A new cleaning platform for the IC and wafer manufacturing sections of the semiconductor industry, has been designed for advanced-front-end-of-line (AFeOL), FeOL, pre-gate, pre-epi, etc., cleans. The platform, aKROS, is best suited for ASIC manufacturing. (See Figure 1)

Chemistries required for these FeOL cleans include HF, SC1 or RCA-B (NH4OH/H2O2/H2O), SC2 or RCA-A (HC1/H2O2/H2O) and DI w/O3 or ozone. These chemistries must be mixed and blended at very precise concentrations to meet manufacturers’ processing needs. For existing IC products, these recipes and processes are well established. As an IC company makes changes in the product, such as a device shrink or a change of process to a system-on-chip (SOC) device, the wet chemical processing requirements will most likely need to be changed. There are a wide variety of reasons why multiple chemical recipes may be needed.

In addition, many of these next-generation devices will be built using new or alternative technologies, including:

* new or different high k gate materials

* optimized cleaning processes using a more dilute or concentrated chemistry

* SiGe strained silicon processes

* “airless” processing of the devices, which require additional wet cleaning tool sets

Established product devices can continue to be manufactured on existing wet chemical tools, but newer devices with advanced designs and materials will need a new wet chemical cleaning tool set. This requirement can be costly, as IC manufacturers will need to continue adding new wet chemical processing tools to improve cleaning processes.

Given these challenges, ASIC manufacturers need an advanced automated tool set that will allow them to use a single wet chemical cleaning platform to build older products, newer products, and products that are in development. Accommodating all product-processing needs in a single platform reduces capital expenditures, fab space requirements, and cost of ownership.

Working with ASIC process engineers, Akrion has designed a single wet chemical cleaning platform capable of handling this need for process flexibility. The new design techniques allow use of different chemistry mixes, concentrations and techniques on a single platform for each different lot of material or devices. The following is a discussion of chemical handling characteristics of an automated wet station capable of supporting the advanced cleaning requirements of present and future ASIC devices.

Chemical Handling Needs

Single-Pass Chemistry

ASIC manufacturers require single-pass (SP) chemistries to develop certain products. The need usually arises because one or more products have some sort of “doped” oxide or film that could potentially contaminate any re-circulated style of bath. Another major reason to use SP chemistry may be that a fresh batch of chemistry is needed to optimize the process demands of that specific device type—for example, the device may require a unique chemical sequence or process.

Filtration Re-circulation System (FRS)

For manufacturers who use one dilution or concentration of chemistries throughout their product line, FRS chemical handling can save and reuse the chemistries that have been selected. Once the wafers have been removed, the chemical manager will automatically adjust the chemistry in the bath to bring it back to its original concentration. This chemical spiking results in a significant saving in chemical usage.

Unique Chemical Management Features of Advanced Automated Wet Station

Many unique features are required for advanced automated wet processing:

*  “airless” processing

* chemical concentration modification

* combination processing

* DI rinsing optimization

Airless processing means exactly what it says; manufacturers process materials without ever exposing those wafers to air. This is a unique feature developed for the many ASIC manufacturers who need to run a device type without re-growth of any native oxide, chemical oxide, etc. As an example, the manufacturer can run a wafer lot through a process of “DIO3, Rinse, DHF, Rinse, SC1, Rinse, SC2, Rinse and Dry” without ever exposing the wafers to air.

Chemical Concentration Modification (CCM)

For complete flexibility of processing, chemical concentration modification is required. This system allows device manufacturers to vary the concentration of the chemistry in any or all of the cleaning chemistries. For instance, while one lot of material is being run through a determined concentration of chemistry, CCM can, while the chemistry is in the bath, either “raise” or “lower” the concentration of any or all of the chemical constituents that make up that specific chemical bath. When the wafers are nearly finished, the bath can be taken back to the original chemical concentration. Recipes can be changed on the fly.

This option also allows a manufacturer to take advantage of chemical etching for particle removal with very precise silicon dioxide etching. As an example, when wafers are placed in an SC1 bath diluted 200:2:1, the chemical concentrations can be modified to 50:2:1 or conversely to 300:2:1 after 30 seconds of processing. When the process is nearing completion, the bath can be returned to its original concentration (if desired). The same can be done for DIO3, DHF or SC2 chemistries.

A component of this CCM system is chemical feedback. If an automated immersion cleaning system incorrectly mixes a chemical bath, it can result in damaged product wafers, wasted chemicals, and extra processing steps that can lengthen cycle times. For example, a product lot may have a chemical recipe that calls for a 50:3:1 ratio of SC1 chemistry (HOH, H2O2 and NH4OH). This chemical blend will have rates of oxide etch and oxide re-growth specific to the device type being processed. If the chemical bath is incorrectly mixed at a 50:1:3 ratio, then the etch rate will be three times higher than specified and the oxide re-growth rate will be one-third of the desired target. Many device types would suffer yield losses of 10 percent or more if this were to happen.

The chemical concentration feedback system of an automated immersion cleaning tool can be used to significantly reduce error risks. The system’s unique combination of software and hardware allows the wafer fab engineer to set up critical chemical sequences and concentrations, and then to verify that chemistries are within specifications before the product wafers enter the bath. The chemical concentration feedback system also offers users the flexibility to mix and modify a variety of process recipes without any hardware modifications. (See Figures 2 and 3)

Combination Processing

Combination processing will allow manufacturers to run two or three chemistries in sequence without a mix-out or blend-out of the chemical bath. This option was designed for use with wafer drying techniques that need to finish with either an HF or an HCl in the final rinse. This can also be combined with the wafer-drying step itself.

Optimized DI Water Rinsing

To enhance yield and save DI water, the DI water-rinsing step needs to be optimized. An advanced wet station speeds up the completion of the rinse of a wafer lot, which saves on the product’s cost of ownership.

Conclusion

With the transition to larger wafer sizes and smaller circuit geometries, chipmakers have more invested in each product wafer than ever before. With so much at stake, every step in the fabrication process is crucial, especially wafer cleaning, the most frequently repeated step in the chip making process. To ensure that wafers are cleaned properly, semiconductor manufacturers should choose an automated immersion cleaning system that allows them to manage both today’s chemical processes as well as those future processes being developed in R&D. A modern automated wet station needs to provide consistent, yet flexible chemical management for the existing product devices as well as handling those recipes being developed for future generations of device types.