Publications

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Substitution of Al by Sc has been predicted and demonstrated to improve the piezoelectric response in AlN for commercial market applications in radio frequency filter technologies. Although cosputtering with multiple targets have achieved Sc incorporation in excess of 40%, industrial processes requiring stable single target sputtering are currently limited. A major concern with sputter deposition of ScAl is the control over the presence of non-c-axis oriented crystal growth, referred to as inclusions here, while simultaneously controlling film stress for suspended microelectromechanical systems (MEMS) structures. In this paper, we describe 12.5% ScAl single target reactive sputter deposition process and establishes a direct relationship between the inclusion occurrences and compressive film stress allowing for the suppression of the c-axis instability on silicon (100) and Ti/TiN/AlCu seeding layers. An initial high film stress, for suppressing inclusions, is then balanced with a lower film stress deposition to control total film stress to prevent Euler buckling of suspended MEMS devices. Contour mode resonators fabricated using these films demonstrate effective coupling coefficients up to 2.7% with figures of merit of 42. Finally, this work provides a method to establish inclusion free films in ScAlN piezoelectric films for good quality factor devices.

Substitution of Al by Sc has been predicted and demonstrated to improve the piezoelectric response in AlN for commercial market applications in radio frequency filter technologies. Although cosputtering with multiple targets have achieved Sc incorporation in excess of 40%, industrial processes requiring stable single target sputtering are currently limited. A major concern with sputter deposition of ScAl is the control over the presence of non-c-axis oriented crystal growth, referred to as inclusions here, while simultaneously controlling film stress for suspended microelectromechanical systems (MEMS) structures. This work describes 12.5% ScAl single target reactive sputter deposition process and establishes a direct relationship between the inclusion occurrences and compressive film stress allowing for the suppression of the c-axis instability on silicon (100) and Ti/TiN/AlCu seeding layers. An initial high film stress, for suppressing inclusions, is then balanced with a lower film stress deposition to control total film stress to prevent Euler buckling of suspended MEMS devices. Contour mode resonators fabricated using these films demonstrate effective coupling coefficients up to 2.7% with figures of merit of 42. This work provides a method to establish inclusion free films in ScAlN piezoelectric films for good quality factor devices.

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We have developed an ambient temperature, SiO₂/Si wafer - scale process for Josephson junctions based on Nb electrodes and Ta x N barriers with tunable electronic properties. The films are fabricated by magnetron sputtering. The electronic properties of the Ta_xN barriers are controlled by adjusting the nitrogen flow during sputtering. This technology offers a scalable alternative to the more traditional junctions based on AlO_x barriers for low - power, high - performance computing.

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This work reports the utilization of a recently developed film, ScAlN, as a silicon etch mask offering significant improvements in high etch selectivity to silicon. Utilization of ScAlN as a fluorine chemistry based deep reactive ion etch mask demonstrated etch selectivity at 23 550:1, four times better than AlN, 11 times better than Al2O3, and 148 times better than silicon dioxide with significantly less resputtering at high bias voltage than either Al2O3 or AlN. Ellipsometry film thickness measurements show less than 0.3 nm/min mask erosion rates for ScAlN. Micromasking of resputtered Al for Al2O3, AlN, and ScAlN etch masks is also reported here, utilizing cross-sectional scanning electron microscope and confocal microscope roughness measurements. With lower etch bias, the reduced etch rate can be optimized to achieve a trench bottom surface roughness that is comparable to SiO2 etch masks. Etch mask selectivity enabled by ScAlN is likely to make significant improvements in microelectromechanical systems, wafer level packaging, and plasma dicing of silicon.

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