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Scratch Defects, Troubleshooting & Root Cause Identification in Wafer Fabs

by | Aug 7, 2020

Wafer scratch defects result in costly rejected die in the semiconductor manufacturing industry across all technology nodes and wafer sizes. Given that scratch defects generate significant yield loss, it is imperative to identify the source of contamination. Scratch Defects typically present as distinct elongated linear patterns of scattered particles or device feature damages caused by contact of a foreign contaminant with the wafer surface. There can be several sources for scratch defects within the wafer manufacturing process. Still, the most common cause is foreign contaminants’ presence on wafer handling robots, wafer buffer cassette components, and FOUPs. As wafer handling takes place from the load ports to the process modules, production wafers may incur contact with the foreign particles at these locations and presents a substantial risk of impacting several production wafers in a short period of time.

Identifying scratch defect sources within manufacturing equipment can be cumbersome and often requires advanced inspection tools and techniques to troubleshoot and eliminate the root cause effectively.  Foamtec International has collaborated with customers to create an inspection tool kit and specialized inspection techniques that significantly reduce time and labor involved in identifying contaminants that cause scratch defects.

The Foamtec International HT4500-FIBINSP tool kit features a Digital Micro Camera (variable zoom 20x-200x with USB connection for video display), a USB extension cable, an adapter kit for machine access ease and flexible connectivity, an LED illuminated magnifying glass, and PolyCHECK inspection wipers and swabs. The tool kit aides in contaminant sample collection for visual and advanced analysis capability. See images below showing the inspection tools Foamtec has developed for quick and effective equipment inspections.

ht4500-fiber-inspection-kit
HT4500-FIBINSP Kit Components

polycheck-wipers-and-swabsHT5291K – PolyCHECK Wiper | HT1501FZW – PolyCHECK Swab

Utilization of the inspection tools and techniques Foamtec has developed offers reduced effort and time required to identify and eliminate scratch defect sources. The key to troubleshooting a wafer scratch defect is understanding wafer movements through the machine. It is recommended to use film-coated investigation wafers to help identify the repeatedly impacted region of the wafer. Once a repeated defect signature is obtained, the investigation should be expanded to cover areas with correlated wafer touchpoints and movements. To adequately inspect the machine’s suspected regions, Foamtec’s inspection tools and techniques can help identify root cause contamination sources quickly.  Rapid root cause identification and correction limits the economic impacts associated with scratch defect events.

Below is a step by step inspection model Foamtec has developed. This method has helped engineers identify and correct wafer scratch defects.

Step 1: Identification of Scratch Defect Signatures and Location of Impact

metrology-mapsFilm Coated Inspection Wafer Metrology Maps

Step 2: Inspection of Suspected Wafer Contact Surfaces using LED Illuminated Magnifying Glass

inspection-of-wafer-and-robotInspection of Wafer Cassette/FOUP | Inspection of FEM Handling Robot | Inspection of FEM Wafer Buffer Cassette

Step 3: Use Digital Micro Camera for Increased Sensitivity Inspection at Suspect Locations

inspection-of-robots-and-fibersInspecting Bottom of FEM Robot End Effector | Identification of Fiber Bundle at End Effector | Removal of Fiber Bundle from End Effector

Step 4: Compare Identified Scratch Defect Sources to Previously Generated Metrology Maps

comparison-of-contaminants-and-metrology-mapsComparison of Identified Contaminants to Metrology Maps to Ensure Root Cause Elimination

Step 5: Use PolyCHECK Inspection Wipers/Swabs to Visually Confirm Contamination & Collect Samples

polycheck-systemsFoamtec PolyCHECK Wipers & Swabs | Samples Collected with PolyCHECK Wiper & Bagged for Lab Analysis

Step 6: Perform Laboratory Analysis of Collected Samples to Identify Root Cause Contaminant Sources

sem-edx-analysis
SEM/EDX Analysis Performed on Collected Samples at Foamtec International’s Contamination Identification Laboratory

ftir-chartsFTIR Contaminant Identification Analysis Result Trace

Fiber contaminants frequently occur, as fiber shedding occurs when typical Polyester fab wipers are used on tool surfaces such as ceramics and metals with sharp corners or surface imperfections that can snag on wiper surfaces. Fibers can be challenging to identify and locate when adhered to machine surfaces. Fibers often cling tightly to surfaces, as static charges easily manipulate them.

In most situations, a fiber may lay flat against a surface until an object with a higher electrostatic charge is introduced, causing the fiber to extend and attempt to transfer to the charged object surface. Foamtec application engineers recorded the behavior of a Polyester fiber found on an upper FEM transfer robot blade. A HT1700 UltraSOLV swab was employed to manipulate and eventually remove the Polyester fiber from the FEM robot blade.  Similar movements of the Polyester Fiber were observed during wafer transfer execution. This Polyester Fiber was identified as the root cause of scratch defects following repeated attempts to identify and correct the source by the engineering technician and tool owners.

The following video shows this phenomenon taking place:

fiber-electrostatic-charge-video-button

If you would like to know more about Foamtec International’s industry leading chamber cleaning and inspection products, or our contaminant analysis services please visit our website or email our staff at contact@foamtecintlwcc.com to get direct support from one of our industry experienced applications engineers.