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Trouble Shooting Particle and Fiber Excursions in Advanced 300MM Wafer Fabs

by | Jul 28, 2015

With the advent of sub 20nm device geometries, wafer fabs face daunting challenges scaling these new nodes to yields that earn acceptable ROI. Ramping production at acceptable yields is imperative due to the much higher equipment costs, especially lithography and metrology that are required for sub 20nm nodes. Among the many impediments to yield are particle defects including scratch defects.

While fabs have invested heavily in metrology in order to stay ahead of such defects, particle excursions are a significant source of yield loss. Fiber related scratch defects are especially difficult to manage as metrology tools often do not catch the defect immediately, causing huge yield loss events to occur more frequently. Furthermore, the time to uncover the root cause failure mechanism is a significant strain on engineering resources. Scratch defects are a perfect example of a problem that resists root cause understanding, even when modern metrology tools are available. The reason is while scratch defects are caused by particle, or more exactly fibers, these fibers often do not remain on test wafers giving the engineer a false sense of confidence.

Ironically, scratch defects are most frequently associated with fibers that are released by clean room wipers which suffer snags and abrasion damage during wet cleans of transfer robots and other wafer contact surfaces. In the wrong place these fibers drag across wafers creating scratch defects in the pattern. And, as mentioned above, these defects are very hard to track down as the fibers or particles do not transfer to the wafer lending identification by test wafers.

* The pictures below are examples of how lint fibers cause scratch defects:

End+Effecotr scratch+map

Lint fiber hanging from end effector / Scratch defect shown on wafer map, caused by hanging lint fiber dragging across wafer on every wafer transfer

The following link is a video showing the effects of electrostatic charge on hanging lint fibers, making it extremely difficult to locate the culprit when troubleshooting fiber excursions

Scratch defects are a very specific fiber related defect; however, clean room polyester wipers, due to very low resistance to abrasion damage, release high quantities of particles and fibers on wafer contact surfaces and especially in wafer transfer robots as pictured below:

To aid wafer fab engineers in understanding the root cause of scratch defects, Foamtec has developed a fiber inspection kit. This kit enables equipment engineers to quickly inspect vacuum chambers, wafer transfer robots, and other wafer contact surfaces to ensure they are clean enough to process production wafers. By using Foamtec’s fiber inspection kit, equipment engineers will be able to reduce particle and fiber contamination that lead to reduced wafer fab yields.

* The pictures below show a step-by-step process how to use the fiber inspection kit to troubleshoot a suspected tool that had resulted in scratch defect related yield loss:

Foamtec Fiber Inspection Kit

Step 1:  Inspect end effector using LED magnifier

Step 2:  Inspect end effector using digital microscope to take picture and document findings

Step 3:  Picture of stuck fibers under end effector   

Step 4:  Stuck fibers being pulled from under end effector showing risk to loss of yield