Survey on Pulsed Electron Beam Deposition Work in Technique and Medicine

Survey on Pulsed Electron Beam Deposition Work in Technique and Medicine

C. Schultheiss, P. Brenner, L. Buth, H. Bluhm

Forschungszentrum Karlsruhe GmbH,Institute for Pulsed Power and Microwave Technology P.O. Box 3640, D-76021 Karlsruhe, Germany

Abstract: The principal benefits of the PED (Pulsed Electron Deposition) coating method using Channel Spark are the simplicity of the system itself and the excellent coating results as for instance conservation of stoichiometry even in complicated alkali-earth alkali compounds within 1 % to 2 % and growth rates up to 1Ǻ per shot. The coating process relies on ablative generated molten droplets of nano scale, which are deposited amorphous onto the surface of the substrate. Even plastic substrates and thin plastic foils can be coated provided the repetition rates are low enough. Since the initial capital and operational costs are low in comparison with PLD (Pulsed Laser Deposition), there is a wide field of actual and future applications of PED which are summarized in this presentation.

INTRODUCTION

An increasing interest of industry is focused on coating systems which base on Pulsed Electron Deposition (PED) technique with special interest to the Channel Spark (CS) [1] system. The deposition method of PED is similar to the Pulsed Laser Deposition technique (PLD) [2] which can be considered as a front-runner because of its ability to prepare epitaxial layers of various multi-component materials on substrates such like ceramic high-Tc-superconductors. The coating process relies on the generation of physical vapor by means of ablative erosion of target material. In the PLD- as well as in PED-case the range of the directed energy into the target material is low and therefore the energy density in a thin layer exceeds 50 kJ/g easily. For UV-laser the range is about 20 nm and for low energetic electrons (~10 keV) the range is around 150 nm. In general the investment and operation costs of PLD are high so that for many applications [3] the PED method with a lower cost structure is considered to be the alternative.

CHANNEL SPARK SYSTEM

In principal the CS-system consists of three functional parts: the gas discharge trigger, the hollow cathode and the dielectric channel which extends into a vessel at anode potential with a gas pressure of 1 Pa. As can be seen in Figure 1, the trigger plasma is generated by a glow discharge, which takes place in a glass tube after charging the capacitors and after the ignition of a gas switch to ground. The plasma density is enhanced by the axial field of a permanent magnet. The middle section of the CS is a conically shaped hollow cathode, which supports the glow discharge to develop more current and allows the current amplification of the discharge to several 100 Amperes.

The glow discharge trigger allows repetition rates of up to 30 Hz. By means of a small artificial leak at hollow cathode site [3] the beam instability could be eliminated which increases the range of the beam from 1 to several centimeters. It also improves the beam stability, measured by the bremsstrahlung X-ray signal coming from the tungsten-target which deviates not more then 10 % from shot to shot. This may be the consequence of a quiet plasma development in the conically shaped hollow cathode without sharp edges etc., which suppresses undesired explosive electron emission (EEE) events.

FIGURE 1. Channel Spark device consisting of glow discharge trigger, hollow cathode and ceramic tube arrangement as used for coating substrates

SURVEY ON ABLATED MATERIALS

Best target materials are compounds of metal oxides with low thermal conductivity. Therefore dielectric glasses, ceramic high-Tc-superconductors, ferromagnetic materials as well as transparent conducting films like SnO₂ are favorite coating materials [4]. Transparency and electric insulating behavior of target material is not an obstacle for PED. Another group of materials are plastics. PTFE gliding films on substrates as well sealing of (cracked) soft PTFE can be used in chemical micro fabrication devices. These mentioned materials also can be used for coating purposes, for generation of nano particles as well as for the generation of nano tubes (carbon) [4].

GLASS FILMS ON POLYMER FOILS

Ablated vapor and molten droplets of fluid- or borosilicate glass adhere very tight at polymer surfaces, because of a melting-up of the plastic. The energy transported by the electron beam is 1–2 J, and each single ablation event heats up the substrate surface to some centigrade corresponding to heat absorption of 0.1 J/cm². By means of the control of the repetition rate or active cooling the average heat transfer to the foil can be limited such that the foil is not damaged. This is realized in a roll-coater supplied with two arrays of 13 CS-system (see Figure 2)

FIGURE 2. Coating of plastic foil in a roll-coater by means of two arrays of each 13 channel sparks (see TABLE)

TABLE. Permeation of oxygen of coated 12µm polyester foil in terms of cm3/m224h at 25°C