which the HF subsequently dissolves. The sequential oxidation and dissolution of the oxide layer efficiently removes both particles and adsorbed metals from the substrate surface. Originally developed by workers at Sony, SCROD cleaning has been shown to remove 87% of aluminum oxide particles, 97% of silicon nitride particles, and 99.5% of polystyrene latex particles from a substrate surface. This is nearly an order of magnitude more efficient particle removal than can be achieved using a standard SC-1 clean. Similarly, studies have shown that metal contamination levels on a substrate surface of less than 1 x 10⁹ can be achieved using SCROD approaches.

Advanced Reticle Cleaning

Traditional methods (RCA) for reticle cleaning degrade their optical properties and reticles can only be cleaned between 2 and 8 times before unacceptable degradation occurs due to surface roughening. Since ozone-based chemistries produce very little surface roughening, they are becoming preferred in reticle cleaning applications.

LIQUOZON^® Dissolved Ozone Delivery Systems

MKS offers the LIQUOZON^® product line containing several different configurations for the generation and delivery of DIO₃. The LIQUOZON VariO3 Ozonated Water Delivery System system is designed for use with multi-chamber single wafer cleaning tools having cleaning applications requiring up to 140 L/min DIO₃ flow and ozone concentrations of 25 - 115 ppm. The LIQUOZON VariO3 Ozonated Water Delivery System system has an integrated analyzer for dissolved ozone that can be used for accurate closed-loop control of the DIO₃ concentration. LIQUOZON VariO3 Ozonated Water Delivery System is specifically designed for the maintenance of stable DIO₃ concentrations, even with varying DIO3 demands. It has an integrated booster pump for low pressure UPW supplies.