Michael David Henry

Platinum Diffusion Barrier Breakdown in a-Si/Au Eutectic Wafer Bonding

Eutectic bonding in semiconductor fabrication requires a large degree of control over the stoichiometry and precision film thickness of the bonding materials. To reduce the migration of the bonding layers, diffusion barriers are typically utilized. Here, we demonstrate that a widely utilized diffusion barrier, Pt, does not prevent migration of Si in Si/Au eutectic bonding. We observe that this barrier breaks down at approximately 375°C, above the Au-Si eutectic temperature (363°C), and encourages consumption of the silicon substrate leading to uncontrolled stoichiometry variations and creation of microvoids. This failure results in reductions of bond strength and hermeticity. As an alternative, silicon dioxide is observed to prevent the silicon diffusion and subsequent substrate loss.

Frequency Trimming of Aluminum Nitride Microresonators Using Rapid Thermal Annealing

To transition aluminum nitride (AlN) microresonator filters into a manufacturable technology, accurate control of the resonator frequency across a wafer is required. This paper describes a postfabrication rapid thermal anneal approach to trim resonator frequency over 27000 ppm with an accuracy of 500 ppm. Measurements made on 22.4 MHz resonators indicate that the effect of annealing on the resonators saturates in 5 min and upshift the resonator frequency super linearly with temperature. We replicate the frequency trimming effect on hermetically sealed wafer level packaged devices to reduce the across-wafer frequency distribution from 22000 to 4000 ppm. We confirm that this postannealing technique is permanent by temperature cycling the resonators from 50°C to 125°C. This technique provides a method to trim AlN microresonator frequency overcoming effects such as thin film variations, which are inherent to microsystems fabrication.

Superconductive Silicon Nanowires Using Gallium Beam Lithography

This work was an early career LDRD investigating the idea of using a focused ion beam (FIB) to implant Ga into silicon to create embedded nanowires and/or fully suspended nanowires. The embedded Ga nanowires demonstrated electrical resistivity of 5 m-cm, conductivity down to 4 K, and acts as an Ohmic silicon contact. The suspended nanowires achieved dimensions down to 20 nm x 30 nm x 10 m with large sensitivity to pressure. These structures then performed well as Pirani gauges. Sputtered niobium was also developed in this research for use as a superconductive coating on the nanowire. Oxidation characteristics of Nb were detailed and a technique to place the Nb under tensile stress resulted in the Nb resisting bulk atmospheric oxidation for up to years.

ScAlN etch mask for highly selective silicon etching

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...

Silicon nanowire pirani sensor fabricated using FIB lithography

As radio frequency microelectromechanical systems (RF-MEMS) mature as a manufacturable technology, packaging of the devices becomes increasingly important. Devices such as aluminum nitride (AlN) RF-filters require packaging which is either hermetic or under vacuum to protect the devices [1]. It then becomes critical to have a measurement of pressure inside the packaged chamber. Typically for this need, Pirani gauges are fabricated using poly silicon or metal patterned on suspended membranes [2]. These type of devices increase die area, add complexity to fabrication flows, and difficulty when attempting to suspend the membranes. In this work we fabricate and characterize a suspended silicon nanowire for use as Pirani gauge by utilizing Ga lithography and plasma reactive ion etching for defining the nanowire geometry and simultaneously releasing the wire. This method benefits from the high surface to volume ratio inherent in the nano regime, decreased thermal conductivity of amorphous silicon (from implantation) and increased electrical conductivity of Ga doping to reduce device area and fabrication complexity of a Pirani gauge.

