Wafer Cleaning Needs Damage Control

Presenters at Semiconductor International's latest technology webcast —Renee Mo of IBM (White Plains, N.Y.), Paul Mertens of IMEC (Leuven, Belgium), and Ahmed Busnaina of Northeastern University (Boston) — spelled out the overriding tool and chemistry challenges. Foremost, the trade-off between complete residue removal and damage has become difficult to manage.

Mo, an FEOL process development engineer at IBM's systems and technology group, discussed renewed interest in solvent cleans for metal gates, where aqueous-based cleaners face corrosion issues.

"Non-uniformity was not a big issue in the past. We typically cranked up the power until we got full removal. Now, however, we have fragile lines that easily damage," said Mertens, program manager for contamination control, cleaning and surface preparation at IMEC. Damage depends on the structure, including unintended removal of high-aspect-ratio features such as gates, line rip out from narrow copper lines and substrate recess. He said one way to reduce megasonic damage is by making sure the energy in cavitation is less than the bonding strength of the finest features on the device. "The major issue is non-uniformity, where you can have damage on one part of the wafer and zero particle removal efficiency on another part of the wafer," as shown in Figure 1 .

1. The greatest disadvantage associated with megasonic cleaning is the non-uniformity of particle removal across the wafer. (Source: IMEC)

One option is high-speed aerosol cleaning, which has demonstrated better uniformity and increased particle removal efficiency in combination with reduced damage. "Better uniformity is probably the biggest benefit," Mertens said.

Busnaina, director of the NSF Center for Nano and Microcontamination Control at Northeastern University, showed modeled and experimental results that provided an explanation for why nanoparticles are more difficult to remove than submicron particles (Fig. 2 ). "Very small particles tend to be caught near the surface at the acoustic boundary layer, and because of the oscillating nature of the megasonics, they can redeposit many times over," he said.

2. Modeling by computational fluid dynamics shows how nanoparticles are not easily swept from the low-flow boundary layer, causing them to redeposit on the wafer. In contrast, larger particles are swept up in higher flows above the boundary layer, so are removed more quickly. (Source: Northeastern University)

"Our studies showed that the same phenomena occurs in deep trenches," Busnaina said. In 5:1-aspect-ratio trenches, it took up to eight minutes of megasonic cleaning to clean the trench. Busnaina said the particles get caught in vortices that are oscillating and tend to fall back into the trench.

Mertens showed that higher particle removal efficiency is typically associated with higher damage for a variety of cleaning tools.

With different low-k dielectric films, Busnaina showed that particle removal is easier in an alkaline chemistry rather than acidic or neutral. Interestingly, alumina particle removal efficiency was always poorest at neutral pH (DI water).

Mertens said that cryogenic aerosol cleaning looks very promising as a method to increase particle removal efficiency. "One concern is that the cleaning occurs at a certain angle, and you will not get symmetric cleaning on two sides of a trench," he said.

Mo voiced her preference for cryogenic cleans over supercritical or laser shock cleaning. "Supercritical cleaning is quite challenging to implement, as is laser cleaning," she said.