Case Study - Yield
Since it is impossible to completely eliminate all vibration in the storage cassettes, optimum inclination is tilted back 1mm to reduce “wafer walking”. As the position of storage cassettes is often not an upright position when retrieved by the robot, it is important that levels are accurate. In this scenario, an auto leveling sensor, similar to the auto vibration sensor, was implemented to monitor level of storage cassettes against a set norm.
Leveling was also performed inside the
difficult-to-access vertical furnace to assure that when wafers are being processed at 1000c+ degrees, they are at the proper inclination. Figure 5 shows that optimally levelled stockers, levelled with an automatice levelling system tilt back 1 degree to prevent ‘wafer walking’.
Auto vibration sensor set new standards for vibration detection Whenever a wafer is moved or something within the vicinity of the wafer moves, resulting vibrations can be a potential cause of defects. Typically only a few of the common sources of wafer vibrations such as robot arm bearings or lead-screw lubrication are investigated. When comparing tools that should have identical yields, but one or more tools have abnormally high defects, investigation of the entire wafer path is key to quantify the full range of conditions to which the wafer is exposed. The diffusion team did just that using the auto vibration sensor (AVS). Once the AVS identified multiple wafer defect causes that were previously overlooked or undetected, engineers at the 200 mm fab conducted a number of corrective actions that included: Routine maintenance such as lubrication, which was previously overlooked
Replacement of failing or substandard parts before yield impact
Service sub-par sub-systems such as noisy fans
Where necessary, implement vibration damping systems such as on push-carts or tools near busy traffic areas
Optimally level storage cassettes to reduce “wafer walking” and mishandling.
Revise standard procedures to reduce wafer vibration where excessive vibration was identified
Document new vibration and levelling wafer procedures for implementation into all future routine diffusion tool PMs
The AVS’ data logging capabilities also allowed the fab to establish new preventative maintenance requirements and process control standards for both the tools and engineers that are now a part of Best Know Methods in preventive maintenance of tools. As a result, all 13 tools are providing yield at acceptable levels, including the four that were at sub-par levels.
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Figure 4a &4b
Figure 5
www.euroasiasemiconductor.com Issue IV 2011
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