Control for Multiple-Source Electron Beam Deposition of Superconducting Thin Films

The objective of this DARPA-sponsored program was to develop physical models and model-based control of electron-beam deposition systems to grow YBCO (Yttrium Barium Copper Oxide) thin films at Stanford University and at 3M.

To make YBCO films of consistent quality, the evaporation rate of yttrium, barium, and copper must be controlled accurately. Sensing for flux control of these component elements was achieved in this program by employing Tunable Diode Laser-based Atomic Absorption (TDLAA).

First, SC Solutions developed and installed an appropriate data acquisition system at 3M. Next, SC designed and implemented experiments to acquire real-time data to help develop a nonlinear dynamic model of the deposition process.

The physics-based model consists of a crucible heat transfer model, an evaporation model, and a transport model. The model incorporates the essential physics, and predicts the sign and magnitude of the gain as well as the dynamics with sufficient accuracy for controller development purposes. The model was calibrated and subsequently validated by comparing the dynamic evaporation rate profile predicted by the model with the AA profile for step changes in the inputs.

Model-based feedback controllers with Java-based GUI were developed and implemented using SC's RT-iCon platform at 3M and at Stanford for yttrium, copper, and barium flux control as well as control of the tape drive.  These real-time controllers met or exceeded the accuracy goal of 3% or better flux and composition control.  An embedded control system has been operational at Stanford University for several years and has enabled production of YBCO films with excellent properties.