A  High Speed Etching Source -  Development and Application

	Introduction   The development and application of a low energy argon ion beam source for high quality depth profiling in an X-ray photoelectron spectrometer designed for routine (QA) analysis is presented. This compact Kaufman ion source, which combines extremely high sputter rates and low ion beam acceleration potentials, reduces ion induced mixing of the surface atoms. The properties of this ion source mean that concentration depth profiling retaining good interface resolution through several hundred nanometer thick layers is readily achieved. Rotation of samples during sputter profiling is shown to improve interface resolution further.   The depth profiling capabilities of this high speed etching source are demonstrated through a number of challenging samples. Atomic concentration profiles through a three layer sample have been performed as a function of ion acceleration voltages, and the effect on interface resolution is discussed. A further example of depth profiling through an inorganic material with a thin metal underlayer on a polymer is shown. Chemical state information is retained from the inorganic oxide layer, with some reduction of the oxide layer observed due to the preferential sputtering of oxygen from the surface during profiling. Data presented shows no decrease in the sputter rate due to positive charging of electrically insulating samples during the sputtering process.
Figure 1: Schematic diagram of the Kaufman ion source	The Kaufman ion source   The Kaufman ion source developed for sputter depth profiling on the AMICUS photoelectron spectrometer is shown as a schematic diagram in Figure 1. Ar+ ions are produced by direct-current electron bombardment of low pressure argon gas in the ionisation chamber. The hot cathode is at the centre of the ionisation chamber, with the anode forming a cylindrical outer boundary to the discharge region. A permanent axial magnetic field is applied to the ionisation chamber, such that the electrons produced at the cathode have an increased path length and therefore ionisation efficiency. Argon ions are extracted from the ionisation chamber by a double grid optical system with the advantage of a much higher ion current density when compared to a traditional single-aperture source operating at the same voltages. The argon ion beam acceleration potential may be selected between 0 - 1000 V, whilst maintaining extremely high sputter rates by virtue of the high ion current density.
Figures 2 & 3: Source Characteristics	Characterisation of the ion source   The operating conditions of the Kaufman ion source have been optimised by measuring the sample current as a function of gas pressure, emission current and ion acceleration voltage, shown in Figure 2 (a, b, and c, respectively).   To characterise the sputter crater produced by the ion source, a large SiO2 / Si sample was sputtered without sample rotation. The sample was then removed and a line scan over the sputter crater performed where the silicon and silicon oxide components were measured, giving the profile shown in Figure 3. From this curve it is clear that the ion source produces a broad beam of ions, creating a 3mm diameter flat bottomed crater. As the AMICUS photoelectron spectrometer has an analysis area greater than this all samples used for sputter depth profiling have been <3mm in diameter to eliminate crater edge effects in the depth profile.
Figure 4: Concentration depth profile through three layer material. (100mA emission, 600V acceleration potential). Sputter Rates (Fig. 4) Al2O3 490Å/min MgF2 754Å/min SiO2 850Å/min Figure 5: Concentration depth profile through TiO2 / Ti on a polymer (100mA emission and 300V acceleration potential)   Figure 6: Ti 2p spectra as a function of sputter depth from the surface from data shown in Figure 5	Sputter Depth Profiles   Figure 4 shows a sputter depth profile through a Al2O3 / MgF2 / SiO2 triple layer sample on Si substrate, where the thickness of each layer has been carefully calibrated using independent techniques. This example demonstrates the extremely high sputter rates that may be achieved using the Kaufman ion source, whilst maintaining good interface resolution. Sample rotation was used during the sputter process to improve the interface resolution. Interface resolution was investigated as a function of ion acceleration voltage, although for this sample no degradation of interface resolution was observed with an increase of acceleration voltage.     Figure 5 shows a depth profile through a TiO2 / Ti layered material on an insulating polymer substrate. The insulating nature of the sample means that it will be susceptible to the build up of positive charge during the sputtering process. In turn this can result in a decrease in the sputter rate of the material. However, high sputter rates were still observed for this sample. Low ion acceleration potentials were used during the profiling to limit the mixing of atoms during profiling.     Chemical state information is retained during the sputtering process, as demonstrated in the Ti 2p region shown in Figure 6. After the first sputter cycle, the TiO2 surface layer is partially reduced due to preferential sputtering of oxygen from the surface. It can also be seen from the concentration depth profile that there is increased oxygen concentration at the polymer/Ti metal interface.
Conclusions   A high speed etching source has been developed for sputter depth profiling samples. The multi-aperture optics provide extremely high argon ion density and therefore high sputter rates over a 3mm diameter. The low ion energy is expected to result in less ion induced mixing of surface atoms during the sputtering process.   This paper was presented at AVS '99, the 46th International Symposium, Seattle, WA, 26th October 1999.

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