When interferation occurs from a grating, light intensity on the wafer will vary between Imin (photoresist unexposed to light) and Imax (photoresist exposed)
In order to obtain a good image of the mask on the resist, Imin should be as small as possible, close to 0, and Imax should be as high as possible, close to 1. Of course this is the ideal case, in reality
MTF is defined as:
High MTF means that there is high contrast between the low intensity and high intensity image
When the grating has lower period (the windows are close to each other), then MTF is lower (the intensity increases in the unexposed area due to the close vicinity of exposed area)
Low MTF (High Imin and low Imax)means lower reproducibility from mask to wafer (the trasferred image will not correspond perfectly with the mask)
Possible solution: the dose needs to be sufficient enough so that the photoresist will dissolve in the regions with Imax (Imax is lower), and this can happen by increasing the developing time (can result in diminshed features for too high exposure)
Dose is equal with the intensity of light multiplied by time. In the picture below is represented the dose that falls on the resist after light passes through a grating. the dose is of course higher in the regions underneath the openings.
if D < D2, then the resist will not dissolve
if D > D1, the resist will dissolve completely
if D2 < D < D1, then the resist will be partially developed
Thus it is difficult to make the structures smaller, because MTF decreases and the contrast is bad, resulting in distorted structures
Substrate preparation
