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.