Accelerated vortex dynamics across the magnetic 3D-to-2D crossover in disordered superconductors

Disorder can have remarkably disparate consequences in superconductors, driving superconductor–insulator transitions in ultrathin films by localizing electron pairs and boosting the supercurrent carrying capacity of thick films by localizing vortices (magnetic flux lines). Though the electronic 3D-to-2D crossover at material thicknesses d ~ ξ (coherence length) is well studied, a similarly consequential magnetic crossover at d ~ Lc (pinning length) that should drastically alter material properties remains largely underexamined. According to collective pinning theory, vortex segments of length Lc bend to adjust to energy wells provided by point defects. Consequently, if d truncates Lc, a change from elastic to rigid vortex dynamics should increase the rate of thermally activated vortex motion S. Here, we characterize the dependence of S on sample thickness in Nb and cuprate films. The results for Nb are consistent with collective pinning theory, whereas creep in the cuprate is strongly influenced by sparse large precipitates. We leverage the sensitivity of S to d to determine the generally unknown scale Lc, establishing a new route for extracting pinning lengths in heterogeneously disordered materials.Superconductors: Vortices and the role of defectsDisorder influences the properties of superconductors, as defects can pin vortices. Thermal energy unpins the vortices, whose creep rate is expected to depend on sample thickness, in particular when the thickness is reduced to below the pinning length. However, the description of pinning in systems with different types of defects is still a matter of debate. Serena Eley at Los Alamos National Laboratory and colleagues systematically studied the thickness dependence of the creep rate of vortices in films of Nb (superconductive critical temperature Tc = 9.2 K) and of a cuprate material (Tc = 92 K). The results unveil the different role of defects in pinning vortices in these materials and show that this approach provides a means of directly accessing the pinning length in heterogeneously disordered materials, such as cuprates.

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

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.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 c...

Sc x Al 1−x N film evaluation using contour mode resonators

Recent literature has focused on improving piezoelectric coupling coefficients by alloying aluminum nitride (AlN) with scandium (Sc). Akiyama et al. showed the highest piezoelectric coefficient increase of nearly four times for a 41% Sc substitution for Al. Thus far, studies mainly focus on material measurements such as x-ray diffraction or piezoelectric constants to assess the material quality. Although these measurements are useful to assess the improvement in the piezoelectric performance of the material, they do not address changes in the coupling coefficient and quality factor. Resonator structures are needed to directly extract these key performance parameters for film assessment. Fabrication integration, however, must be minimized to avoid obscuring film performance by extrinsic device effects. In this work, we assess a film evaluation tool using contour mode resonators (CMRs) to directly extract resonator performance for film comparison. Resonators formed from AlN, Sc0.06Al0.94N, and Sc0.125Al0.875N films are compared to demonstrate the method.

Sc(0.06)Al(0.94)N film evaluation using contour mode resonators

Recent literature has focused on improving piezoelectric coupling coefficients by alloying aluminum nitride (AlN) with scandium (Sc). Akiyama et al. showed the highest d_33 piezoelectric coefficient increase of >4x at a 41% Sc substitution for Al. Thus far, studies mainly focus on material measurements such as x-ray diffraction or piezoelectric constants to assess the material quality. Although these measurements are useful to assess the improvement in the piezoelectric performance of the material, they do not address improvements in the figure-of-merit (FOM) of resonators (i.e., coupling coefficient times quality factor). Resonator structures are needed to directly extract these key performance parameters for film assessment. Fabrication integration, however, must be minimized to avoid obscuring film performance by extrinsic device effects.

Materials Study of NbN and Ta x N Thin Films for SNS Josephson Junctions

Properties of NbN and Ta_xN thin films grown at ambient temperatures on SiO₂/Si substrates by reactive-pulsed laser deposition and reactive magnetron sputtering (MS) as a function of N₂ gas flow were investigated. Both techniques produced films with smooth surfaces, where the surface roughness did not depend on the N₂ gas flow during growth. High crystalline quality, (111) oriented NbN films with T_c up to 11 K were produced by both techniques for N contents near 50%. The low temperature transport properties of the Ta_xN films depended upon both the N₂ partial pressure used during growth and the film thickness. The root mean square surface roughness of Ta_xN films grown by MS increased as the film thickness decreased down to 10 nm.