Many applications use a manufacturing process known as thin-film sputtering deposition. The typical sputtering process uses an ion beam to impact the surface of a sputter material such as gold, silver, or other metal or metallic oxide. In so doing, some of the sputter-material atoms get knocked into free space. The atoms bond with a substrate material to form a thin coating that may be only a few atoms thick.

In the area of strain sensors, the sputtering-deposition technique lets manufacturers form sensors directly on the stressed substrate. The sensor becomes an integral part of the assembly, instead of being bonded to the stressed surface as are foil, resistive, and silicon strain gages. Combined with a Wheatstone bridge, thin-film sputtering-deposition strain gages eliminate many of the problems seen with these other measuring techniques, such as bonding separation or creep.

Almost any material can be used as a substrate for the sensor including stainless steel, Inconel, Hastaloy, aluminium, sapphire, and titanium. The process begins by preparing the surface of the substrate with a diamond slurry to remove all surface pinholes and cracks. A dielectric layer is first deposited on the substrate to insulate circuit power from the underlying metal. Then a thin film of resistive alloy is sputtered over the dielectric layer. This layer is laser trimmed under power to produce the balanced resistors of the Wheatstone bridge. Wires attached to bonding pads applied to the circuit provide power egress. An encapsulation layer coats the final assembly to protect the thin film.

Thin-film strain gages have become the preferred means of measuring strain in critical applications where the need is for small size, robust performance, long-term stability, and superior accuracy. One such area is the medical field, where engineers often use tube sensors to monitor pressure in pump systems. Patients may suffer disastrous consequences if the delivery of fluids to the body is interrupted by a pinched tube or pump failure. Tube sensors measure the force exerted by the expanding wall of a polyurethane or PVC tube. A pinched tube creates a backpressure that forces the tube to expand, boosting pressure on the sensor to trigger a blocked tube alarm. A tube with lower-than-normal pressure may indicate a stopped pump or disconnected tube. MD

Strain Measurement Devices Inc. (www.smdsensors.com) supplied information for this column.

Edited by Robert Repas