Formation of periodic relief structures in thin chromium films using laser interference lithography

Abstract

Subject of study. The formation process of contact masks in thin films of photoresist and chromium using laser interference lithography was investigated. Hence, the photoresist films were structured by exposure in the interference field followed by the transfer of the formed microrelief to the chromium film using a plasma method. Method. The investigation is based on the use of laser interference lithography for the formation of protective masks in the layer of high-resolution photoresist. The photoresist was deposited via spin coating and subsequently dried. Lloyd’s interference scheme was used for photoresist exposure. After exposure, the photoresist was developed in a standard base developer specified by the manufacturer. The distribution nonuniformity of thickness of the photoresist film with microrelief over the substrate surface after development was determined as a ratio of difference in photoresist thicknesses at the substrate edges obtained using profilometry to the maximum thickness. Microrelief in the photoresist layer and chromium film was investigated via scanning probe microscopy. The etching uniformity and process of the microrelief transfer to the chromium film via the protective mask in the photoresist layer were investigated using a setup for reactive ion etching in inductively coupled plasma. Main results. The periodic relief structures in chromium films were obtained for the first time via laser interference lithography for photoresist structuring and transfer of microrelief by reactive ion etching. The obtained depth of microrelief in the photoresist layer corresponded to 350 nm. This depth is sufficient for microrelief transfer to the metal film. We established that the interference field in the recording area enables the formation of the microrelief in the photoresist layer with a coefficient of film thickness nonuniformity of 5% over the surface of the substrate with a diameter of 25 mm. Practical significance. The conducted investigation is initial and the aim involved identifying the principal possibility of formation of contact masks in metal films using laser interference lithography. If the lithographic setup is modified, this method can be applied to form contact masks for diffractive optical elements on substrates with sizes exceeding 25×25mm.