Tatjana Dankovic

Microelectromechanical system-based vacuum gauge for measuring pressure and outgassing rates in miniaturized vacuum microelectronic devices

Cr/Au meander-shaped resistors were fabricated on 0.2 μm thick square-shaped silicon nitride diaphragms with diaphragm dimensions ranging from 0.775 to 2.275 mm. The performance of these sensors was measured in a vacuum chamber as a function of resistor powers from 0.5 to 3 mW at pressures ranging from 2×10−4 to 760 Torr. The lengths of the meander-shaped resistors increase from 5.5 mm for the 0.775 mm diaphragm to 33.6 mm for the 2.275 mm diaphragm devices. It is shown that the pressure dependence of the devices is governed by the kinetic gas theory and that the devices can measure pressures from about 10 to 1×10−3 Torr. At a resistor power of 2 mW at 760 Torr, the 2.275 mm diaphragm device, which has the largest sensing area, exhibits the highest sensitivity.

Comparisons between Membrane, Bridge and Cantilever Miniaturized Resistive Vacuum Gauges

Using bulk micromachining, meander-shaped resistor elements consisting of 20 nm Cr and 200 nm Au were fabricated on 1 μm thick silicon nitride membranes, bridges, and cantilevers. The resistance change as a function of pressure depends strongly on the thermal resistance of the two metal lines connecting the heated resistor to the silicon bulk (cold junction) and on the thermal resistance of the silicon nitride. Relative resistance changes ranging from about 3% (small membrane) to 20% (bridge) per mW of input power were obtained when operating the devices in constant voltage mode. The pressure where maximum sensitivity of these gauges occurs depends on the distance ‘d’ between the periphery of the heated resistor element and the silicon cold junction. Devices with ‘d’ ranging from 50 μm to 1,200 μm were fabricated. Assuming that pressures can be reliably measured above the 10% and below the 90% points of the resistance versus pressure curve, the range of these devices is about two orders of magnitude. By integrating two devices, one with d = 65 μm and one with d = 1,200 μm on the same chip and connecting them in series, the range can be increased by about a factor of three. By fabricating the cantilever devices so that they curl upon release, it will be shown that these devices also exhibit larger range due to varying ‘d’.

11.5: A MEMS-based vacuum gauge for measuring pressure and out-gassing rates in miniaturized vacuum microelectronic devices

Cr/Au meander-shaped resistors were fabricated on 200nm thick square silicon nitride diaphragms on silicon with diaphragm dimensions of 0.775mm, 1.025mm, 1.275mm, 1.775mm, and 2.275mm. At 760Torr, the resistors were heated to about 20°C–60°C above ambient by Joules heating and the resistances were monitored as a function of pressure from 760Torr to 2×10−4Torr. As expected from kinetic theory, the sensitivity of the gauges shifted from high to low pressure with increasing diaphragm size (distance from the heat source to the cold junction). Measurements were taken at powers ranging from 0.6mW to 6mW. The sensors were applied to measure potential vacuum bursts due to outgassing events in field emitters and to measure the pressure inside of glass capillaries.

Integration of a MEMS-type vacuum pump with a MEMS-type Pirani pressure gauge

This paper describes the use of a miniature Pirani pressure sensor to measure the properties of a miniature vacuum pump. The construction and fabrication process of the integrated device is presented. Results of characterization of both MEMS devices performed separately in reference vacuum system and after their integration are shown.

A MEMS-based resistive vacuum gauge with voltage readout

A 20nm thick Ni resistor element was fabricated on a 1μm thick, 400μm wide silicon nitride bridge via bulk micromachining. By applying a given power to the resistor its temperature increases, as the pressure decreases, since fewer gas particles are available to transfer heat away from the resistor. Rather than sensing the resistance change, a thin film thermocouple consisting of 20nm Ni and 20nm Cr was fabricated in the center of the resistor element, allowing the direct measurement of a voltage signal. It will be shown that by fabricating two gauges with different ds, but identical thermal resistances, the measurable pressure range can be extended from two orders of magnitude to about 4 orders of magnitude. One of these gauges was incorporated into a miniature vacuum chamber with a volume of about 1cm3 to observe potential leakage and pressure bursts from operating a field emitter device in close proximity.

Thermal-based MEMS vacuum gauges for measuring pressures from 10 −2 Torr to 10 −6 Torr

Bulk micromachined Pirani gauges were designed and fabricated using an H-shaped 1 μm thick silicon nitride support structure and 30 nm of Ni for the meander resistors and connecting metal lines. The distance d between the resistor element and the cold junction was chosen to be 1200 μm. From theoretical and experimental considerations, it is shown that pressures below 10-4 Torr can be measured if the thermal resistances of the two metal lines which connect the resistor element to the cold junction and the thermal support structures are about or larger than 1×106 K/W. Also, the distance d between the resistor element and the cold junction needs to be near or larger than 1000 μm.

Fabrication of plastic micro-channels for microfluidics solvent extraction

In this work plastic micro channel systems were investigated as a potential device for micro solvent extraction of rare earth elements. The proposed microfluidic structures are made by laser welding of three layers of inexpensive thermoplastic films which form separate paths (top and bottom channels) for each of the immiscible fluids. The middle layer is perforated in order to provide contact between two fluids and to enable the extraction process. Experiments were performed to show that two different immiscible fluids (water and 1-octanol) can flow through the fabricated device and exit at separate outlets without mixing even when those fluids get into close contact within the main channel. Experimental results for single devices show that immiscible fluids can be brought into intimate contact and then separated with compliant polymeric microfluidic devices. The transfer of a compound from one immiscible fluid to the other was verified by dye exchange between the immiscible fluids. The same fabrication method is a promising technique for fabrication of massively parallel systems with larger throughput.Copyright

Electrostatically Actuated Compliant Microvalve

We present an electrostatically operated normally opened microvalve for gas flow control. The valve is made of thermoplastic materials and uses a new fabrication method. The voltage required to close the 0.9 mm wide microvalve was ∼350 V for fluid (air) pressure ∼1kPa.A new technique [1] has been developed to fabricate valves, micromixer [2, 3] and other microfluidic structures, by patterned welding of compliant thermoplastic films using Universal Laser System CO2 laser. The normally opened valve is electrostatically actuated by applying the voltage on its metalized thermoplastic surfaces. There is no movable membrane or cantilever which closes the fluid path as reported in many other electrostatic microvalves. The walls of the channel collapse toward each other when sufficient voltage is applied, thus effectively closing the fluid path. The material used is 1.4 μm thick Mylar™ sheets (DuPont) coated on one side with ∼10nm of gold.Copyright