Sensor Performance Key to Process Improvement

Consumer demand for higher quality and lower cost has driven improvements in automated manufacturing. Semiconductor manufacturing is a prime example where precision position feedback is critical to product quality, and the speed of processing is critical to cost. The level of technology integration the semiconductor industry utilizes is finding its way into nearly every automated manufacturing process, regardless of the cleanliness of the environment or the end product being manufactured.

Eddy current, noncontact displacement sensors can provide submicron resolution in harsh environments, as well as in the sterility of a semiconductor cleanroom.

How they work
Eddy current sensors are very different from the common inductive proximity sensor. Where proximity sensors typically employ a colpitts oscillator circuit, eddy current displacement sensors use a fixed-frequency crystal oscillator and bridge circuit, creating a variable impedance transducer. The bridge is tuned to be in balance with the wire wound coil located in the probe. The coil is typically excited with 1MHz RF. The resulting oscillating electromagnetic field induces eddy current in any electrically conductive material -- or target -- that enters the field.

The induced eddy current produces a corresponding electromagnetic field, which reacts with the probe field, causing an impedance change in the probe circuit. The resulting bridge imbalance is linearized and amplified, producing an analog voltage or current output proportional to the distance between the probe and the target.

Measurement
Unlike an inductive proximity sensor, eddy current technology provides a linearized analog output proportional to the distance from the sensor tip to the target. When referencing from the fixed sensor position, dimensions can be determined. In progressive die-stamping operations, accurate dimensioning of such things as boss height and bend angle can be determined.

Position
With submicron resolution, eddy current sensors are used in stage positioning in semiconductor process tools. This same precision has been applied to insert location in plastic molding processes.

Displacement
Being a robust technology that can handle high temperatures and pressures, they have been used in plastic injection molding applications monitoring things like core shift and mold separation during injection.

Go/no-go
In some instances, the resolution and hysteresis of an inductive proximity sensor typically used in go/no-go applications just will not provide reliable process control. Eddy current sensors have been used in a wide variety of go/no-go applications. A common application is thread detection in automotive parts. Another example is part location prior to automated welding or staking operations.