F. Santagata

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.

Fully back-end TSV process by Cu electro-less plating for 3D smart sensor systems

A fully back-end process for high-aspect ratio through-silicon vias (TSVs) for 3D smart sensor systems is developed. Atomic layer deposition of TiN provides a highly conformal barrier as well as a seed layer for metal plating. Cu electro-less plating on the chemically activated TiN surfaces is applied to uniformly fill the TSVs in a significantly shorter time (2 h for 300 μm deep and 20 μm wide TSVs) than with Cu bottom-up electroplating (>20 h). The process is CMOS compatible and can be performed after the last metalization step, making it a fully back-end process (VIA-last approach). Wafers containing metal interconnections on both sides are in fact used as demonstrator. Four-terminal 3D Kelvin structures are fabricated and characterized. An average resistance value of 650 mΩ is measured for 300 μm deep TSVs with an aspect ratio of 15. The crosstalk between adjacent TSVs is also measured by means of S-parameters characterization on dedicated RF test structures. The closest TSVs (75 μm) show a reciprocal crosstalk of less than −20 dB at 30 GHz.

CNT bundles growth on microhotplates for direct measurement of their thermal properties

Vertically aligned Carbon Nanotubes (CNT) arrays were successfully grown on top of a freestanding microhotplate, to investigate the thermal dissipation properties of CNT bundles and their applicability as heat exchanger. Two CNT configurations are employed: a group of six bundles, each with a diameter of 20 μm, and a single CNT bundle with a diameter of 200 μm. In both configurations the bundles are 70 μm high. The microhotplate consists of a platinum thin film microheater integrated on a freestanding silicon nitride membrane. The microhotplate is used as heat source and as temperature sensor. Results show that at 300 °C, 20% and 31% of power can be saved with the circular six and single bundle configurations, respectively.

Miniaturized particulate matter sensor for portable air quality monitoring devices

This paper presents a MEMS particulate matter (PM) sensor for portable air quality detection. The small size (5.2 mm × 4.2 mm × 1 mm) of the sensor allows integration in portable devices. Furthermore, an unsophisticated fabrication process is required which results in higher yield and consequently lower cost. The principle used to detect PM is based on light scattering in a micro fabricated chamber, formed by two micro machined silicon chips, one containing a laser diode and the other a photodiode, mounted on top of each other. The presence of PM in the micro fabricated chamber is detected by an increase of the light scattered and sensed by the photodiode. The principle is validated by exposing the sensor to tobacco smoke, which is a common PM source. Preliminary measurements show that the sensor is capable of detecting the presence of tobacco smoke. The sensor output (1.235V) is in a factor of 5% higher in the presence of tobacco smoke than in clean air.

Carbon Nanotube based heat-sink for solid state lighting

A new carbon-nanotube-based (CNTs) heat sink is developed. Due to their high thermal conductivity and aspect ratio, CNT boundles are used as fins of the heat sink. Fins as high as 300 μm with an aspect ratio of 30 are fabricated. For the thermal characterization of the heat sink, a microheater is integrated with the heat-sink and it is also used as temperature sensor. It is realized by using the low doped silicon bulk as electrical resistor. The sensor shows a sensitivity of 0.6 Ω/K. A thermal characterization is performed to evaluate the heat dissipated by the CNT-based heat-sink. Results show that up to 18% of power reduction can be achieved with the proposed CNTs-based heat-sink configuration.