Detecting Aliasing Between Cross and Machine Direction Variations by Variable Sampling Rate

It is well known that use of scanning sensors in the presence of high frequency machine direction process variations gives rise to aliasing effects that cannot be avoided when designing algorithms that seek to separate cross machine (CD) variations from those in machine direction (MD). These effects are particularly troublesome when machine direction fluctuations have a strong component close to the scanning frequency of the sensor and then often lead to machine direction variations being perceived as cross machine profile effects. It is also possible for a changing profile to be misinterpreted as a periodic MD variation. The method presented here assists in determining whether such observed variations are in MID or CD. The method makes use of variable rate scanning to distinguish between artifacts related to the scanning rate and true process fluctuations.