All microscope stages, from any manufacturer, will present some mechanical errors. Because they repeat movements to the sub-micron level, even tiny errors affect accuracy, especially over longer travel ranges. It’s the correction of these errors, together with precise in-house machining and expertise in stage assembly, that underpins Prior’s reputation as a global leader in XY stage manufacturing.

In this article, we look at the main causes of mechanical errors in microscope stages and how they are mitigated to ensure the accurate, repeatable movements that are essential for microscopy applications such as semiconductor scanning or high-content screening.

Microscope stage design: why mechanical errors are unavoidable

Most motorized microscope stages have a similar format: the sample is positioned in the middle of the stage, and left-to-right (X) and forward-to-back (Y) movements are driven by actuators positioned at the edges. This means there is some distance between the motors and the area of interest: any inherent difference in levels of accuracy in the ball screws will be amplified by this distance. Even the most mechanically accurate ball screws will have slight differences which affect repeatability at the sub-micron level.

XY stages use two motors that operate on different planes and this will introduce skew. Over long travel ranges – say 300 mm – a slight error will have an incremental effect. Instead of moving in a completely straight line, the stage moves at a slight angle, introducing a radius.

These angular movement errors are known as Abbe errors.

As an example, if we move 100 mm at the edge of the stage where the ball screw is fitted, the skew means that the actual distance traveled by the object at the center of the stage is 100.003 mm. Fortunately, this 3 µm error can be corrected to ensure that the stage provides an accuracy that would be impossible to achieve through mechanical means alone.

For the user of the stage, the error correction works seamlessly and they can be confident that the stage will move precisely the required distance.

Expert, precision manufacturing

Although mechanical errors are unavoidable, Prior’s expertise in designing and manufacturing stages means that localized errors can be resolved during mechanical assembly, leaving only errors affecting the entire length of the travel range needing to be corrected. Additional corrections would add time to the manufacturing process for no discernable benefit over these travel ranges.

Because our stages are machined in our UK factory, we maintain complete control over the quality and precision of the entire manufacturing process.

Correcting for errors: Prior Scientific’s Expertise

In the section above, we outlined the accuracy and repeatability errors that affect all standard motorized microscope stages from any manufacturer. In this section we cover the steps Prior Scientific takes to correct for these errors to deliver accurate, stable and reliable movement. Error correction is a standard method used by controllers and software to recognize and adapt to the errors so they do not affect the accuracy or repeatability of any experiments.

Depending on the type of stage and the application involved, Prior offers two types of error correction:

4-number correction

Unsurprisingly, this method uses four generated numbers to correct the motion of the stage. The four numbers are the X error, the Y error, and two skew errors. These values are generated individually for each stage during testing and calibration. Each stage will have unique mechanical errors, and these four correction numbers are stored electronically with the stage at the point of manufacture.

4 number correction provides a best fit straight line through X and a best fit straight line through Y. This is an appropriate level of correction for most applications in stages with ranges up to those of Prior’s H101A stage (114 x 75 mm).

For stages with longer travel ranges – such as Prior’s H116 or H112 – however, a different correction method is required:

Full stage mapping

As described above, 4-number correction applies an average best fit across a line of travel. Over longer travel ranges, full-stage mapping provides greater accuracy. This method still provides correction values, but these are applied for points over the full area of travel of the stage at regular intervals, known as mapped locations.  An average correction value for positions between these points is also applied, and the distance between the mapped locations is set to a sufficiently small distance so that this averaging does not affect stage performance. This ensures that positional correction is applied at the resolution of the stage, without the requirement to map every single possible position.

In full stage mapping, stages are mapped during test and calibration using a specially manufactured grid, called a graticule. The graticule is segmented into finer intersections, each of which is mapped as a known position, and the error is recorded. This is the basis for defining the mapped locations, and setting the distance between them for averaging.

This method is more precise over longer travel ranges because instead of correcting an average across a line of travel, the correction factors are applied to an exact point. Longer travel ranges are more likely to have localized positional variation, so full mapping ensures that Abbe errors are not observable by the end user. In semiconductor wafer scanning, which can involve scanning over distances of 300mm or more a high magnifications, this higher level of correction is essential.

Plug and play

In a Prior III ProScan  stage, all correction data is stored on the stage in an EEPROM (Electrically Erasable Programmable Read Only Memory). When connected to a ProScan III controller, the EEPROM is read and the controller automatically applies the correction factors. This means that if a system including a Prior stage needs to be upgraded, the correction factors determined during manufacture will be retained if, for example, the controller needs to be replaced. Prior stages have extremely long lifetimes including a 5-year warranty, and storing correction factors with the stage ensure that Prior systems can evolve with customer needs without requiring recalibration.

Precise, reliable movements – whatever your application

Prior’s motorized XY stages are compatible with most brands of microscope available on the market and offer a choice of movement range and load capacity to suit most life science and industrial applications. Contact us to discuss your application.