Copper Metallization by Thermal Deposition Processes

A plasma is generally described as a 4th state of matter in which proportionally ionized particles (electrons and ions) are present and thus form an electrically conductive medium. However, such a mixture is only qualified as a plasma if the negative and positive charges are in equilibrium, i.e. the plasma as a whole is in an electrically quasi-neutral state. Thermal plasmas, which are used for the application of thermal coating processes under atmospheric pressure, exist from a threshold value of electrical conductivity and have a temperature of approx. 8000K. The ignition of a plasma is achieved e.g. by a high voltage pulse of about 9kV, the plasma can then be stabilized with a constant current of several hundred amperes at a low applied voltage. The technical arrangement of a thermal coating system consists basically of a nozzle and counter electrode between which the arc rotates and thus ionises the process gas flowing through. The use of a Laval nozzle leads in the following to thermal energy of the medium being converted into kinetic energy. The coating material, e.g. in powder form, is then injected into the accelerated gases and accelerated and melted by the gas flow. When the coating material hits the substrate, the particles solidify and the coating is generated. The process characteristics and properties of the coating are influenced by numerous factors.

In the first step the choice of plasma gas determines the physical properties of the plasma. Argon is selected as the carrier gas for power electronic components. In comparison to molecular gases, no dissociation of the molecules is necessary, which means that an overall lower ionization energy and electrical voltage is required during the process. Thus the critical threshold value for semiconductor devices of 50V is not exceeded.

Previous investigations dealt with the qualification of an alternative process for the additive generation of a die-top system to realize the topside contacting of power semiconductors in the copper thick wire bonding process. The investigated coating process with a thermal argon plasma allows a selective copper deposition with layer heights starting from approx. 40 µm on any substrate material by using masking. Extensive parameter studies allow the definition of a process window which allows a damage-free metallization on wafer level.

A further focus is the development of a flexible structuring concept to realize the metallization of fine structures, such as gate contacts on IGBTs.

The developed basics thus form a basis for the further application of thermal deposition processes in power electronics.

The controlled melting of small powder sizes allows the coating of temperature critical components (such as semiconductor components, capacitors, etc.).

By means of suitable diagnostics a precise parameterization of the plasma process and the sample handling can be carried out, so that a non-destructive coating with optimal coating quality is possible despite high thermal stress. The main influencing variables to be controlled are defined as follows:

– Particle temperature: controlled by gas quantity and composition, secondary by plasma power

– Particle velocity: depending on gas volume and temperature

– Surface temperature: Handling distance and travel speed

By means of thermal post-treatment in a reducing atmosphere, layer qualities with over 60% conductivity compared to pure copper and high mechanical characteristics can be achieved.