CHARACTERIZATION OF CONTRAST MECHANISMS OF THE PHOTOELECTRON EMISSION MICROSCOPE

K. Siegrist1, V. Ballarotto2, M. Breban3, E. D. Williams3

1Optical Technology Division, NIST, GaithersburgMD20899

2Laboratory for Physical Sciences, College ParkMD20740

3Physics Department, University of Maryland, College ParkMD20742

Supported by the Laboratory for Physical Sciences and University of Maryland MRSEC

        Photoelectron emission microscopy (PEEM) is a potentially powerful technique for real-time imaging of organic, metal and semiconductor electronic devices. This non-intrusive surface-sensitive technique has primarily been used to investigate nanometer-scale surface chemistry and morphology, but its broad imaging capabilities make it well adapted for imaging  of micron-scale electronic devices.  This application has not yet been realized due to lack of understanding of the contrast mechanisms relevant to micron-scale imaging of devices. In this work, the characterization of PEEM image contrast and in particular, the contrast effects generated by local electric fields at the surface of a sample, were investigated. Such fields arise from a number of causes, and include topographic features, which give rise to local surface fields under the influence of the strong accelerating field of the microscope,  and externally applied potentials, which can generate local fields stronger than the accelerating field at the surface of the device under study.  These local fields at the sample surface can have strong impact on the trajectories of the just-emitted extremely low energy photoelectrons, thereby changing image intensity.  Test structures were fabricated for investigating the contrast effects induced by carefully controlled contributions of the field sources mentioned above. Two sets of topographic samples, consisting of steps of varying height, were fabricated of titanium and of nickel. Topographically similar samples with the capability of biasing the fabricated steps were constructed to allow intentional variation of electric fields at the sample surface.  Finally, samples having entrenched features with the same biasing capability, but without associated surface topography, were constructed. Some of these bias-capable samples were subsequently covered with oxide layers to investigate PEEM imaging of buried devices. Analysis of PEEM images of these test structures was combined with numerical calculations of the sample surface field configuration and ray tracing simulations, in order to understand and quantify the contrast effects generated by electric fields at the sample surface. Application of this approach to the analysis of contrast generated by step height, magnitude of applied voltage, and the imaging of buried interfaces are presented.