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Description
Molybdenum (Mo) thin films find application as a bottom electrode material for acoustic resonators due to their favorable acoustic and physical properties. For subsequent overgrowth with piezoelectric materials such as AlN, which are necessary for acoustic resonators, high quality and low roughness of Mo (110) thin films are essential. Direct deposition of Mo on silicon (Si) is challenging, as interfacial reactions can lead to the formation of Mo-Si compounds, degrading the film quality and negatively impacting the growth of any subsequent functional layers. One method to suppress this compound formation is the use of a buffer layer, such as aluminum nitride (AlN), which is also known to promote (110) oriented growth of Mo. In this work, an optimized process for the deposition of highly crystalline Mo with low roughness on AlN (0002) buffer layers using pulsed DC magnetron sputtering is presented. Two differently grown types of AlN (0002) buffers were investigated. The first approach relies on an $N_2$ based growth process, leading to the common columnar type of AlN growth with a roughness of 2 nm RMS. The second approach uses a two-step growth process, where an initial $N_2$-grown AlN template is followed by $NH_3$-based growth, enhancing lateral growth and column coalescence, leading to a reduction of roughness to 0.35 nm RMS. Overgrowing both these templates with Mo leads to drastically different results: While the overgrowth of the $N_2$ grown AlN with Mo results in RC FWHM of 0.74° and a roughness close to 2 nm RMS, the overgrowth of the bilayer AlN leads to RC FWHM of 0.35° and roughness beneath 0.5 nm RMS. Using this Si-AlN-Mo template, subsequent high quality AlN growth resulted in an outstanding RC FWHM of 0.74° and an RMS roughness of 1.9 nm. These results show the importance of AlN buffer layers for achieving high-quality Mo electrodes, providing a suitable template for acoustic resonator structures.