Wet cleaning

The wet cleaning market has been evolving to keep pace with the advances in integrated circuit designs.

This includes the enhancement of batch immersion systems and emergence of single wafer cleaning solutions. Although each process type has certain advantages and may be preferred for specific applications, the synergy that exists between them means that it may be advantageous for a semiconductor capital equipment company to possess both technologies. There are often more than 100 cleaning steps involved in IC device manufacturing, and traditionally these cleans have been performed using immersion or batch spray systems. In immersion processing large numbers of wafers are immersed in tanks filled with chemical mixtures for specific periods of time at set temperatures. This method has proven very effective over the past 40 years. Although the techniques used in batch immersion processing have been refined with the use of dilute chemistries, in situ processing, megasonics and surface tension gradient dryers, they are still very much related to the RCA clean developed in the 1960s.

Process of record Manufacturing of semiconductors is often a highly complex process consisting of many steps designed to produce consistent, repeatable results. Manufacturers establish a Ôprocess of recordÕ that details the requisite steps to achieve these results at plants all over the world. Once a process of record has been established, manufacturers are often unwilling to deviate from it. Most front-end-of-line (transistor building) processing is currently done with batch technology, either immersion or spray processing. In most cases, there has been no compelling reason to change from the proven process of record. Critical cleans are currently dominated by batch immersion systems that combine traditional cleaning chemistries with megasonics. Megasonic energy lifts the particles away from the wafer's surface. Single wafer processes have attracted some of the market share for this sector. As devices continue to shrink, it is likely that use of the single wafer process will increase because of the heightened process control that it offers. As devices become more and more sophisticated, technological hurdles arise that allow new technologies to be implemented. Single wafer is such a technology. The platform will start making inroads into FEOL because of the new challenges generated by leading-edge device manufacturing.

Choice of methods The hot acid application is one area of the manufacturing sequence where batch immersion is now and will probably remain the method of choice. Heated chemical mixtures, such as a sulphuric and peroxide mix (SPM) and phosphoric acid are used for a variety of steps in the typical device manufacturing. They are used to dissolve away photoresist and nitride compounds used as masks and barriers for the device structures. Immersion works very well for these purposes and it is likely that photoresist stripping and nitride etching will be done in this way for the foreseeable future. The high temperatures and extended cycle time needed for these steps do not lend themselves to single wafer platforms. Major improvements to current process capabilities Ð perhaps the development of new chemical formulations Ð will be necessary for single wafer processing to be useful for these purposes. This is particularly true of the nitride etch process, which has an extremely long cycle time. Batch immersion processes have a significant throughput advantage over single wafer processes. For most processes, two or more times as many wafers per hour can be handled by batch immersion. Consequently, fewer tool sets are needed and capital expense is lower. Batch immersion systems also typically provide for more uptime than their more sophisticated single wafer competitors. More uptime equates to more production and lower cost of ownership. Today's advanced immersion tool sets, such as Akrion's GAMA Series systems, have greatly increased in capability compared with the manual dip-and-dunk wet stations used in previous decades. GAMA Series tools provide very high throughput and are proven for 0.09µm devices. Advances such as in situ tanks (combined process and rinse), chemical dilution, new chemical formulations (such as DIO3 Ð ozonated deionised water) and shortened cycle times have enhanced today's batch systems. Future improvements are likely that will allow GAMA and other batch systems to be used for even smaller features. Recirculation of the chemical baths used in batch immersion processes, coupled with low dilutions, also gives an advantage to immersion processes when it comes to chemical costs. Chemicals can be used for several lots, rather than discarded with each lot, as is the case with single wafer processing. Processing changes are generally not made unless the current methods reach technology roadblocks. For some applications, single wafer cleaners offer inherent technical benefits that cannot be obtained in batch cleaners. Examples of single wafer technical capability and control benefits include: • side selective chemical etching; • very quick transition times between chemical and rinse steps; and • ability to improve megasonic uniformity because of the elimination of interference from tanks, carriers and other wafers. Productivity is also a factor regarding the single wafer versus batch tool decision. In general, batch tools have an inherent advantage regarding throughput, but single wafer tools have an advantage regarding cycle time. Depending on the type of fab and product mix, the throughput or cycle time can be a more important factor. Also, the single wafer tools are relatively competitive for throughput when a single step cleaning process is used. For multiple step cleaning processes, the disadvantage for single wafer grows. There are three segments where single wafer cleaning tools have established considerable market share: post deposition and other particle removal cleans; BEOL post etch/ash cleans (Al and Cu); and backside trace Cu contamination removal. Scrubbers have long been a common method of removing post deposition and other types of particles. They remove particles in a simple, cost-effective manner. Single wafer spray tools have gained considerable market share for BEOL Cu post etch/ash cleans and are beginning to be used for leading edge BEOL Al cleans. Akrion's Goldfinger Mach2 and Mach2HP provide benefits for the post deposition and BEOL post etch/ash applications due to the GF megasonics and Sahara dry technologies. The GF megasonics provide improved sound control versus standard methods, allowing use in the presence of sensitive structures. For example, current methods do not use megasonics in the presence of Al lines. The result is that some defects remain after the standard chemical cleans. Using Goldfinger megasonics during these cleans eliminates these defects and improves yield. The Sahara dry is based on the rotational Marangoni (Rotagoni) concept developed at IMEC. This product provides the same watermark-free performance as the batch Marangoni tools, but in a single wafer platform and in much shorter time periods. For BEOL post etch/ash applications where vias and trenches can result in watermarks, this technology is enabling. There are obvious benefits for manufacturers to outfit their fabs with a capital equipment company capable of providing both batch and single wafer tool sets. The costs for service are greatly reduced, as the manufacturer will be dealing with one company with one point of contact. The supplier will be able to provide better support at a lower cost by having a service organisation that is trained on both types of equipment. Additionally, the costs of maintaining spare parts stocks will be reduced.

Supplier impartiality There will also be advantages in using a supplier who can understand the customerÕs entire cleaning processes. Process development and process support will be enhanced if one company can offer both types of product platforms. The more the supplier knows about the customer's process, the more helpful he can be when it comes to solving problems. Partnering of this type should result in the ability to immediately see the problem areas and identify the solutions required. A major advantage for the IC manufacturer is the impartiality of a supplier who can provide both batch and single wafer solutions. The supplier is more likely to provide the right system for each individual application, rather than, for example, attempting to do a process with a batch immersion tool set, when a single wafer platform provides a better solution. There will also be a cost saving associated with the purchasing volumes resulting from buying more tools from a single supplier.

Growth in single wafer The evidence suggests that batch immersion processing will continue to be a major method for many manufacturing steps. But, as devices continue to shrink and layers continue to expand, it is obvious that single wafer technology will play a larger role. More cleaning applications will migrate to the single wafer platform. According to Dataquest figures, batch immersion market share will be reduced from 61% in 2004 to approximately 58% in 2008, while the market for single wafer processors will increase from 20% to about 23%. Obstacles will have to be overcome before single wafer platforms can supplant batch technology for all processes. Not only in today's market, but also in the foreseeable future, manufacturers will benefit from working with a company that can understand and offer both types of technology.