Self-Actuated MEMS Nanomechanical Platform for High Temperature Testing Using Tantalum as a New Structural Material

Thin film materials usually have nanosized grains, which greatly enhances their room-temperature strength but may also increase creep. However, the size dependence of creep behavior is not well understood due to rapid grain growth at relatively low or even room temperatures. Recent development in thermally stable nanocrystalline alloy systems enables an opportunity to investigate the true effect of grain size on thin film creep. There is an urgent need to develop a reliable apparatus for thin film creep testing.

Compared with conventional nanomechanical test methods, MEMS-based approaches feature simple stress state, high through sample preparation and parallel testing. They eliminate misalignment and greatly enhance flexibility with testing protocols. Conventional MEMS nanomechanical test platforms use polycrystalline silicon (polysilicon) as the structural material and V-shaped thermal actuators. However, these platforms are actuated by Joule heating and operate at temperatures 300-500 °C higher than the substrate due to polysilicon’s low coefficient of thermal expansion (CTE, 𝛼𝑆𝑖=2.7 𝜇𝜖/°C). This causes heat flow to the specimen, complicating interpretation of the measurements. Also, only a few different materials, typically Au and Al, have been batch fabricated with these platforms.

The goal of this thesis is to develop a MEMS creep test platform that enables isothermal creep testing of many different materials. The realization implements thermal actuators (TAs) to apply a force to a specimen using the refractory metal 𝛼-Ta as a new structural material. Compared with silicon, Ta has more than doubled CTE (𝛼𝑇𝑎=5.9 𝜇𝜖/°C). Isothermal self-actuation is demonstrated simply by raising the ambient temperature due to the CTE mismatch between Ta and silicon substrate.

First, I address the well-known poor etchability of 𝛼-Ta films. Vertical sidewalls are obtained using conventional parallel-plate reactive ion etch (RIE) configuration for a 2.5 μm thick film. Then, the compressive stress build-up in Ta during a buffered hydrofluoric acid (BHF) release is discovered and attributed to, for the first time, hydrogen injection. The stress is largely recovered by a high vacuum anneal. After that, free-standing Ta TAs are fabricated following conventional surface micromachining methods and demonstrated by both conventional actuation by Jouleheating and passive self-actuation. Compared with polysilicon, the Ta TAs operate at 16 times lower actuation voltage and half the temperature change with the same structural design. The mechanical properties and grain size of Ta structural layer are found to remain stable for anneals up to 1000 °C. I also develop a new sacrificial material, AlN, whose selective removal is hydrogen free. In this way, the compressive stress build-up and high pre-stress prior to test is avoided. Then, using the Ta TAs and AlN sacrificial layer, an integrated platform with co-fabricated specimen is modelled, fabricated and tested. The Young’s modulus and creep behavior at 82 °C of a 110 nm thick Au specimen is measured. In principle, it can test creep at temperatures possibly as high as 700 °C. The process flow to build this platform is also, in principle, compatible with a wide variety of sputtered films. Furthermore, it is possible to test electrodeposited thin film materials. To show this, integration of the platform with an electrodeposited thermally stable Ni-W alloy is investigated. Telephone cord buckle delamination is observed due to hydrogen generation during the low-current-efficiency electrodeposition process. Two processing methods are demonstrated to obtain good adhesion between the platform and Ni-W film.