This is a side view through the film thickness, not a top view: x runs along the plane, z runs through it, and the substrate sits below. The hot spot sits just under the top surface on purpose — that is where a device deposits heat — so the pattern is deliberately not centred between the two surfaces. Isotherms are clipped at the film’s two faces because this model only describes the film itself. Their aspect ratio in this plane is √(k∥/k⊥). Raise the anisotropy and watch the isotherms flatten into blades that run along the film instead of diving into the substrate — the geometric reason a heat spreader works and a through-stack path does not.
Constant-property anisotropic diffusion drawn in the x–z (through-thickness) section: isotherm axes scale as the square root of each direction’s diffusivity. The film surfaces and substrate are sketched to place the geometry; real analyses solve the layered problem with boundary conditions and interface resistances rather than free-space ellipses.