Case Study - Yield
Figure 1
handling, storage cassettes and the setups inside the vertical furnace where access was particularly difficult.
Identify wafer transport and process locations that do not meet the level standard and those that are level to standard to assure that all wafer handoffs and wafer processing are uniformly administered on all tools.
Using the highest yield tool as the “golden reference”, establish a baseline vibration signature for all wafer movements within the tool and use that baseline signature as a comparison reference for all similar moves on all other tools, with particular attention given to the lowest yield tools.
Test all tools and identify locations where wafer movement does not conform to the established standard for vibration.
In cases where wafer movements in “test” tools exceed the “reference” signature standards, determine the root cause of that excursion and correct it.
White-paper or document as Best Known Method the newly established levelling and vibration test standards and schedule the levelling and vibration testing into the all ongoing routine tool PM activities.
Vibration characterization tool Placed either in standard cassettes, storage cassettes or in wafer boats that typically hold 150 wafers, the wireless vibration sensor moved through the entire diffusion tool, monitoring
three-axis accelerations and vibrations at all locations in the tool in which the wafer travelled. Using Bluetooth technology, the wireless sensor provided wireless real-time data on tool vibration as it moved through the process. With companion software displaying data on a GUI, engineers were able to overlay vibration fingerprints and analyse vibration data. After carefully recording known good tool and using those golden move vibration signatures as a “reference” for all other tools, it was ultimately determined that there was no one specific cause, but rather many cumulative effects that contributed to poor yield in some tools in the diffusion department.
Results Problem 1: Some robot moves exhibited excessive vibration.
Action: Perform routine maintenance such as lead-screw lubrication or identify and replace failing component.
Example 1 – Red trace reference signature of good wafer movement, green trace test signature: Shown above (Fig 1) are two overlaid vibration signatures of a stage move. Frequency is shown in the left plane and Time Domain on the right. The red trace is the reference signature and the green trace is the test signature. The two red lines and the red bar are the go no/go threshold.
Results show that the stage move on the test tool clearly failed, indicating possible defect sources and required maintenance. Also, note
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www.euroasiasemiconductor.com Issue IV 2011
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