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4Wave is the pioneer in a new deposition technology called Biased Target Deposition. BTD is a hybrid between IBD and conventional sputter deposition that combines the best of each technique. BTD is uniquely suited to demanding applications requiring atomically engineered thin films and interfaces as it offers a large range of process pressures, control of adatom energies, and excellent uniformity and repeatability. High performance multilayer devices such as giant magnetoresistive multilayers, optical interference filters, and gate dielectric stacks are particularly well suited to BTD.
- High output, low-energy plasma source.
- Independent target biasing for alloy composition control.
- Substrate plasma cleaning, etching oxidation, nitridization.
- Water cooled, tilting, rotating, pulsed DC biased stage.
What is a Biased Target Deposition (BTD)?
In BTD, a low energy ion source (typically of the end-Hall or closed-drift Hall type) is directed at a negatively biased sputtering target. The maximum energy (typically < 30 eV) of the ions is less than the sputter threshold of the vacuum system materials. No effort is made to capture all of the ions on the target because ions that miss the target do not generate unwanted sputtering. In practice, the ion beam can be much broader than the target to improve illumination uniformity. A plasma sheath develops at the surface of the negatively biased target that accelerates positive ions entering the sheath toward the target to produce sputtering.
Because the sheath is very small (~ 2 mm) compared to the spacing between the ion source and target, the target bias has no substantial effect on the ion trajectories from source to target. Hence, for constant source operation, the illumination profile and the ion current reaching the target are nearly independent of the target voltage. A grounded shield surrounds the target to prevent undesired sputtering of the target mounting hardware that is also biased. DC, RF or pulsed DC target bias is used depending on the target material and desired process. A large range of target voltages (~100 to 2000 V) can be used while maintaining reasonable deposition rates. The selection of the target voltage, by virtue of its impact on adatom energies, has a profound impact on the atomic scale mixing at thin film interfaces and the overall roughness of the growing film. In addition, the ion source is capable of operating over a broad range of process pressures (~10-4 to 5×10-3 Torr), allowing control of the adatom scattering from the background gas.
A second, low energy ion source (an assist source) is directed at the substrate to modify the properties of the growing film. Non-reactive assisting ion energies of order 5-15 eV are useful in creating smooth films. Reactive assist ions can be used (e.g. ions of O2 and N2) to create dielectric films from metallic targets. This source can also be used to etch, clean and modify surfaces prior to deposition.