A Study of Photoelectron Emission Microscopy Contrast Mechanisms Relevant to Microelectronics

We report on a quantitative investigation of doping-induced contrast and topography-induced contrast in photoelectron emission microscopy (PEEM). Calibration samples were fabricated using standard photolithography and focused ion beam writing to test both types of contrast. Using a near-threshold light source, we find that the doping-induced contrast increases monotonically with B concentration over the measured range of 1017 – 2x1020 cm-3. The variation in doping-induced contrast as incident photon energy is varied was also investigated. Optimal doping-induced contrast and PEEM sensitivity is achieved by imaging with photon energy slightly above the highest nominal photothreshold of interest. The photoemission model, based on near-threshold emission, used to describe doping-induce contrast gives good agreement with the measured intensities. Thus, measuring the relative intensity ratio provides a robust technique for determining doping levels. Topography-induced contrast was investigated by imaging Ti samples of various step heights (75, 150, 290, and 550 nm). Image data suggests that edge contrast increases with step height. Numerical simulations show that non-uniform electrostatic fields at step edge are responsible for this contrast. Experimentally, we systematically vary the lateral field strength and show that edge contrast can be controlled. This technique could be useful in failure analysis by identifying breaks in metal lines.