The Next-Generation Wafer
Source : Technical data published by ESPEC (April 2005)
Vibration frequency of cargo resulting from truck transportaion
Analysis (theoretical examination) of steady-state vibration from transit was carried out based on published frequencies encountered during truck transportation. Frequency (horizontal axis) is plotted against vibration energy (vertical axis) and is represented by a power spectral density (PSD).
The results show PSD peaking at around 33Hz (vibrations per second). The rate of vibration acceleration during transportation by truck meanwhile is around ±1G (9.8m/s2), which is continuously transmitted to the cargo.
Based on these results, the dependence of natural frequencies on wafer thickness was examined assuming two hypothetical support patterns within a FOSB, as shown in the following diagrams.
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Examination of the correlation between wafer thickness and natural frequencies (when using support patterns 1 and 2) indicates that, as in the case of gravitational sag, a thickness of approximately 1,800μm would have the same natural frequency as a 300mm wafer with a thickness of 775µm.
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The graphs show simulated displacement values for a 450mm wafer with a thickness of 925μm experiencing sine wave oscillation at a rate of acceleration of 1G. There are higher levels of distortion than anticipated around the resonant frequency in the case of both support patterns 1 and 2.
Once resonance oscillation occurs during transportation by truck, wafers could potentially come into contact with surfaces other than the support points, including adjacent wafers. As with 300mm wafers, it will be necessary to examine whether current FOSB support approaches would be sufficient for 450mm wafers. New FOSB support techniques are likely to be needed for 450mm wafers and are expected to be thickness dependent.
