GIGAHERTZ SCANNING ELECTRON ACOUSTIC MICROSCOPY

Abstract

In Scanning Electron Acoustic Microscopy (SEAM) an electron beam with the beam current impacting a target site is modulated at acoustic or ultrasonic frequencies. The absorbed energy of the electrons results in a time-varying thermal expansion that leads to acoustic and ultrasonic waves to be launched into the substrate. The waves are detected using a piezoelectric transducer. As the electron beam is scanned, SEAM produces images that are indicative of the target thermal properties and ultrasonic wave properties. SEAM has been shown to produce images that are useful in characterizing thermal and elastic properties, in the MHz frequency range. The main contrast mechanism is understood to be due to heat induced acoustic waves. The resolution of SEAM imaging is determined by the thermal generation volume, which is a function of the electron beam energy, electron beam focus diameter, thermal diffusion lengths, and the ultrasonic wavelength. The research presented in this thesis is motivated to extend the SEAM technique to the gigahertz range in order to realize nanometer scale SEAM resolution. Gigahertz SEAM uses a gigahertz modulated electron beam to generate a thermal acoustic wave and uses a piezoelectric thin film of aluminum-nitride (AlN) as the transducer. The resonance frequency of the AlN transducer is in the 1-2GHz range, enabling the GHz-SEAM to be investigated. In this thesis, the theoretical model for the generating acoustic wave is developed as a function of the electron beam modulation frequency. This model demonstrates the potential to reach 10-100nm resolution with GHz-SEAM. SEAM is validated with an electron beam modulated using an electrostatic parallel plate deflector, at both MHz and GHz frequencies. PZT transducers are used at MHz frequencies and the measured output voltages on the transducers agree with theory. AlN transducers are used to sense the SEAM signals at GHz modulation. The signal amplitude suffers from considerable RF coupling, although data suggest that we can observe GHz-SEAM signals. The thesis proposes new measurement approaches to increase the SNR in GHz SEAM experiments in the future.