Area-selective deposition of ferromagnetic cobalt films




Zhang, Zizhuo

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Cobalt is a classic ferromagnetic material and finds applications in magnetic random access memory devices. As an emerging patterning technique, area-selective atomic layer deposition (AS-ALD) of cobalt films has the advantages of low cost and low film damage compared with the current patterning approach of photolithography followed by etching of cobalt films. This research explores the routes to realize the AS-ALD of ferromagnetic cobalt films. Carbon-free Co films have been deposited on MgO(001) and SiO₂/Si substrates by low temperature thermal ALD of CoO and subsequent low temperature atomic deuterium reduction of CoO. While high temperature D₂ reduction of CoO results in rough and disconnected Co islands, low temperature D reduction of CoO produces smooth and continuous Co films. The Co films produced by low temperature D reduction exhibit a smaller coercivity than the Co films produced by high temperature D₂ reduction. Reduction conditions affect the microstructure and the magnetic properties of the reduced Co films. The process-structure-property relationship has been elucidated. The increase of reduction temperature, partial pressure of deuterium, and reaction time increases the average grain size and coercivity of the reduced Co films. Co films that are grown on substrates with lower dewetting tendency, such as MgO and Al₂O₃, show smaller average grain size and smaller coercivity. Polystyrene (PS) has been proposed to work as a passivation material to realize the AS-ALD of cobalt films. PS is effective in inhibiting the nucleation and film growth of CoO on oxide substrates. Micro-patterns and nano-patterns of CoO have been grown by AS-ALD through PS templates produced by photolithography and PS templates created by directed self-assembly of diblock copolymer, respectively. CoO patterns can be further reduced to form Co patterns without deformation.


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