Abstract
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’.
Highlights
Small vacuum systems are gaining importance for several applications such as micro electromechanical systems-based (MEMS) resonators, RF switches and energy harvesters
Meander-shaped Cr (20 nm)/Au (200 nm) resistors with a surface area of 6.3 × 10−4 cm2 were fabricated on 1 μm thick, LPCVD deposited, silicon nitride membranes, bridges and cantilevers using silicon bulk micromachining
At atmospheric pressure (760 Torr) a constant voltage was applied to the resistors and the current was adjusted so that about 2 mW of power was dissipated causing the resistors to heat to several degrees above room temperature
Summary
Small vacuum systems are gaining importance for several applications such as micro electromechanical systems-based (MEMS) resonators, RF switches and energy harvesters. The most popular miniaturized vacuum sensor is the Pirani sensor due to its relative ease of manufacturing, its wide pressure range and low power consumption. It consists of micromachined resistors in single element form (micro wires) or meander-shape. These resistors are free standing or fabricated on thermally insulating supports, such as membranes, bridges or cantilevers. As the resistor elements are powered, ranging from several microwatts to milliwatts, they heat up by several degrees near atmospheric pressures, increasing their resistance. A similar result is obtained with the cantilever device which, due to built-in stress gradients in the nitride film, forms an out-of-plane cylinder resulting in a distributed ‘d’ device
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