Exceptionally strong metallic films unlock new possibilities for MEMS devices
Microelectromechanical systems, or MEMS, are microscale devices that can function as highly efficient sensors, controllers, actuators, switches, and mechanical filters in a broad range of products. However, their superior capabilities require novel engineering designs, materials integration, and unique mechanical and electrical properties. Specially shaped thin films can provide an appropriate backbone for these systems. Silicon, which offers scalable fabrication and performance at acceptable levels, has been, to date, the material of choice for most MEMS devices. However, in their intrinsic state, silicon-based devices cannot provide sufficient electrical conductivity, mechanical strength, and structural integrity at elevated temperatures. As a result, many desired MEMS applications, such as power generators and critical components of the “Internet of Things,” remain out of reach.
Researchers at Johns Hopkins University took this problem to task and used DC sputter deposition to fabricate solid-solution nickel-molybdenum-tungsten (Ni-Mo-W) alloy films. The resulting structure, a 111 textured, nanotwinned, single-phase solid solution, exhibited unprecedented mechanical, thermal, and electrical properties that elevated this material above comparable thin films. Kevin J. Hemker led the research effort, along with his mechanical engineering colleagues Gi-Dong Sim, K. Madhav Reddy, Kevin Y. Xie, Gianna M. Valentino, and Timothy P. Weihs. Together with Jessica A. Krogstad at the University of Illinois at Urbana-Champaign, the researchers published their achievement in a recent issue of Science Advances.
Hemker described the research effort as a highly collaborative effort that yielded unexpected properties. “Motivated by the benefits to be realized by the development of metal MEMS materials, we set out to deposit Ni alloys with compositions that would provide good strength, high conductivity and excellent dimensional stability,” he says. “We added Mo and W to reduce the coefficient of thermal expansion, and only later realized that these additions also significantly lowered the stacking fault energy, which led to the ubiquitous formation of nanotwins during sputtering and ultrahigh tensile strength and an excellent balance of properties.”
The superior properties of the Ni-Mo-W films stem from their unique underlying microstructures. The alloy composition far exceeds the maximum solubility limit. Nevertheless, the high-rate sputter deposition resulted in supersaturated, densely packed, void-free, crystallographically textured columnar microstructures. As an added benefit, the material was discovered to contain a high concentration of nanotwins and stacking faults within the grains. These planar defects reinforce and strengthen the films. Dislocations that govern plasticity are forced to bow between twin boundaries, which significantly increases the film’s tensile strength to an unprecedented value.
Since MEMS devices also encounter a wide range of thermal environments and dynamic electropotential loads, the researchers evaluated the thermal and mechanical durability of the Ni-Mo-W alloy. The microstructure of the sputter-deposited films was unaffected by temperatures as high as 600°C. This stability strongly contrasts with pure nanocrystalline nickel films that exhibit grain coarsening at 200°C. The presence of nanotwins contributed exceptional durability of the films at high temperatures and under very high mechanical loads (3.1 GPa). The material’s electrical resistivity (111.7 mΩ∙cm) is comparable with that of bulk nickel alloys, demonstrating that the nanotwins increase strength without degrading the electrical conductivity. Moreover, the coefficient of thermal expansion was measured to be lower than for pure nickel films and comparable with commercial substrates, thus reducing thermal stresses and distortions. Taken as a whole, the full balance of properties of the sputter-deposited Ni-Mo-W films was shown to hold great potential.
As consumer products and medical devices shrink and gain new capabilities, interest in MEMS devices, two-dimensional electronics, and microscale sensors and energy harvesters expands beyond fundamental curiosity and transforms into demands for practical solutions. These intricate systems require a suite of novel materials, which must become durable in the face of extreme mechanical, thermal, and chemical conditions. The efforts by Hemker and his colleagues yielded ultra-strong, electrically conductive metallic films that possess a unique balance of properties, and hold great potential for future MEMS applications and the design of intricate multifunctional nanomachines.
Read the article in Science Advances.