Low energy ion implantation (LEII)
Fig 1. shows a sketch of the LEII setup. An ultrahigh vacuum chamber houses a Kaufman type hot filament, broad beam ion source. The source can operate all inert and many reactive gases at low gas fluxes. The sample is temperature controlled by a boron nitride heater and a thermocouple. It can be exchanged without severely affecting the vacuum conditions by a sample transfer system (not shown). A rotary and linear motion feedthrough is installed on the ion source flange. It mounts a Faraday cup and a filament heater. The Faraday cup serves to control of the ion flux density that is supplied to the sample from the ion source. The filament heater is employed to minimize the discontinuity of the heat flux onto the sample that occurs when starting and stopping the ion nitriding treatment by swaying the motion feedthrough. The residual gas is controlled by a residual gas analyzer and a needle valve / mass flow controlled gas inlet. With this setup the important LEII parameters can be varied and controlled independently.
Important LEII parameters:Tab. 1
oxygen partial pressure
0.5 – 2.0
0.01 – 1.0
RT – 800
< 3 x 10-5 – 3 x 10-3
0 – 300
The LEII system is particularly employed to study transport phenomena during ion nitriding of austenitic stainless steel and aluminum. Special emphasis is given to a combination of LEII and the ion beam analytical method elastic recoil detection analysis (ERDA).
Fig. 2 / Fig. 3
This combined experiment allows the determination of multi-elemental depth profiles during a well defined ion nitriding treatment. Hence, it provides unique insight into the ion nitriding kinetics. As an example Fig. 4 depicts the evolution of nitrogen and oxygen depth profiles during ion nitriding of pure aluminum at different oxygen partial pressures p(O2). Ion nitriding has been performed for 15 min at a sample temperature of 500°C, an ion energy of 1 keV, and a current density of 0.2 mA/cm2. The p(O2) has been varied from 3 x 10-3 Pa, over 3 x 10-4 Pa, down to less than 3 x 10-5 Pa. Ion nitriding starts at time zero after exposing the samples for 10 min to the respective p(O2).
The data validate that the interplay of sputtering and oxidation is a key parameter for ion nitriding of materials that form dense and stable surface oxide layers (e.g. stainless steel and aluminum). For a successful ion nitriding treatment of these materials the oxide removal rate caused by sputtering should be significantly higher than the initial oxide growth rate. Otherwise a surface oxide layer with stationary thickness establishes that most likely will be detrimental to the nitriding result.