Ancient Secrets Revealed with the Aid of Motion Control

Have you heard of synchrotron X-rays before? Kudos to you if you had, but it was a new term for me. For those of you in the same boat as I was until just recently, synchrotron X-rays are a type of X-ray fluorescence imaging that can reveal traces of chemical elements left behind in fossils or even ancient documents. An example of the use of synchrotron X-rays include examination of a 150 million year old Archaeopteryx fossil to gain clues into the evolution of dinosaurs into birds.

The National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory allows scientists to probe nanostructures as small as a few atoms using the infrared, UV and x-ray light produced by a synchrotron.  The NSLS produces such light by accelerating electrons inside one of two football-field sized rings at close to the speed of light. When the light is focused on a specific sample, it produces an image of its most minute properties on a detector for analysis.

So now that we all have a basic understanding of synchrotron X-rays, where does motion control fit into this? The Brookhaven National Laboratory has begun using a Galil motion controller to control the velocity of a linear stage that moves a silicon substrate during the development of a Multilayer Laue Lens (MLL) - the lens used to focus synchrotron X-rays.

NSLS worked with CVD Equipment Corp. and Galil Motion Control to create a magnetron sputtering system that deposits thousands of ultra-thin layers of two different materials onto silicon substrates to create the MLL. Coating thicknesses range to 100 micrometers with up to 62,000 layers in a stack, with the thinnest layer less than 1 nm.

During production, the substrates are loaded onto a linear-translation stage that rides on a stationary base and rail assembly, and is controlled by Galil’s DMC-4020 2-axis motion controller. The controller sends signals to an amplifier and receives feedback from a high resolution encoder to move the car one-dimensionally back and forth throughout the 23-foot, ultra-high vacuum chamber that contains nine magnetron sputtering guns and four cryogenic pumps. The car travels at defined speeds from .01″/second to 9″/second, with maximum acceleration reaching and maintaining no less than 5″/second².

Typically, the coating process involves the deposition of several thousands of layers over as many as 100,000 nonstop cycles over a period of six days.

The controller was able to perform such repeatable, precision movements because its sinusoidal commutation mode assures that a smooth sinusoidal signal (resolved into 16-bits) is sent to the amplifier. This, plus the incorporation of a linear amplifier instead of a switching one, enabled the DMC-4020 to reduce the velocity ripple to 0.0025%.

Another key feature is that all communication between the DMC-4020 and the host computer is via Ethernet. This enabled the host computer to send commands to the controller to commence a new cycle the moment after receiving signals from the controller announcing the completion of a cycle.

To ensure accuracy and repeatability, the DMC-4020 recorded actual position, position error, velocity and torque every twenty seconds of each cycle to provide data logging and check for system errors.

Brookhaven is currently in the process of building a new synchrotron, the NSLS-II which, when completed, will be capable of producing X-rays more than 10,000 times brighter than its predecessor. To achieve this, the specially coated Multilayer Laue Lens will be used for precise steering and focusing of the X-rays to about 1 nm of the item being analyzed.