Case Study - Yield


traffic fab intersection and the red trace is of a tool away from the busy intersection. In this case, the tool next to the intersection was identified as needing additional vibration damping systems.


Example 4a: Do the carts transporting wafers to and from the tool need vibration damping? Smooth cart transport to and from tools should not be over looked.


Figure 3


the low frequency spike around 15.6Hz. That is a common indicator of a slower motion source most likely requiring lubrication (failing higher speed components typically have higher frequency spikes).


Problem 2: Miscellaneous vibration sources such as passing carts, the carts themselves and the position of storage cassettes in relation to fans, expose wafers to varying degrees of vibration.


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Action: Identify cause of vibration and take corrective action where necessary. For example, tools located close to busy aisles may require extra vibration damping or some tools fans may need to be retrofit with quieter fans.


Results show that the


stage move on the test tool clearly failed,


indicating possible defect sources and required


maintenance


Example 2 – Red trace tool fans in OFF position; green trace tool fans in ON position: The effects of systems or events such as fans should not be overlooked when evaluating the total tool for defect causes. In figure 2 the test signature in red shows Z axis wafer vibration when fans in storage areas fans are OFF. The green trace is the same storage cassette with the fans ON. When the cassettes are not leveled in a tilted back position, as shown in figure 1 above, the risk of “wafer walking” increased (wafers that move can be broken or scratched when the robot retrieves them).


Note that the Time Domain signatures look similar to those in figure 1. However, the Frequency signatures show spikes in the higher frequency range, indicating that the vibration sources originate from faster motions like failing ball-bearings.


Example 3 - Red trace tool not by busy aisle, green trace tool by busy aisle:


Some tools, due to their physical location such as next to a busy aisle with carts continually passing, will be exposed to more vibration than tools further away from busy aisles. In figure 3, the green trace shows a tool next to a high-


In figure 4a, the X, Y and Z axis as well as the RMS of the three are all displayed. Although the RED vibration-suppressed cart seems to show less vibration than the standard cart, especially in the Y axis, it is good practice to run statistical reports from the raw CSV file data of the vibration wafer outputs at 1000 times per second to quantify the vibration energy exposure.


Example 4b: Quantifying vibration “Test” and “Reference” vibration signatures:


In figure 4b, test and reference signatures shown in example 4a are compared for standard deviation using SPC reports provided with the vibration wafers software application. Although figure 4a visually depicts a general tendency for the enhanced cart (red trace) to be smoother than the standard cart (green trace), by applying statistical reporting to the raw CSV file format output, the actual difference is quantified. This figure clearly quantifies that the


standard deviation of the enhanced cart is measurably less in all three axis than that of the standard cart, thus proving that the vibration- suppressing retrofitted carts do offer greater vibration damping. (The vibration wafer software application comes with many SPC reports for advanced vibration analysis through multiple tests and reference signatures at one time.)


Problem 3: Stocker cassettes were not all leveled when tilted back, resulting in incidents of “wafer walking” caused by tool vibrations.


Action: Level all stocker cassettes tilting back 1mm


Example 5: For a comprehensive program in addressing wafer handling issues associated with excessive vibration, leveling wafer cassettes to established standards should be completed prior to conducting vibration analysis.


www.euroasiasemiconductor.com  Issue IV 2011


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