Investigation of emission instabilities of liquid metal alloy ion sources

A detailed investigation of the emission instabilities and their frequency spectra for Ga, AuGeSi and AuGe Liquid Metal Alloy Ion Sources (LMAIS) was carried out. The ion sources with an extracting electrode (triode scheme) as well as without an extractor (diode scheme) were applied. The ion current coming from the collector electrode was amplified and converted into a voltage. The amplifier has a -3 dB edge at 40 MHz and converting factor of 200 mV/µA. The frequency spectra from 0 to 50 MHz were measured using a HP8591 spectrum analyzer with a resolution of 300 kHz. Simultaneously the corresponding oscillograms of the emission current were obtained using a digital oscilloscope. The measured spectra are correlated with these oscillograms. The current for which pulses appear superimposed on the d.c. level of current is well predicted by existing theory. The pulses are believed to be the result of droplet emission, and their terminal frequency appears to coincide with the frequency of vibrations of the sides of the liquid cone at high currents. The emitted droplets screen the source tip. This leads to decreasing of the effective electric field and fluctuation of the emission process. The following disintegration of the droplets into smaller ones aggravate the noise. The fluctuation depends nonlinearly on the emission current. This dependence shows an S shape, at low emission current the noise is negligible, at 40-60 µA the fluctuation goes up rapidly and higher than 70 µA it comes to a saturation. The onset of the pulses (at about 30 µA) is also predictable from a criterion developed by Mair for the critical current. At 100 µA emission current the typical fluctuation is about 5 µA(rms). The alloy sources were investigated also at temperatures from 400 to 1100 °C and the dependence of the fluctuations of the emission on the temperature were obtained. With increasing temperature a stronger fluctuation was registered. Better understanding of the instabilities that develop on the liquid anode can be useful for deposition purposes.