By Ruth DeJule

WaferNews Technical Editor

One of the biggest trends in wafer cleaning today is single wafer wet cleans at the critical pre-gate level. Strictly speaking, PVA brushes (scrubbers) used in post-CMP cleans can be considered a single wafer cleaning process, and some fabs use single wafer etching for backside thinning and layer removal.

But for critical cleans, single wafer cleaning is new. Megasonics, which speeds up the cleaning process, has been added for cleaning particles, metal contaminants, and residue from surfaces containing fine geometries.

Verteq recently released the Goldfinger Mach2 single wafer cleaner, and more models are expected soon. At 180nm and smaller, traditional cleaning technologies can damage device structures, forcing a reduction in applied energy and/or time. However, smaller particles require higher energies to break free from the wafer surface. One solution is the application of megasonic technology.

Though there are a number of theories as to how megasonics works, it is clear that, if done right, megasonics produces good results. According to Mark Beck, CEO of ProSys, Campbell, CA., megasonics is a different way of boiling water. Instead of heat, megasonics changes localized pressure in the fluid a million times a second, creating a void (cavitation).

During cycling, gases are pushed in and out of the void until it collapses, releasing the energy required to free small surface particles. Higher frequencies increase cavitation with one megahertz producing bubbles one micron in diameter. Therefore, higher frequencies should be more effective for cleaning devices with higher aspect ratios - but not necessarily. A 1.5 or 2MHz frequency will produce smaller bubbles, but their potential energy and corresponding ability to remove a particle will also decrease, noted Beck.

Timing is the biggest challenge in single wafer cleaning. It is clear that batch cleaning times of 20 to 50 minutes using standard RCA cleaning must be reduced to one to two minutes, according to Paul Mertens, leader of the ultra clean processing group at IMEC in Belgium. In this short timeframe of about one minute, cleaning, rinsing, and drying have to be completed.

One approach is to reduce the number of chemistries, and therefore steps, to one or two. In the past IMEC has demonstrated success with a two-step process using ozonated water with HF treatment. More recently it has demonstrated success with mixtures containing ammonium hydroxide and peroxide with chelating agents for single chemistries.

To reduce drying times, spin dry comes to mind as the fastest, simplest approach. However, it has been insufficient for critical levels, leaving residue on the wafer and watermarks on exposed silicon surfaces. Most critical are surfaces containing hydrophobic regions such as bare silicon or some of the low-k layers. IMEC has concentrated efforts on Lineagoni (linear motion) and Rotagoni (rotational motion) drying technologies.

Each applies the Marangoni effect, a surface tension gradient effect that creates a force on parts of the liquid, resulting in very effective local scale liquid removal, while minimizing evaporation, noted Mertens. The results from tests on 200mm wafers have been impressive - a 30 to 40X improvement over batch drying times, from 7-10 minutes to 10 to 20 seconds.

Current dry cleaning methods are often used for surface preparation, not effective in removing particles and metal contaminants. But with the introduction of porous low-k materials or air gaps, this may change since liquids can alter material characteristics of porous films, according to Mertens.

But at this point, single wafer wet cleaning is taking off with the introduction of Verteq's Goldfinger Mach2, a cleaning system that provides both single-wafer megasonic cleaning and surface tension gradient drying in the same module. Other single wafer clean systems are due out in the next couple of months.