Low Magnetic Field Tables
Certain applications require that our positioning tables have minimal external magnetic fields. Some of these, such as mapping the field strengths of “wigglers” for synchrotron studies, take place at atmospheric pressure, while others (E-beam and focused ion beam systems, for example) require high vacuum preparation as well. We have employed a sensitive flux-gate gaussmeter to map the field strength around our positioning tables. Since the primary construction material is non-magnetic aluminum, the overall field strength is low. The worst component proved to be the small Nd-Fe-B magnet used to activate our Hall-effect limit sensors. The stepping or servo motors proved to be the only other significant field source, although their strength is quite low (in retrospect, effective motor design requires that the field be kept internal to the motor). Both the leadscrew and the ways, which are fabricated from tool steel (leadscrews are optionally available in 304 stainless) showed no clearly discernible magnetic field. The gaussmeter resolution is <0.5 mG, but the earth’s field, together with fields resulting from building materials requires differential measurements as stage components are moved relative to the flux-gate detector.
For moderate sensitivity applications, we have prepared a 1 gauss level map of the fields surrounding our tables. The motor’s field strength fell to this level at a cylinder 1.5″ from the motor surface, and 3.5″ from the motor ends. On the stage top surface, the limit magnet fell to the one gauss level on a hemisphere of 1½” radius, centered above the magnet location. Our limit sensors can easily be replaced with opto-interrupters, eliminating this field source.
In many cases, the motor’s external field strength will have fallen to acceptable levels at the point of interest on the user’s payload. Alternate procedures would include extending the motor shaft(s) and thereby moving the motor(s) away from the stage, and enclosing the motor in a can constructed from mu-metal (this can would have to be somewhat larger than the motor to avoid saturating the mu-metal). In high vacuum X-Y systems, custom spline-drive tables can be supplied which allow both motors to be located outside the vacuum chamber. While the steel components (leadscrew and ways) are prime candidates for increased field levels, the hardening process takes these materials above their Curie point. As mentioned, leadscrews can be provided in 304 stainless, and the rod and ball ways can be made from the mildly magnetic 440C stainless alloy. We regularly design custom stages with piezoelectric actuators, which have no external magnetic fields (see the e-beam probing application on page 8). Our air bearing stages are another means of minimizing ferrous components, although they are not vacuum compatible.
In some applications, typically involving electron beams, sensitivity to magnetic fields can be severe. While the testing methodology is rigorous, we can provide stage measurements below the 0.005 Gauss level. As mentioned above, mu-metal shielding and other design techniques may be required. Given the fact that the external field drops off as 1/r3, appropriate positioning of stage components can also be quite useful.