By Hank Hogan
FRANKLIN, Mass.— When it comes to contamination control, even a light touch can cause problems.
For example, as semiconductor feature sizes shrink, so too can the wavelength of light used. Because shorter wavelengths have more energy, it increases the need to measure and control airborne molecular contamination (AMC). Today's state-of-the-art 193-nanometer (nm) light sources and the accompanying process nodes require parts-per-billion AMC control.
That's expected to shrink as much as a thousand-fold within the next few years, according to a prediction from an industry workshop recently conducted by Boston-suburbia-based Extraction Systems Inc. The reason for the tightened specification is that 193-nm technology might be around much longer than originally thought.
“People want to push 193 down a couple more nodes, and if that's the case, you're going to be very, very limited in the resolution,” explains Kim Dean, project manager for lithography at Internationatal Sematech, a research consortium based in Austin, Texas. “So, if you have any other detractors from critical dimension control, whether from contamination or variability in the track, it just won't make it.”
Just when these restrictive AMC levels will be needed is anyone's guess. Semiconductor process nodes last about two years. The smallest attainable with 193-nm light is thought to be the 45-nm node, which is half the feature size of the most advanced chip process available today. It's several nodes below where 193-nm was originally expected to play out.
As to why 193-nm might be the light source of choice comes down to optics and economics. The industry consensus is that using 193-nm technology will be at least as cost-effective as the alternatives. What's more, the downward limit of 193-nm processing can be extended through immersion lithography. This approach substitutes water or another transparent liquid for the air between the wafer and the final optics in a photolithography tool.
That changes the optical characteristics and lowers the minimum feature size that can be produced. It's a trick microscopists have used for years. Initial immersion lithography results are promising, and a decision about 193-nm immersion should be made early next year.
The use of 193-nm light for such small features means tighter AMC controls are needed and more sensitive real-time contaminant monitoring is required. The company is working on both real-time monitors that identify when a filter needs to be changed, as well as precise tools for characterization and qualification of equipment and processes. The company has recently introduced a monitor capable of measuring organic contaminants at the parts-per-trillion level.
The reasons are simple.
“That's what the optics require,” says John Higley, Extraction's vice president of sales and marketing. “We need to understand at extremely low levels if there is contamination going on and to be able to identify and quantify that.”