The development of SMD’s proprietary thin-film process technology has yielded economic and performance benefits, enabling the company to considerably expand its served applications to include medical, industrial, automotive, and process control applications. Strain Measurement Devices’ thin-film sensors have significant advantages over silicon, foil strain gauge, capacitance, and other sensing technologies, at competitive prices.
There are many advantages to the thin film method of deposition; one of the major benefits is the true molecular bond between the fixture (transducer) and the deposited strain sensing films. This flexure/dielectric interface is devoid of organic materials and oxygen which could absorb moisture or react in time. Sputtered thin film strain gauges are immune to the problems of organic backing materials found in bonded foil strain gauges. The lack of organics also means that elevated temperature operations to 250°C is within reach of these devices.
Thin film strain gauges are directly deposited onto sensing fixtures (e.g., pressure transducer diaphragms or load cells) in a vacuum deposition chamber by sputtering. The sputtering system also provides process control so that films may be deposited with good batch-to-batch repeat-ability. Significantly tighter control of bridge resistance and zero balance are routinely achieved in production than are possible with conventional strain gauges. Long-term stability on thin film pressure transducers have been measured at better than 0.02% of full range output in an accelerated six-year lift test. Film deposition rate is approximately linear with input power and the rate may be slowed sufficiently so that very thin films can be deposited by integrating low powers over short periods of time. By modifying the sputtering parameters, strain gauge bridges can be fabricated which meet demanding insulation resistance specifications such as CENELEC (500Vac).
The technology which has been used for many years for the production of ultra-stable precision resistors is the strain gauge technology of the future. Some of the more interesting applications of the thin film sensor technology is within the medical field. When delivery of fluids to the body via infusion pumps, insulin pumps, internal 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 repeat-ability, ability to survive the rough handling and accuracy required to be successful in these pump applications.