Application of Focused Ion Beams in Materials Research

With the invention of the liquid metal ion source (LMIS) in the sixties the focused ion beam (FIB) technique started an impressive development from the laboratory level to high performance industrial equipments. At present, the FIB is a very useful and versatile tool in microelectronics industry for mask and integrated circuit repair and modification, failure analysis or TEM specimen preparation, as well as in the material science for radiation damage and sputtering investigations, for grain size and distribution analysis in metals and alloys, for the formation of silicides or the fabrication of micro-tools. A modern FIB column which is operated with gallium liquid metal ion souces (LMIS) only reaches a spot size in the range of 10 nm and current densities of more than 10 A/cm2 .

For special purposes in the field of research and development, like writing ion implantation or ion mixing in the µm- or sub-µm range other ion species than Ga are needed. Therefore alloy LMIS are used. The energy distribution of the ions from an alloy LMIS is one of the determining factors for the performance of the source in the FIB equipment, in other words, the obtainable spot size. Different source materials like the alloys Au73Ge27 (Tm = 366°C), Au77Ge14Si9 (Tm = 365°C), Co36Nd64 (Tm = 566°C), Er69Ni31 (Tm = 765°C), and Er70Fe22Ni5Cr3 (Tm = 862°C) were investigated and compared with respect to the energy spread of the different ion species depending on the operation parameters emission current I, ion mass m and temperature T. For single charged ions the predicted dependence of the energy spread according to DE µ I2/3 m1/3 T1/2 found for Ga could be confirmed in reasonable agreement. Due to the ion mass, for doubly charged ions a weaker and for clusters a stronger slope was found. The temperature dependence of the source behaviour is strongly related to the surface tension coefficient of the used alloy.

The alloy LMIS`s discussed above have been used in the Rossendorf FIB system IMSA-100 especially for writing implantation to fabricate sub-µm pattern without any lithographic steps. So a Co-FIB was applied for the ion beam synthesis of CoSi2 micro-structures with a minimum feature size of 60 nm. Additionally, the possibility of varying the current density with the FIB by changing the pixel dwell-time was used for radiation damage investigations in Si and SiC at elevated implantation temperatures. Furthermore, a broad spectrum of ions was employed to study the sputtering process depending on temperature, angle of incidence and ion mass on a couple of target materials using the volume loss method. The effective erosion properties of the FIB were exploided in fabricating micro-tools with dimensions smaller than 40 µm from WC(Co) and HSS steel. The application of the FIB as a scanning ion microscope with high topographic resolution of the sample surface can be used to study the grain size and distribution of metals and alloys as well as micoelectronic structures on the surface and also, after sputtering, in deeper layers. The sputtered holes can also be imaged after polishing with the FIB and tilding as a cross section by SEM. All the examples underline the importance of a FIB in modern research.