Piezo flexure stages are positioning devices with a high degree of resolution. These motion stages contain a piezoelectric actuator that converts an electrical signal into displacement based on the reverse piezoelectric effect, which is the ability of a piezo electric material to elongate when a voltage is applied. When a voltage (e.g., 0-150 VDC) is applied to the actuator, the stage moves in proportion to the applied voltage. Yet, the exact position remains unknown, because piezo electric actuators are inherently non-linear and exhibit hysteresis.
Fig 1 Schematic view of a Piezo Flexure Stage
Fig 2 Piezo stage with MTI Cap probe embedded
Fig 3 Piezo stage in-situ wafer quality
Embedding a displacement sensor in the stage enables precision measurement of this motion. By using displacement as feedback, a stage controller can move the piezo stage to an exact position. Typically, the displacement sensor is either a capacitive probe or a strain gauge. Capacitive sensors measure absolute distance directly and without contact. Strain gauge sensors are bonded to the structure of the stage, derive distance information from strain, and are categorized as either piezo resistive or metal foil.
Capacitive sensors provide very high linearity and the best long-term stability. Strain gauge sensors cost less than capacitive sensors and are sufficient for many applications. However, strain gauge sensors cannot match the linearity and stability that is associated with the direct measurement of capacitive sensors. To highlight these differences, the basic operating principles of both types of displacement sensors are explained below.
How Capacitive Probes Work
Capacitive probes use electric fields to measure the distance from the sensor face to the target surface. A guard ring steers the direction of sensing practically straight toward the target, ignoring the potential influences of adjacent surfaces that could disrupt the reading. Consequently, only the target area is measured. The guard ring minimizes external influences and helps to produce a linear sensing field.
Typically, the targets for capacitive probes are conductive and grounded. The electric field senses only the surface layer of the conductive target and is neither affected by the type of metal nor its magnetic properties. In addition, capacitive sensing is not adversely affected by temperature, vacuum, air pressure, or magnetic fields.