L. Mele

Mechanical Design and Characterization for MEMS Thin-Film Packaging

In this paper, a thin-film packaging approach is developed. It is meant to provide microelectromechanical systems (MEMS) devices with hermetic encapsulation that is sufficiently strong for transfer molding. A flat slab structure supported by columns is considered as basic geometry for the mechanical model. It takes into account both the plate deflection and the stress at the interface with the columns. To verify the model validity, thin-film packages are fabricated using silicon nitride as material for the capping layer. Both high- and low-temperature processes are used to fabricate the packages. The packages differ for the diameter of the columns (from 2 μm to 28 μm), the distances between columns (from 20 μm to 100 μm), and the capping layer thickness (from 3 μm to 7 μm). The packages are tested at different pressures up to 12.5 MPa (125 bar). Failure points agree well with the mechanical model. The largest package fabricated is a square package of 300 μm side length and with four columns (10 μm diameter) in the middle. It withstands a pressure of 10 MPa with a thin SiN capping layer with a thickness of 6 μm. Moreover, the packages are carried through grinding, dicing, and transfer molding, demonstrating that the presented thin-film encapsulation approach is robust enough for commercial first-level packaging.

An analytical model and verification for MEMS Pirani gauges

A new analytical model for the design of micromachined Pirani gauges operating in constant current mode is presented. This model expresses the pressure range as a closed-form analytical function of the design variables such as geometry and biasing. Furthermore, it yields simplified expressions for other performance parameters such as the sensitivity, output swing and power consumption. A Pirani gauge has been designed according to the presented model and has been fabricated and characterized in order to verify the validity of the model. The measurements match the theory closely. The model will be useful to designers who need to trade off performance against the costs of chip area and biasing power.

A Tube-Shaped Buried Pirani Gauge for Low Detection Limit With Small Footprint

We present a micromachined Pirani gauge that combines low detection limit and strongly reduced footprint. It consists of a tube-shaped resistor that is buried in the silicon substrate. The choice of the tube geometry gives the resistor a very high structural rigidity. This enables the fabrication of much longer resistors, thus shifting the detection limit toward lower pressures. In addition, since the resistor is buried under the silicon surface, its footprint is kept very small. The high stiffness allowed the fabrication of a 3-mm-long and 1.8-μm-thick poly-Si tube with a 1-μm gap without buckling and/or stiction problems. It shows a detection limit of 0.1 Pa for a noise level of 50 μV, and it has a footprint of only 0.012 mm2. This is an improvement of at least 20 times compared with Pirani gauges with the same detection limit. Pirani tubes of 1.6- and 0.4-mm lengths have also been designed, fabricated, and tested. The 0.4-mm-long tube shows a low pressure limit of 2 Pa, whereas the tube of 1.6 mm shows a low pressure limit of 0.2 Pa. The measured transfer functions correspond very well to the 1-D analytical model.

An all-in-one nanoreactor for high-resolution microscopy on nanomaterials at high pressures

We present a new MEMS nanoreactor fully integrated on a single die. It enables atomic-scale imaging of nanostructured materials under the high pressures and temperatures that are typical for many industrial applications (14 bar and 660 °C). The reactor can therefore be used to study the behavior of e.g. catalysts in a transmission electron microscope (TEM). It has a shallow channel (0.5 µm), which is made with surface micromachining techniques and contains pillars that prevent bulging. Integrated with the channel are very thin windows (15 nm) and a resistive heater. The reactor is very transparent, enabling the imaging of atomic lattice fringes with a spacing down to at least 0.15 nm.

Wafer-level assembly and sealing of a MEMS nanoreactor for in situ microscopy

This paper presents a new process for the fabrication of MEMS-based nanoreactors for in situ atomic-scale imaging of nanoparticles under relevant industrial conditions. The fabrication of the device is completed fully at wafer level in an ISO 5 clean room and it is based on silicon fusion bonding and thin film encapsulation for sealed lateral electrical feedthroughs. The fabrication process considerably improves the performances of previous nanoreactors. The wafer-level assembly allows faster preparation of devices, hydrocarbon contamination is no longer observed and the control of the channel height leads to a better flow reproducibility. The channel is shown to be sufficiently hermetic to work in the vacuum of a transmission electron microscope while a pressure of 100 kPa is maintained inside the nanoreactor. The transparency is demonstrated by the atomic scale imaging of YBCO nanoparticles, with a line spacing resolution of 0.19 nm.

A silicon carbide MEMS microhotplate for nanomaterial characterization in TEM

We report a SiC MEMS microhotplate designed for high temperature characterization of nanomaterials in transmission electron microscopes (TEMs). The microhotplate integrates, for the first time, a microheater of doped polycrystalline silicon carbide (poly-SiC) and electron-transparent windows of amorphous SiC (a-SiCx) on a freestanding membrane of undoped poly-SiC. Our work focuses on the development of the SiC layers by LPCVD, as well as on their combination in the fabrication process. The microhotplates were demonstrated to operate at temperatures well beyond 700°C.

Sputtered molybdenum as conductive material for high-temperature microhotplates

This paper presents a fabrication process for high-temperature MEMS microhotplates that uses sputtered molybdenum as a conductive material. Molybdenum has a high melting point (2693°C bulk) and is simpler to deposit and pattern in larger series than platinum. Molybdenum is sensitive to oxidation above 300°C, so during fabrication it is protected by PECVD silicon oxide and then covered by LPCVD SiN. The electrical resistivity is linear with the temperature up to 700°C at least. Molybdenum microhotplate has a higher maximum operating temperature than platinum which is demonstrated by the observation of the boiling of barium carbonate (BaCO3) microcrystals at 1360°C. Annealing at 1100°C is effective in extending the operating range. The molybdenum microhotplate performs far better than platinum also in terms of long-term resistance drift.

Self-cleaning mass calibration of a thermogravimetric device using a thin-film molybdenum

This paper presents a self-cleaning mass calibration procedure for a thermogravimetric (TC) device using molybdenum (Mo). A Mo thin-film is deposited by sputtering and patterned with known geometry on the device sample area using a standard lithography step thus giving accurate control of the mass of the sample under investigation. The device resonance frequency is measured while the temperature of the sample area is increased from room temperature to about 923 K using the integrated heater. First the Mo oxidizes. Then, at temperatures above 773 K the Mo trioxide (MoO3) evaporates. This causes a shift in resonance frequency that can be linked to the known initial mass of the Mo. An advantage of this method is that, the Mo leaves the device clean and ready for TG analysis (TGA) of other samples.

Low power PECVD SIC delay lines for optical coherence tomography in the visible

The scanning delay line is a key component of time-domain optical coherence tomography systems. It has evolved since its inception towards higher scan rates and simpler implementation. However, existing approaches still suffer from drawbacks in terms of size, cost and complexity, and are therefore not suitable for implementation using integrated optics. In this paper we report a rapid scanning delay line based on the thermo-optic effect of amorphous silicon carbide for application in the visible. The reported device attained line scan rates of 50 Hz and demonstrated a scan range of 0.4 mm. The devices use 40 times less power than similar architectures based on silicon.

Wafer level encapsulation techniques for a MEMS microreactor with integrated heat exchanger

This paper presents a MEMS chemical microreactor with integrated heat exchanger designed and fabricated by means of wafer level encapsulation techniques like low temperature (400°C) silicon fusion bonding and thin film encapsulation. The fabrication method results in a leak tight reaction cavity under 1 atm pressure difference during operation. Furthermore, the surface micromachining process leads to scale down the height of the microchannels and to achieve close contact between hot and cold microchannels along their whole length, maximizing heat exchanging capability of the device.