Electron Beam Deposition for YBCO Thin Films

The team of SC Solutions and Stanford University have developed physical models and model-based controls for electron-beam deposition systems to grow YBCO films at Stanford. To make consistent YBCO films, the evaporation rate of yttrium, barium and copper must be controlled accurately. One method of controlling the amounts of these elements is through laser atomic absorption (AA) sensing, and then independent power control of the electron beams. The closed-loop TDLAA e-beam controller developed by SC eliminated the need for the tedious calibration procedure and provides repeatable performance for a wider range of deposition rates. The multivariable controller developed for control of all three elements was implemented and produced YBCO films with excellent properties. The system was delivered with a Java Graphical User Interface (GUI), and has been operational for more than two years.

Electron beam deposition uses the heat generated by electron beams impacting the solid surfaces to evaporate yttrium, barium, and copper for physical vapor deposition (PVD) of YBCO thin films which have high-temperature superconducting properties. The three metals are evaporated by the electron beam guns while atomic oxygen is pumped in for oxidation of the elements. The schematic of the set-up is shown below. One of the important factors in growing YBCO crystals is to have the desired stoichiometry. Due to this constraint the flux of the three metals must be sensed and controlled accurately.

Schematic of electron beam PVD of YBCO Thin Films.

Closed-loop Feedback Control for Electron Beam PVD

Control strategy for electron beam PVD of YBCO Thin Films.

The figure below shows the closed-loop system response to step inputs, and comparison with model predictions.

Closed-loop step response for yttrium source flux controls.

The controller has shown excellent tracking performance during the runs. The normalized tracking error is shown above along for a typical run. The normalized tracking error is defined as the ratio of the mean laser signal to the setpoint, subtracted from unity.

Closed-loop controller performance and resulting tracking error.

The ternary plot below shows the composition for a series of runs. The YBCO thin films prodeuced in these runs are all in the desired region of the ternary plot and are capable of exhibiting high critical current densities (i.e., J_c of MA/cm²). One factor which contribute to variation in the composition is that the physical dimensions of the evaporation plume vary throughout the run which changes the atomic flux to the substrate.

Ternary plot showing YBCO composition data.

This work was funded by a DARPA contract as part of the Virtual Integrated Prototyping (VIP) Program.