Reactive Ion Basics

Definition of reactive ion technology

Reactive ion technology is closely related to the plasma ion etching technology, at least when it comes to the generation of the plasma, but because plasma ion etching and reactive ion etching are using two different mechanisms, there are a number of differences between these two processes:

• Plasma ion etching relies mainly on the kinetic energy of the ions hitting the target, while this effect is undesirable in reactive ion etching
• For the same reason, plasma ion etching relies on hot plasmas in order to archive the necessary kinetic energies, while in reactive ion etching a cold plasma (with thermally cold ions) is desirable in order to minimise the effect of kinetic ions
• Plasma ion etching typically uses inert process gases like Argon to avoid chemical reactions between the ions and the target, while reactive ion etching is based on the chemical reaction between the ions and the target, thus using chemically very active process gases
• While plasma ion etching typically uses the same process gas for different target materials, the process gas or gas mixtures for reactive ion etching are selected and optimised based on the target material
• Plasma ion etching usually also makes use of vacuum ultraviolet (VUV) light to enhance the cleaning / etching effect, while this is not always desired or required in reactive ion etching
• In case the target consists of different materials, the effect of plasma ion etching on these materials is almost the same, while with reactive ion etching it is possible to only target selected materials
• Because reactive ion technology relies on chemical reactions and not on mechanical cleaning via hot ions, there is almost no heat transfer to the target, the only effective source of heat are possible exothermic chemical reactions

Reactive ion technology applications

Based on the differences listed above, there are a number of applications where reactive ion technology outperforms plasma ion etching, as well as several applications for which plasma ion etching is not suitable at all:

• Photoresist stripping

  Reactive ion technology is highly efficient when it comes to photoresist stripping / ashing. As an example, for the commercially widely used photoresist SU-8, depending on the surface area etching rates between 200 μm / h and 20 μm / min are possible, with total photoresist layer thickness of > 1 mm not posing a problem.

• MEMS production

  Due to the high etching rates and the fact that metals like Nickel, Nickel-Iron, Gold, Copper and others are not affected by the process, reactive ion technology is also especially well suited for the production of MEMS (MicroElectroMechanicalSystems). As the process does not leave any organic residues or solvents, cleaning processes can be drastically reduced.

• Surface activation

  Because reactive ion etching plants are designed to handle aggressive process gases, they are very well suited for the surface activation of plastics / polymers which are otherwise difficult to treat, for example to enable later glueing / bonding or printing of these parts.

• Plasma-enhanced CVD

  For the same reason, reactive ion etching plants are also very suitable for plasma-enhanced CVD (ChemicalVapourDeposition), especially when used for aggressive gases or gas mixtures.

• Silicon wafer treatment

  Reactive ion etching plants also offer an outstanding performance when it comes to semiconductor back-end applications, like wafer thinning and stress relief.

• Chip decapsulation

  Due to the high selectivity of the reactive ion etching process, these plants are especially well suited for chip decapsulation, e.g. for quality control or failure analysis. Due to the high selectivity epoxies and other encapsulation materials are etched at high rates, while lead frames, bonding wires and the silicon structures are left almost unaffected.

• Flexible PCB manufacturing

  Another application where reactive ion etching plants outperform other processes is the de-smearing and drilling of flexible PCBs (PrintedCircuitBoards).