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High fidelity

Ensuring nanopositioner accuracy requires sensor monitoring and careful x-y stage design.

From oemagazine June 2002
31 June 2002, SPIE Newsroom. DOI: 10.1117/2.5200206.0007

A nanopositioner is a positioning device with nanometer or even sub-nanometer resolution, precision, and accuracy. Flexure mechanisms commonly provide the ultra-precise motion. When the required movement is on the order of tens of micrometers, the flexure stages (nanopositioning stages or nanopositioners) are driven by piezoelectric devices.

To function properly, a nanopositioner must be able to accurately track its motion. Measurement accuracy should not be confused with measurement precision, though both are needed in a good nanopositioner. Accuracy refers to the trueness of the measurement, while precision refers to the number of significant digits to which the instrument measures. An instrument can be no more precise than allowed by its absolute accuracy. Position accuracy is achieved by a closed-loop configuration in which the reading of a position sensor is fed back to control electronics that maintain the position. Capacitive sensors are very high-resolution, non-contact sensors and are one of the best choices as position sensors.

The position accuracy of a stage primarily depends on two factors: the sensor linearity and the mechanical design of the stage. The linearity of the sensor is usually checked with an interferometer. Deviations from linearity are roughly parabolic. To achieve high-position accuracy, the sensor reading is compensated by a microprocessor using higher order (fourth to sixth) polynomial corrections. The difference between the actual distance and the measured distance forms a residual curve (see figure). A linearity error of 0.1%—for example, 100 nm of uncertainty over a 100-µm range of motion—is easily achieved. Although 0.1% linearity error over the entire range is low, local variations in the residual gradients can be significant though still within the quoted value. The use of a linear position sensor and higher-order polynomial fits can reduce errors to as low as 0.01%.

An accurate nanopositioner shows minimal deviation from linearity for a plot of actual versus commanded position. Microprocessor compensation can correct performance.

stage design

The mechanical design of the stage is crucial to achieving positional accuracy. A proper design can minimize position errors. Several factors can affect position accuracy and are associated more with mechanical design than with the electronics. Some stage designs incorporate two similar piezos to produce motion in one axis. This approach increases the stiffness of the stage and results in a faster response. Any two piezos expand differently with the same voltage, however, resulting into unwanted stage rotation that may not be detected by the motion sensors. A single piezo design provides higher accuracy.

The error resulting from the offset of measurement from the motion axis (Abbe error) can be more serious. If a sample mounted on an x-y-z stage is far from the x-y sensor position, there can be a difference in its actual x-y position and its measured position. To minimize Abbe errors and thus to increase position accuracy, one needs to either minimize the offset of the two axes or calibrate the nanopositioner motion at the actual position of the sample.

Temperature changes can also affect accuracy. Even a change as small as 1°C can cause the center of a nanopositioner (with a range of motion, for example, of 100 µm) to shift by as much as 1 µm. This shift cannot be detected by the motion sensors. Choosing a material with a low coefficient of thermal expansion (CTE), such as Invar, is one method for minimizing thermal drift. Sometimes low CTE materials are not the best choice; it may be more important to match the nanopositioner to the material of the surrounding support structure. Feedback control can also monitor and correct for thermal effects. Alternative mounting schemes such as kinematic mounting can minimize the thermal drift, which is usually a long-term effect.

Ensuring the accuracy of nanopositioners can be challenging. By remaining aware of the primary error sources and compensating for them, however, designers can engineer effective devices. oe

Katerina Moloni

Katerina Moloni is vice president of marketing at Piezomax Technologies, Middleton, WI.