Thin Film Sensor Deposition
Thin film sensor deposition process is nothing new to industry. Many applications, from the manufacture of complex microprocessors to the manufacture of precision resistors for strain gauges, use this technique. In the case of strain gauges, thin film sensor deposition directly onto the stressed substrate is an option to foil strain gauges epoxied to the substrate, resistive strain gauges and silicon strain gauges that eliminates many of the problems of these later technologies. The Wheatstone bridge improved and popularized by Sir Charles Wheatstone in 1843 is well known but the application of thin film vacuum deposition to this old tried-and-true circuit is not so well understood.
Strain Measurement Devices has been using thin film vacuum deposition process to bond strain gauges directly to 15-4 stainless steel, Inconel, sapphire, and titanium for 30 years. The process begins by preparing the surface of the substrate with a diamond slurry to remove all surface pinholes and cracks. The next step is the deposition of an oxide layer to insulate the circuit from the metal substrate. Following this, a thin film resistive alloy is sputtered over the oxide layer. This latter film is laser trimmed under power to produce the four resistors of the Wheatstone bridge. Bonding pads are applied and wired to the circuit to provide a power egress and the whole thing is coated with an encapsulation to protect the thin film (see representation below).
Over time, the thin film strain gauge has proven itself to be the preferred means for measuring strain in critical applications where small size, robust performance, long term stability and superior accuracy are required. One of the more interesting applications of the thin film sensor technology is in the medical field. When delivery of fluids to the body via infusion pumps, insulin pumps, enteral feed pumps, and wound irrigation systems is interrupted by a pinched tube or pump failure there may be disastrous consequences. Engineers often use what they call ‘tube sensors’ to monitor pressure in these pump systems by measuring the force exerted onto a sensor pressed against the expanding walls of a polyurethane or PVC tubing or they place the sensor behind the pump to record pressures as the pump backs up against the sensor during operation. Thin film sensors have the repeatability, ability to survive the rough handling and accuracy required to be successful in these pump applications.
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