Wafer Storage Contaminants See the Light

One ongoing problem of wafer storage and handling is the accumulation of organic contaminants on silicon wafers during storage and shipping. Assuming storage is accomplished in a particle-free environment, contaminants originating from storage are primarily organic contaminants and moisture.

Currently, the most common solution to this problem is a surface cleaning procedure using a sequence of wet cleaning steps. Other options include a hot plate anneal process in ambient air and exposure to UV radiation in ambient air. New to this arena is the use of white-light illumination in ambient air to control the organic contamination of silicon surfaces.

An experiment conducted by Pennsylvania State University (University Park, Pa.), LETI (Grenoble, France) and QC Solutions Inc. (North Billerica, Mass.) used a rapid optical surface treatment (ROST) using white-light illumination from a halogen lamp in ambient air to control the organic contaminants accumulated during wafer storage and handling. The results indicate that ROST effectiveness depends on the type of organic contaminants accumulated on the surface. It also depends on the storage time and ambient in which the wafer is stored.

The ROST system used includes a 600 W halogen lamp. The wafer surface was exposed in an enclosed chamber in ambient air. The closed-loop wafer temperature control system allowed presetting the temperature to 300°C. Time of exposure was varied from 10 to 300 sec.

ROST was found to be as effective as other methods of organic contamination removal, including wet treatments, when the silicon surface needed to be "refreshed" following shipment or short-term storage. However, it was not as effective as standard wet cleans in the case of wafers stored for a prolonged period of time and frequently exposed to ambient air.

This treatment applied prior to gate oxidation had a beneficial effect on the reliability of thin gate oxides. Complete results are presented in the January issue of the Journal of The Electrochemical Society.

Experimental results verified that a 60 sec ROST treatment was effective in removing light hydrocarbons from silicon surfaces that were adsorbed during wafer shipping and storage. The ROST treatment left the surface in its prior-to-shipment condition, but free from organic contamination accumulated in the shipping box. Therefore, ROST effectively "refreshed" the silicon surface.

In the next stage of this experiment, the researchers compared ROST with three other methods. The first method consisted of a hot plate anneal in the ambient air at 300°C for 5 min. The second involved a 5 min exposure to UV radiation in ambient air. The third consisted of a conventional APM wet surface treatment. All four methods caused a similar decrease of the contact angle on the wafers removed from the recently opened shipping box. This indicates that all four of the methods have similar organic removal efficiency.

However, the ease of implementation of each is not the same. For example, the UV treatment not only requires a 5 min long treatment, but also must be performed in a vented enclosure because of the generation of ozone. The hot plate treatment also takes a relatively long time, and a physical contact of the back surface of the wafer with a hot plate may result in wafer contamination. Finally, the APM clean, although the most effective among those methods, requires a prolonged immersion in the hot solution followed by 5 min water rinse and wafer dry.

In this context, ROST offered the least time consumption and was the easiest to implement for light organic removal. However, it was observed that the ROST cleaning action is not effective on the wafers stored for a prolonged period of time in frequently opened boxes — this was also true for the hot plate treatment as well as UV/air. But the wet APM clean displayed the same effectiveness in removing heavier organics from the aged wafer surfaces compared with the light hydrocarbon surfaces.

The final part of the experiment applied ROST as a surface treatment prior to thermal growth of the gate oxide. It was determined that cleaning action during lamp exposure was predominantly thermally driven oxidation. In fact, the exposure of wafers "out of the box" to the ROST process immediately prior to oxidation had a beneficial effect of the electrical integrity of subsequently grown oxide.

Since ROST was found to effectively "refresh" silicon surfaces following wafer shipment as well as IPA — drying along with the finding that, when applied prior to gate oxidation, it has a positive effect on the electrical integrity of gate oxide — it has potential in incoming wafer processing applications.

Additional new results of joint Penn State-LETI experiments on lamp cleaning will be presented in the forthcoming proceedings of the symposium Ultra Clean Processing of Silicon Surfaces (UCPSS 2002).

For additional information on clean processing, go to www.semiconductor.net/clean