Nanopatterning of organic and inorganic materials by holographic lithography and plasma etching

We report on the nanopatterning of semiconductors and conjugated polymers to produce photonic band gap structures. Poly(p-phenylenevinylene) (PPV) thin films prepared by spin coating on ITO-coated glass, GaAs and Si substrates were successfully processed. The technological process for the patterning was based on U.V. holographic lithography (364 nm line of Ar+ Laser) and plasma etching. First a photoresist layer was deposited on the sample surface and exposed to an interference pattern in a "corner cube" interferometer, with multiple exposure capability, in order to realize pattern of lines, squares, pillars and holes. The shape of the pillars and holes, produced by double exposure, can be changed by rotating the sample by angles in the range between 30 and 90 degrees. Each step in the mask production procedure was carefully controlled by AFM measurements. Then the photoresist patterns were transferred to the substrate materials by means of a plasma etching in a parallel plate reactor. In the case of PPV, a long optimization of the Argon plasma parameters was done in order to balance the photoresist versus PPV etching rate and to prevent over-etching of the PPV. This problem is inherent to the organic nature and similar chemical characteristics of the two materials and it makes the mask transfer process in the PPV very critical. The resulting patterned region consisted of 200nm wide parallel lines with a periodicity of 400nm in the case of single exposure, whereas ordered two dimensional arrays of pillars (250nm in diameter) were produced with a double exposure of the photoresist mask. A CHa/H2/Ar and CF4 based reactive ion etching was used to transfer the 2D holographic photoresist patterns respectively to the substrates of GaAs and Si, in order to realize two-dimensional matrices of pillars and holes for photonic band-gap applications and optical devices based on matrices of dots or anti-dots.