E. Iervolino

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.

Thermogravimetric device with integrated thermal actuators

This paper presents the first MEMS device for ThermoGravimetric Analysis (TGA) with integrated thermal actuators. It consists of a sensing cantilever paddle connected to two separated thermal actuators, one at each side of the cantilever. Moreover, it has an integrated thermocouple that allows combined TGA and calorimetric measurements. To demonstrate the device performance TGA of copper sulfate pentahydrate (CuSO4·5H2O) samples has been performed. The operation range for the TGA device is 40 pg up to 0.1 µg for the mass and 25 up to 650 °C for the temperature. The mass sensitivity is about 200 Hz for a 1 ng sample on the 10 kHz resonance frequency.

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.

MEMS for Thermogravimetry: Fully Integrated Device for Inspection of Nanomasses

This paper presents a microelectromechanical-systems device for thermogravimetric analysis (TGA) with integrated thermal actuators. It consists of a sensing cantilever paddle connected to two separated thermal actuators, one at each side of the cantilever. An integrated thermocouple allows to measure directly the temperature difference between the heater at the tip of the cantilever paddle and the device silicon frame. The cantilever paddle vibration amplitude (frequency) is measured with an integrated piezoresistor. The temperature dependence of the resonance frequency on local heating with the integrated heater is investigated. Mass and temperature calibrations are performed from 0 to 6 ng and from 300 K to 823 K, respectively. To demonstrate the device performance, TGA of polyamide 6 and paraffin samples is carried out. TGA can be performed with the presented device in the temperature range from 298 K up to 920 K for sample masses as small as 0.8 ng. The mass sensitivity is about 164 Hz/ng at ambient temperature.

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.

A novel 3-D tube-shaped buried poly-Si Pirani gauge for extended dynamic range with small footprint

We have developed a novel micromachined Pirani vacuum gauge employing a tube-shaped 3-D structure with enhanced rigidity and very low footprint because it is completely buried inside the silicon substrate. A CMOS-compatible three masks process was used to realize this new design, which allowed the fabrication, without stiction problems of a 3.2 mm long Pirani poly-Si tube with a 1.5 µm gap. Compared to previous gauges, the dynamic range is 100 times larger, ranging from 20 mPa to 10 kPa with a maximum sensitivity of 17.5 mV/decade.

Thermal analysis of peptides with a calorimeterchip

In this paper we describe a simple method to measure the thermal conductivity and thermal diffusivity of three related biochemicals: an amino acid, a peptide and a protein in aqueous solution using a liquid calorimeter chip. The measurements are performed by applying an AC voltage to an integrated heater at 10 mHz and 1 Hz and measuring the output voltage of the thermopile. The biochemicals solution tested included: L-lysine dissolved in water at different concentrations (50; 10; 1; 0.1 wt%); α-mating factor dissolved in water at 0.1 wt% and the bovine serum albumin (BSA) at 1 wt%.