The Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP has been developing electron beam technology as an alternative beam tool for devarnishing.
Fraunhofer FEP says that precise, selective devarnishing of layers from a substrate plays an important role in numerous industrial production processes. The manufacture of precision resistors, sensor fabrication, and production of electronic displays and monitors can be mentioned here as key examples.
- A typical job would be to etch operational electronic layers such as resists on plastic, ceramic, or glass substrates at the micron-scale in order to trim characteristics to the desired level, such as precise balancing of electrical resistances, setting sensor values, as well as defining the smallest units of operation.
It is important during this step that devarnishing of the layer be as residue-free as possible while causing a minimum of thermal and mechanical stress to the carrier substrate, which can be a real challenge particularly in the case of plastics.
Beam tools offer crucial advantages here, as they provide the necessary accuracy without contacting the work piece during processing. The laser is a tool that has become standard in many fields of application. It ablates or blasts away the intended areas of the layer by intense inputs of pulsed energy.
According to Fraunhofer FEP the specialised properties of electron beam technology open up several important advantages compared to other processes for devarnishing layers.
In contrast to the laser, whose energy is quickly absorbed at the surface (especially in the case of metallic layers), absorption of the electron beam takes place in the bulk of the layer. This enables the penetration depth of the beam to be exactly set according to layer thicknesses that are present. The irradiated bulk is thereby heated directly rather than relying on indirect thermal conduction processes and is removed from the beam track as molten liquid.
In this respect, the electron beam does not differentiate between optically transparent and optically absorptive layers, so that one and the same beam source can be used for both types of materials. Thanks to the selective depth mentioned above and being able to very quickly direct the continuous beam, thermal stresses on the substrate can be kept very small. This enables it to be used on flexible plastic substrates. For micron-level work, the electron beam can be steered and diverted about 10 to 15 times faster than a laser beam at the same working distance.
Figure: Fraunhofer FEP - E-beam devarnishing
The biggest disadvantage frequently mentioned about electron beam technology is the necessity of using vacuum engineering.
However, a vacuum actually delivers some important prerequisites for precise devarnishing of thin layers: the absence of air prevents oxidation of adjacent areas during thermal processing, experience indicates trimming resistances is considerably more accurate and reproducible without the presence of humidity, and contamination of the substrate is reduced.
Benjamin Graffel, scientist working in Electron Beam Processes at Fraunhofer FEP, said, "The electron beam diameter can be matched to the application, which expands its possibilities for utilization even more. The diameter can even get down to the nanometer range and is being employed especially for high-precision electron-beam devarnishing by means of locally induced gas-phase etching." Benjamin added, "This is already being used for repairing lithography masks in microelectronics, for example."