Investigations of the isotropic etch of an ICP source for silicon microlens mold fabrication

This work investigates the isotropic etching properties of an inductively coupled plasma (ICP) etcher for masked and maskless etching steps in reference to fabrication of a silicon microlens mold. Using the described method a wide range of lens geometries and lens arrays with 100% fill factor can be achieved. The silicon etching is performed with a continuous SF 6 based ICP. Analysis of the etching profile was done by SEM inspection and by optical interferometric measurements. For the masked etching step a consistent picture of the profile evolution is obtained, including a relation between the etching depth, the radius of curvature of the profile, the etching time and the size of the mask opening. For the maskless etching step, the optimal etch is purely isotropic. Within the tested process parameter range the maskless etching showed a non-isotropic behavior, which results in lens aberrations, crystal orientation dependence, poor uniformity and roughness. Generally a trade-off between the individual etching behaviors is found, but useful etching recipes can be found for a range of different lens geometries.