Ion acoustic imaging of microstructures using a focused ion beam system

Full text: In the last decades focused ion beams have become very useful tool for many tasks in sub-micron technology. For imaging and controlling of subsurface microstructures for instance Scanning Electron Acoustic Microscopy (SEAM) can be used. SEAM has limited resolution in the range of some microns. The aim of the present work is the development of a Scanning Ion Acoustic Microscope (SEAM) based on a focused ion beam (FIB) system that can provide sub-micron resolution and can be used as a tool for analysis, device repair and modification. The basic principle of ion acoustic effect is acoustic wave generation due to periodical heating of a sample by intensity modulated focused ion beam. The acoustic wave can be detected on the near surface of the sample. One of the factors that define the resolution of scanning acoustic microscope is the thermal diffusion length in the solid. It depends on material properties and beam pulse frequency and can be decreased by increasing of the frequency. This work is a continuation of the first experiments carried out using the IMSA - 100 focused ion beam system in Research Center Rossendorf. Ga+ and Au+ focused ion beams with an energy of 30-35 keV and maximum current of about 3 nA was used. The ion beam was pulsed with variable frequency up to 1 MHz. Acoustic waves were detected by a PZT piezoelectric transducer with an integrated preamplifier. In order to obtain the best condition the pulse frequency of the ion beam was chosen equal to resonance frequency of PZT plate. The beam power was about 100 μW yielding a PZT signal of about 50 -100 nV. In spite of the low amplitude of the signal first two-dimensional acoustic images were obtained using a digital image scanning system. Several PZT detectors were used at their resonance frequencies 80 kHz, 272 kHz, 632 kHz. Lateral resolution of the first images was about 20 μm at 80 kHz and about 6 μm at 632 kHz on silicon. The resolution can be improved by increasing the pulse frequency. At a frequency above 20 MHz it will reach the sub-micron range on a silicon sample. In this work images of a control sample of a glass with the thickness of 100 microns were obtained as an evidence of possibility of buried structure detection. The glass was harrowed from two sides The picture got with the secondary electrons emission method doesn't permit to see a scratch on the backside of the glass. On the ultrasound picture it's possible to see a scratch on the backside of the class. That shows advantage of the given method to detect of the hidden internal defects