What is Porous Silicon Fabrication?

The Porous Silicon Fabrication technology uses selective formation and etching of porous silicon on standard silicon wafers to create micro-structures that are useful for a variety of MEMS devices. Posifa’s Porous Silicon Fabrication platform has been seamlessly integrated into standard off-the-shelf CMOS processes eliminating the need for more costly traditional MEMS processing that requires specialized equipment and facilities.. The benefits of such integration are two-fold. First, leveraging the proven reliability, quality, and scalability of CMOS fabrication, Posifa can provide the most economical sensing solutions to the most demanding, high-volume applications. Second, proven analog and digital components can be readily combined with sensing elements to deliver more compact and powerful solutions. Posifa has developed a portfolio of patents around Porous Silicon Fabrication and will continue to bring innovative products from this platform.

How Does Posifa’s MEMS Thermal Flow Sensor Work?

The MEMS thermal flow sensor die (i.e. the sensing element) is comprised of a resistive heater, and two clusters of thermocouples (thermopiles), each positioned symmetrically up- and downstream of the heater. The heater and the hot junctions of the thermopiles reside on a thermal isolation base that is built into the silicon substrate. The thermal isolation base does use open cavity or thin membrane, allowing the sensor to be resistant to clogging and pressure shock.

The thermopiles generate millivolt output when the heater (excited by either constant voltage or current) heats up the thermal isolation base (i.e. the Seebeck effect). When there is no flow across the sensor die, the differential output from the thermopiles is effectively zero. In a flow medium, the upstream thermopile is cooled by the flow medium. While the downstream one is also cooled it is heated at the same time due to heat transport from the heater in the flow direction. The resulting differential signal (downstream minus upstream) will correlate with the mass flow rate of the medium.

How Posifa‘s MEMS Micro-Pirani Vacuum Sensors Work

Based on the principle that the thermal conductivity of gases is proportional to pressure in the low vacuum range, Posifa’s micro-Pirani sensor measures tiny changes in the resistance of a heated element resulting from heat conducted to the surrounding gas. The heated element (also the measurement resistor) consists of a platinum thin film resistor residing on a thermal insulating membrane. The membrane is suspended over a micro-machined cavity that has a bottom surface parallel to the membrane, ensuring repeatable measurement of heat transfer. Because it needs only a small volume of gas to achieve accurate measurements, Posifa’s micro-Pirani sensors deliver ultra-low power consumption and fast response time. A reference resistor, built with the same material as the measurement resistor (ensuring the same thermal coefficient of resistance), resides on the sensor substrate and can be used by the sensor circuitry for ambient temperature compensation.