The main figure of merit for the resist is the resist contrast - it shows the quality of resist
Contrast shows the ability of the resist to separate the light from the dark portions
Is extracted from the contrast curve slope (the exposure time is increased, the resist thickness is measured after each increase in exposure time, the curve resist thickness versus exposure dose = intensity *time is drawn)
Dose D1 - represents the dose for which the resist does not start to dissolve in the developer
Dose D2 - represents the dose for which the resist is totally dissolved in the developer
For intermediary doses, the resist is only partially dissolved
It is preferred that the slope to be as steep as possible
The contrast is defined as the curve slope:
In the next picture two resists are compared:
Red - good resist contrast
Blue - bad resist contrast
Higher dose D2 is required for the resist with bad contrast in order to reach complete dissolution of photoresist
The resist with bad contrast cannot be used for very small features production
Another picture shows what is the effect of obtaining poor contrast resist on the structure made:
Poor resist contrast
Sloped walls
Swelling
Poor contrast
Good resist contrast
Sharp walls
No swelling
Good contrast
In the next picture the ideal situation (ideal source and ideal resist in the left ) is compared to the real one (real source, real resist in the right side)
Because the dose for a real source does not decrease abruptly from D1 to D2, the resist thickness will also have a slope
With increasing the time in the developer, the resist will be removed, which can cause the final features to be smaller than intended
The real resist is characterized by a contrast, thus if it has poor contrast the slope in the resist thickness will decrease
Substrate preparation
