During the PEB process, the correlation between different layers needs to be considered. There are tiny variations between the layers caused by not only the exposure process but also the PEB process. First, we cut the photoresist to $k$ layers. The size of each is $n$ by $m$. Equation (9) can be written as Eq. (11) in three-dimensions (3-D). Display Formula
$Ti,j,ln+1\u2212D\delta t2(\delta x)2(Ti\u22121,j,ln+1\u22122Ti,j,ln+1+Ti+1,j,ln+1)\u2212D\delta t2(\delta y)2(Ti,j\u22121,ln+1\u22122Ti,j,ln+1+Ti,j+1,ln+1)\u2212D\delta t2(\delta z)2(Ti,j,l\u22121n+1\u22122Ti,j,ln+1+Ti,j,l+1n+1)=Ti,j,ln+D\delta t2(\delta x)2(Ti\u22121,j,ln\u22122Ti,j,ln+Ti+1,j,ln)+D\delta t2(\delta y)2(Ti,j\u22121,ln\u22122Ti,j,ln+Ti,j+1,ln)+D\delta t2(\delta z)2(Ti,j,l\u22121n\u22122Ti,j,ln+Ti,j,l+1n).$(11)