Ion Beam Deposition, Film Modification and Synthesis
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steel in a fraction of a second. At energies of 1-5 keV and relatively low currents (mA) are the sources used for surface analysis and depth profiling. The so-called "processing" ion sources typically operate at energies of 20 - 3,000 eV, with currents in the milliampere to ampere range. These sources include the Kaufman-type source, the electron cyclotron resonance (ECR) source, the Hall effect source, and other related or hybrid designs. The rest of this article is devoted to these low energy, fairly high current sources and their applications. Ion beams can be used to sputter etch or "mill" samples, either for removing surface contamination or a particular layer from a sample, or for the larger scale machining or thinning of parts (Figure la). In conjunction with a reactive gas, the etch rate can be dramatically increased over sputtering alone. ECR sources are particularly best suited to this type of reactive etching because they have no hot internal surfaces and can be constructed of materials that are inert in the reactive gas of interest. An ion beam directed onto a target will sputter the target material, causing the formation of thin films of target atoms on nearby surfaces (Figure l b ) . This technique, known as sputter deposition, can also be done in a reactive atmosphere, causing the formation of a compound thin film (an oxide or nitride, for example). Ion beams used for thin film materials processing operate at p r e s s u r e s low enough that both the incident ions and the sputtered atoms are in a free-flight, or line-of-sight mode. The pressure is low e n o u g h that collisions with the background gas are rare, and most of the ions or sputtered atoms pass right through the background gas. This indep e n d e n c e of the ion beam from the background gas allows them to be used
c o n c u r r e n t l y w i t h e i t h e r o t h e r ion sources or other deposition or modification techniques in the same chamber. For example, an ion beam is used in conjunction with an e-beam evaporation source in Figure 2. In this arrangement, the ion beam is directed at the sample, a n d can be operated either independently from the evaporator (to clean the surface or etch a layer) or else concurrently with the evaporator. In the latter case, the ions incident on the sample surface during the actual film deposition can be used to systematically alter the properties of the deposited material. This process, known as ionassisted deposition (IAD) or ion-beamassisted deposition (IBAD), is one of the most interesting applications of ion beams to materials processing. Controlling the ion energy, arrival rate (as compared to the arrival rate of depositing atoms), angle of incidence and species allows altering various properties of the deposited film. In addition, as IBAD is a non-equilibrium process, new metastable states a n d c o m p o u n d s can be formed that would be difficult or impossible to form using other processing techniques. It is, of course, possible to deposit the material in Figure 2 by other means. One example u
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