A mechanism of the penetration limit for producing holes in poly(4-vinyl phenol) films by inkjet etching

A penetration limit has been experimentally demonstrated for inkjet etching of holes in thin polymer layers. A mechanism combining the competing coffee ring flow, polymer dissolution and diffusion into the solvent drop, and the interaction between the contact line during evaporation and the softened deformable polymer, is proposed to explain the existence of such a penetration limit. The height-averaged velocity of the coffee ring flow within the evaporating sessile drop is calculated during the initial stage of this etching process when the spherical cap geometry assumption is valid. This is compared with the diffusion velocity of the disentangled polymer into the solvent. The two competing flows are used to elucidate why a hole could be formed initially. The complex wetting dynamics of the receding contact line is included to explain the via hole profile evolution in the later stage of the etching process and the existence of a penetration limit. These two stages are differentiated by the drop volume with respect to the volume of the via hole produced by the preceding drop. The competition between the coffee ring flow transferring polymer away from the central region and the polymer diffusion within the solvent drop is postulated to contribute to either via hole formation or a penetration limit, depending on which one of the two processes is dominant within the solvent evaporation time scale.