Non-penetration condition at solid wall

I impose a Navier slip boundary condition at an inclined wall to solve the liquid slip flow over hydrophobic surface. But the slip velocity penetrates into solid wall because uN.x + vN.y = 0 is not satisfied. How to implement the non-penetration condition (uN.x + vN.y = 0) at solid boundary. I have tried many methods, but they does not work.
Many thanks.