The depth of focus is an important parameter to measure the exposure process window. It marks the relationship between the imaging quality of the exposure system and the position of the wafer surface. Within the depth of focus range, the quality of exposure imaging can be guaranteed. The depth of focus during exposure must be much larger than the unevenness of the wafer surface. Only in this way can the yield of the photolithography process be guaranteed.
As shown in Figure 1, in order to simplify the calculation, only the 0th and 1st order diffracted light from the mask is imaged on the wafer through the projection lens. , The focus depth is defined as 2P0P, which is:
DOF=2P0P=λ/2(NA)< p>Among them, λ=the exposure wavelength, NA=the numerical aperture of the optical system
Calculation of the optical path of the spherical wavefront (P0 is the focal point of the wavefront, ie R0P0=RP0)