Multi-stack wafer bonding is one of the most promising fabrication techniques for creating three-dimensional (3D) microstructures. However, there are several bonding issues that have to be faced and overcome to build multilayered structures successfully. Among these are: (1) chemical residues on surfaces to be bonded originating from the fabrication processes prior to bonding; (2) increased stiffness due to multiple bonded wafers and/or thick wafers; (3) bonding tool effects; (4) defect propagation to other wafer-levels after high-temperature annealing cycles. The problems and the solutions presented here are readily applicable to any microelectromechanical systems project involving the fabrication of multi-stack structures of two or more wafers containing intricate geometries and large etched areas.

Multi-stack silicon-direct wafer bonding for 3D MEMS manufacturing

In Part I of this two-part paper, the design approach for a microturbopump was presented. This second part describes the fabrication and experimental characterization of the demo microturbopump device, which includes hydrostatic bearings, a spiral-groove viscous pump, and a multistage microturbine. The device is composed of five wafers: one glass wafer, one silicon-on-insulator (SOI) wafer, and three silicon wafers. The silicon and SOI wafers are patterned using shallow and deep reactive ion etching (total of 14 masks), while the Pyrex glass wafer was ultrasonically drilled. Anodic bonding, fusion bonding, and manual assembly with alignment structures were then used to complete the device and enclose the 4-mm-diameter rotor. The approach allowed the microfabrication of unique interdigitated blade rows in the microturbine and interchangeable parts for flexible testing. After completion of the device, bearings were first tested in static and dynamic conditions. Then, the turbine was characterized with compressed air only and spun up to 330 000 r/min, producing 0.38 W of mechanical power. The pump performance map was also completely characterized for speeds up to 120 000 r/min, showing a maximum pump flow rate of 9 mg/s and maximum pressure rise of 240 kPa. In a turbopump system performance test using compressed air to the turbine and water in the pump, the rotor was spun up to 116 000 r/min, which corresponds to 25 m/s in tip speed. At this condition, the turbine produced 0.073 W of mechanical power with 41 kPa of differential pressure and 24 mg/s of flow rate, and the pump pressurized water by 88 kPa with a flow rate of 4 mg/s, maintaining constant efficiency of 7.2% over the operating range. Out of the total power produced by the turbine, 10% was consumed by the viscous pump, while the rest was dissipated by other components through viscous drag. The system-level predictions by models introduced in Part I also match the measured performance, suggesting that a valid design basis has been established for this type of rotating micromachine.

https://crn2.3it.usherbrooke.ca/guide_sb/publications/ref_1782_2011_lee_a_silicon_microturbopump_part2.pdf

A Silicon Microturbopump for a Rankine-Cycle Power-Generation Microsystem—Part II: Fabrication and Characterization

Atomic MEMS technology is an emerging multidisciplinary subject which benefits from different fields, including microfabrication, laser technique and atomic physics, etc. This paper gives an overview of atomic MEMS and discusses the challenges faced in the design and manufacture of atomic MEMS devices.

Review of atomic MEMS: driving technologies and challenges

A microscale jet ejector driven by ethanol vapor is designed and tested to induce a suction draft using a supersonic converging–diverging micronozzle. A three-dimensional axisymmetric nozzle is fabricated using electro-discharge machining to produce a throat diameter of 187 µm with an expansion ratio of 3:1. The motive nozzle achieves a design mass flow efficiency of 93% compared to isentropic calculations. Two different ejector area ratios are compared using ethanol vapor and nitrogen gas separately to motivate and entrain ambient air. The experimental data indicate that the ejector can produce a sufficient suction draft to satisfy both microengine mass flow and power off-take requirements to enable its substitution for high-speed microscale pumping turbomachinery.

https://iopscience.iop.org/article/10.1088/0960-1317/20/4/045019/pdf

Experimental results for a microscale ethanol vapor jet ejector

Due to several special characteristics of magnetic suspension, bearingless drives may be suitable for turbine application. These characteristics range from operability without lubrication, the absence of mechanical friction and of sliding seals to variable stiffness and damping, inherent bearing diagnostics function, low maintenance and long life time. However, there are several drawbacks which might be critical to turbine operation. Using a bearingless disk drive prototype, experiments with a turbine and compressor wheel at high rotational speeds shall provide conclusive information about the rotor behavior over the entire speed range. The results will show if, and under which assumptions, the use of bearingless disk drives for micro turbine applications seems feasible.

Suitability investigation of a bearingless disk drive for micro turbine applications

A microelectromechanical system (MEMS) turbocharger has been designed, fabricated, and tested as part of a Massachusetts Institute of Technology program aimed at producing a microfabricated gas turbine engine for portable power applications. A gas turbine engine requires high-speed high-efficiency turbomachinery operating at tip speeds of several hundred meters per second. This MEMS turbocharger serves to demonstrate these requirements. The turbocharger's silicon rotor, which is supported on hydrostatic gas thrust and journal bearings in a silicon stator housing, was spun to 480 000 rpm, corresponding to a tip speed of 200 m/s. This paper discusses critical fabrication processes that enabled the capabilities of this device. Operational issues and test results are also presented. The turbocharger's compressor demonstrated a pressure ratio of 1.21 at a mass flow rate of 0.13 g/s, with a combined compressor-turbine spool efficiency of 0.24. Under these conditions, the turbine produced about 5 W of power. Results from the simultaneous operation of a spinning rotor and burning combustor within the microscale turbocharger are also presented. Experimental results compare well with analytical models and computations.

Linhvu Ho

Papers

Fabrication and Testing of a High-Speed Microscale Turbocharger