Reduction of Spikes on the Sides of Patterned Thin Films for Magnetic Tunnel Junction Based Molecular Device Fabrication

Abstract

Sputter thin film deposition after photolithography often produces unwanted spikes along the side edges. These spikes are a significant issue for the development of magnetic tunnel junction (MTJ)-based memory and molecular spintronics devices, microelectronics, and microelectromechanical systems because they influence the properties of the other films deposited on the top. Our molecular spintronics devices that utilizes MTJ as the testbed are almost short-lived and encountered high background current that masked the effect of molecular transport channels placed along the sides of MTJs. Therefore, tapered thin film edges are critically needed in devices. Here, we report a very cost efficient and fast way of creating an optimum photoresist profile for the production of ‘spike-free’ patterned films. This approach is based on performing a soaking in the photoresist developer after baking and before the UV exposure. However, the success of this method depends on multiple factors accounted for during photolithography photoresist thickness (spin speed), baking temperature, soaking time and exposure time. Our recent experiments systematically studied the effect of these factors by following the L9 experimental scheme of the Taguchi Design of experiment (TDOE). The L9 experimental scheme effectively accommodated the study of four photolithography factors, each with three levels. After performing photolithography as per L9 TDOE, we conducted sputtering thin film deposition of 20 nm Tantalum. Then we conducted an atomic force microscope (AFM) study of thin film patterns and measured the spikes along the edges of the deposited Tantalum. We utilized spike height as the desired property and chose “smaller the better” criteria for TDOE analysis. TDOE enabled us to understand the relative importance of the parameters, relationship amongst the parameters, and impact of the various levels of the parameters on the edge profile of the thin film patterns. We discovered that baking temperature was the most influential parameter; presoak time and photoresist thickness were two other influential factors; exposure time was the least effective factor. We also found that 4000 rpm, 100 C soft baking, 60 s soaking and 15 s UV exposure yielded the best results. Finally, the paper also discusses the interdependence of selected factors, and impact of the individual levels of each factor. This study is expected to benefit MEMS and micro/nanoelectronics device researchers because it attempts at finding a cheaper and faster alternative to creating an optimum photoresist profile.