Reactive sputter deposition of piezoelectric Sc0.12Al0.88N for contour mode resonators

Abstract

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.

Thermal Conductivity of Aluminum Scandium Nitride for 5G Mobile Applications and Beyond

“…These values are an order of magnitude lower than that for the AlN film with a similar thickness and microstructure (∼51 W m −1 K −1 ) and 2 orders of magnitude lower than a single-crystal epitaxial film of AlN (∼320 W m −1 K −1 ). 28 It should be noted that the average columnar (lateral) grain size resulting from the reactive sputter deposition process 18,29 was consistent (∼35 nm; Figure 1c) among all the AlN and Al 1−x Sc x N films investigated in this work. Consequently, from a practical perspective, the reduction in thermal conductivity underscores the intrinsic trade-off between piezoelectric and thermal performance in the Al 1−x Sc x N solid solution.…”

“…These stress values were determined via wafer curvature measurements . As reported elsewhere, the density of AOGs tended to be lower in films with higher substrate bias …”

“…17 As reported elsewhere, the density of AOGs tended to be lower in films with higher substrate bias. 18 Figure 1 shows representative transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. As shown in Figure 1c, the columnar grain size of the c-axis-textured AlN and Al 1−x Sc x N films were ) has shown that films with smaller rocking curve linewidths (full width at halfmaximum, FWHM) exhibit high k t 2 .…”

Dissipation Analysis Methods and Q-Enhancement Strategies in Piezoelectric MEMS Laterally Vibrating Resonators: A Review

“…Although it has been reported that the k eff 2 can be increased by about four times compared to un-doped AlN for a doping concentration of 40%, the improvement comes at the cost of significant reduction in Q [ 129 ]. Possible reasons for degradation in Q are related to the formation of inclusions and film stress during the film deposition process [ 130 ]. Besides optimizing the process parameters to avoid inclusions, as well as controlling film stress, an interesting Q-enhancement strategy in terms of materials advances has been to combines the ScAlN film with single crystal silicon to realize high-Q ScAlN-on-Si TPoS LVRs [ 131 ].…”

Microwave Acoustic Devices: Recent Advances and Outlook

“…1(c)-(e)], since it was first introduced, has found its way into commercial products at moderate doping concentrations [30]. Extensive research has gone into reactive sputtering of Sc-doped AlN targets that can produce uniform doping concentration and stress across wafers [33]. Tradeoffs between doping levels and experimentally attainable Q have been renewed every year, producing higher and higher k 2 with nearly no sacrifice of Q [28].…”