The team of SC Solutions, HRL Laboratories and UCLA have developed and implemented model-based controllers for depositing reproducible III-V thin films by molecular beam epitaxy (MBE). To achieve and control the atomic scale features that are required, processing must be founded upon a fundamental understanding of the atom-by-atom assembly of these engineering structures. For purposes of control, the problem is to develop a model which relates morphology variables at the atomistic scale and sensor variables at the device scale to the control variables at the reactor scale.
Epitaxial growth of a single layer includes the following processes: deposition and diffusion of adatoms, nucleation of islands through collisions of adatoms, and attachment of adatoms to the island leading to island growth and coalescence.
The control problem addressed here is to grow a specified number of layers such that the resulting surface meets a specified "roughness'' criteria. The control uses a RHEED or PEO (photoemission) sensor to measure step edge density which signifies roughness. The figure below shows the MBE chamber and the associated sensors used for this work. The sensors are Reflection High Energy Electron Diffraction (RHEED), Photoemission Oscillation sensor (PEO), Absorption Band-Edge Spectroscopy (ABES, for temperature), Reflection Mass Spectroscopy (REMS) sensor, all of which are in-situ sensors, and Scanning Tunneling Microscopy (STM) which is an ex-situ sensor.
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| Schematic of the MBE system. |
The variables that strongly effect the layer growth are the substrate temperature and adatom flux. In the MBE reactor, it is not possible to rapidly change the diffusion (by controlling surface temperature) over the time period of typical 5-10 monolayer growth because of the slow thermal dynamics of the substrate. Hence, substrate temperature is useful as a "run-to-run" control variable. Flux, can be rapidly changed by adjusting the effusion cell cracker valves or shutters, and more slowly changed by controlling the cell temperature. Hence, flux is the effective control variable. A change in flux will effect the deposition time to achieve a desired coverage, i.e., decreasing flux increases the deposition time to reach a coverage goal, thereby lowering the step edge density.
Surface conditions are monitored from RHEED and PEO signals. The oscillation period of the RHEED signal is directly related to the growth rate of layers of atoms on he surface. The PEO sensor scans a smaller area of the wafer but is more sensitive to surface changes yielding accurate period data at the beginning of a growth cycle, is more robsut to changing growth conditions, and can be used with wafer rotation. However, the PEO is a relatively new sensor compared to RHEED, the latter being used in period control described below.
The oscillation period of the RHEED signal was controlled during III-V growth. Using the error between the desired period and the period calculated from the RHEED signal, the controller was able to adjust in real-time the set-point temperature of the Group III material effusion cell.
The model-based controller algorithm used results from a KMC simulation model of atom-by-atom film growth together with a dynamic thermal model of the effusion cell. The controller, which controls growth from layer to layer, has two important features:
The figure below shows details of the RHEED oscillation controller including the various control inputs and sensor outputs and computer configuration.
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The figure below shows the results of a control experiment performed on the MBE system. The controller shows good performance by keeping the oscillation period close to the reference period. The experimental data and the simulation results are in good agreement.
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This work was funded by a DARPA contract as part of the Virtual Integrated Prototyping (VIP) Program.