AVS 59th Annual International Symposium and Exhibition
    Applied Surface Science Tuesday Sessions
       Session AS-TuP

Paper AS-TuP9
The Development of Charged Particle Lenses for High Spatial Resolution XPS Studies

Tuesday, October 30, 2012, 6:00 pm, Room Central Hall

Session: Applied Surface Science Poster Session
Presenter: R. Walker, Shimadzu Research Laboratory (Europe) LTD, UK
Correspondent: Click to Email
The development of selected area and imaging XPS made significant advances with the adoption of Magnetic Snorkel lenses. This lens type of an unconventional design, projects from its main bulk an axial magnetic field that enables focusing of charged particles with low optical aberrations. Simulation techniques, calculate typical lens aberration coefficients of spherical, Cs=3mm and chromatic, Cc=8mm. In the XPS application original theoretical expressions used to characterize this lens type, have been re-modeled, to use present day data and predict some future improvements with this and other lens types.The spatial resolution of present commercial instruments (3μm) is limited by signal to noise. With the large collection angles involved, it is shown that astigmatism is a major contribution that limits the spatial resolution. Furthermore the spatial resolution of the detected signals is insufficient to resolve the stigmatic foci of the astigmatism present in the XPS image.Snorkel lenses are positioned under the specimen and to enable sufficient working distance between the lens and the specimen surface, the lens is highly excited. Typical working distances measure 15mm. By refining the lens coil and iron circuit design, the lens excitation is increased to a maximum and a lens working distance of 19mm is achieved. Consequently with the Snorkel lens positioned in this manner, the thickness of sample will remain limited. A Snorkel lens may also be positioned above the specimen surface so that previous spatial constraints imposed on the specimen form no longer apply. The lens is conical in shape with a bore to focus the photoelectrons through. It will be shown that a Snorkel lens with a 45º access to the specimen surface and at a 2mm working distance has been experimentally proved with very low lens aberration coefficients (Cs, Cc) of 1mm. Calculations show that with only small increases to existing laboratory x-ray powers, it is possible to use this lens to image XPS spatial resolutions of 1μm with present day count rates. Snorkel lenses can have limitations with specimens that magnetize, particularly specimens with topography. Substitution of an Electrostatic accelerating lens for a Magnetic Snorkel lens may result in only a small deterioration of the detected photo electron photoelectron spatial resolution. The ultimate spatial resolution of an image exhibiting the same signal to noise is calculated to be 7µm (Electrostatic) as apposed to 3µm (present day Magnetic). Details of an accelerating electrostatic lens are shown, that is proved to operate reliably with no electrical breakdown.