Stanford University Reports Manufacture of Model Superconducting Magnetic Bearing

The Stanford University team for the Satellite Test of the Equivalence Principle (STEP) experiment has successfully patterned and etched a model superconducting magnetic bearing for their instrument. The bearing is about 10 centimeters long on a substrate 2.5 cm in diameter, and has four circuits, one in each quadrant, totaling 152 traces. The niobium traces are about 250 microns wide by 0.5 micron thick. Two of the circuits will be used for performance tests. This result validates the concept of the exposure system, a critical path item for manufacture of the STEP accelerometers.

The photograph below (somewhat enhanced) shows the traces parallel to the cylinder axis and the contact pads at the end of the bearing. The quartz between the traces was lightly etched by the acid bath and appears white. The color fringes are caused by interference in the photoresist mask, which had not been removed for the photo.

The manufacture of this bearing was made possible by improvements to the cylinder exposure system that writes the image onto a cylinder with an ultraviolet beam focussed through a microscope. The improvement was a hardware change that reduced the likelihood of a rare electronic timing error. The exposure system is now accurate to about +/- 5 microns (out of a ~25000 micron image) at rotation speeds up to about 7 Hz. The accuracy limitation is apparently due to mechanical positioning errors, while the speed limitation is from electronic timing inside the computer. Both limitations are capable of significant improvement, which should give accuracy much less than the spot size of ~2 microns. At the present limit of speed, we can expose a 5 cm bearing in about 4-5 hours, or a small (1 cm) bearing in 30 minutes. These are long but acceptable exposure times for manufacturing small quantities of bearings.

In the STEP experiment similar bearings will be used to support a cylindrical test mass. Currents are injected at the contact pads and flow along the traces. The magnetic field of these currents is excluded by the superconducting coating on the interior surface of the test mass, providing a friction-free suspension of the test mass.