Advanced diffuser technology helps reduce vent-up times while maintaining wafer integrity on vacuum tools loadlock chambers.

Wafer throughput and particle counts are key metrics for any semiconductor manufacturer’s yield enhancement programs. Recent advancements in diffuser technology have helped manufacturers enhance these metrics while improving the attributes for most vacuum processes. These processes include dry etch, chemical vapor deposition (CVD), physical vapor deposition (PVD), rapid thermalprocessing (RTP), and Epitaxial deposition (Epi).

Execution of membrane diffuser technology dramatically decreases required vent time and has become a highly effective tool upgrade option. An early implementation of this technology was used on 200mm batch-style loadlocks that had an inherently large internal volume. The loadlock was prone to long vent cycles to prevent particle contamination.

As the industry transitioned to a 300mm wafer platform, factories increased their development of single-wafer loadlocks (SWLL) in an effort to boost tool throughput. Gas diffusers with ultra-fine filtration membranes solved these issues. Compared to the 200mm batch-style loadlocks, the SWLLs had extremely low internal volumes and were designed to cycle vacuum to atmosphere very quickly. With the low volumes inherent in the SWLL, the velocity of the incoming vent gas became critical, since any particles on the bottom of the loadlock chamber would easily sweep onto the wafer should theybe hit with a high velocity gas. Particles are typicallypresent in the loadlock due to mechanical wafer handlingdevices and environmental exposure. Gas diffusersallowed a large, uniform volumetric flowrateof gas into the loadlock chamber at low downstreamgas velocities.

While now standard on most 300mm loadlocks, the majority of 200mm tools in the field do not utilize membrane diffusers. Typically a screen, frit, and/or soft vent procedure is used to control the flow into the loadlock. However, these tools can now be retrofitted with membrane diffuser technology. The result is a large reduction in particle count while maintaining throughput levels at a low cost with minimal downtime.

STANDARD 200MM TOOL VENTING TECHNOLOGY
The method most widely established to control particle disturbance on 200mm semiconductor vacuum process tool platforms is a two-step venting process, which implements a “soft” vent followed by a standard vent. The soft vent is typically conducted using a second line equipped with a flow restrictor to minimize the flowrate and bleed gas into the chamber until a certain pressure is reached inside the loadlock. This helps reduce the disturbance of particles. Once a set pressure is reachedin the loadlock, a second valveis actuated to complete theventing process and bring thepressure of the loadlock toatmosphere. Depending on thevolume of the loadlock chamber,the soft vent stage alonecan take anywhere from a fewseconds to several minutes tocomplete.

This method is often acceptable. However, there are cases where 200mm tool owners are required to increase wafer throughput due to capacity constraints or to enhance overall equipment effectiveness. While many of the critical variables that influence wafer throughput are fixed (such as the process times, robot speed, and loadlock pump down speed), the time to vent up the loadlock may become the rate-limiting step to wafer throughput. This is especially true with shorter process times or if dual-batch loadlocks are not working in parallel. One approach is to provide a rapid pressure increase by boosting the flowrate of gas. However, with a standard screen or open porous material, the gas velocity at the chamber entrance will be high and non-uniform, resulting in the disturbance of unwanted particles that have settled in the chamber.

The situation also occurs where vent-up time is not a throughput limiting step. In this case, the tool owner is faced with more stringent particle requirements or observes a spike in particles on the wafers in the loadlock. The focus then becomes yield enhancement and the goal is to reduce the particle adders on the wafers. Common approaches to theparticle problem on installedsystem loadlocks have includedcomplete loader rebuilds,performing additional series ofwet cleans, upstream filterreplacements, and screen diffuserreplacements, whichoften do not yield the desiredgoal.