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Introduction |
Specifications |
Operation modes |
XPEEM and LEEM methods
The microscope is composed by five different parts, that are labeled (1) to (5) in the picture below. From the front (preparation chamber), the illumination column is located on the right hand side, the imaging column on the left hand side, the beam separator in the center of the instrument (in between both columns) and the objective in the main chamber above the transfer optics.
Figure 1: The French X-PEEM microscope.
(1) The Main Chamber
The sample is mounted on a vertical manipulator (facing down) in front of the objective lens. The main chamber is connected to the beamline toward a CF40 port. The incoming photon beam illuminates the sample in grazing incidence (16° to the hoerzontal plane). An XY manipulator allows the precise positioning of the sample (±5mm with an accuracy of 0.5µm). Four CF40 portviews can be used for evaporators, and can host Omicron EMF 3/4 or Elmitec evaporators. Note that in order to prevent a drastic contamination of the objective lens, evaporation of volatile materials such as Ga, As, Sb, organo-metallic and organic molecules are forbidden in the main chamber. Such evaporation can be performed in the preparation chamber connected to the microscope. The samples can be annealed in-situ up to 1900K and cooled down to 150°. The nominal base pressure in the main chamber is bellow 1·10-10 mbar. The microscope can be also operated as a gas flow reactor, a maximum pressure of 1·10-5 mbar is allowed.
(2) The transfer optics and the beam separator:
This part connects the illumination column and the imaging column to the main chamber. The magnetic beam separator deflects the electrons issued from the sample to the imaging column. It allows also to separate incident and reflected beams in the LEEM and LEED mode.
(3) The imaging column:
The imaging column consists of two lenses (Field and Intermediate lenses) and a set of three projective lenses (P1, P2 and P3). The electrons are projected onto a chevron channel plate/screen detector (Burle Co.). The final images are collected using 12 bits CCD video camera (PCO SensiCam).
(4) The illumination column:
A 20 keV electron beam is produced and focused in the illumination column, consisting of a LaB6 electron gun and a set of three lenses. The electrons energy can be varied in a range of -5 to 500 eV with an energy spread of 0.1 eV. The illuminated area on the sample is about 80µm.
(5) The sample preparation chamber:
A basic and small preparation chamber is connected to the microscope. This chamber is equipped with a fast load lock that allows transferring samples in 30 min from the air to the vacuum. This preparation chamber serves mainly for basic sample preparations as degassing, annealing, sputtering, gas exposure.
In the LEEM mode, one can schematically describe the path followed by the electrons from the illumination to the detector: a 20 keV electron beam is produced and focused in the illumination column (4). The electron beam is then deflected by the beam separator and transferred through the objective lens via the transfer lenses (2). The incoming electrons are decelerated by the electric field in front of the objective lens before they reach the sample surface (1). An intermediate image of the sample surface created by elastically backscattered electrons is set by the objective and transfer lenses to the center of the beam separator (2). This intermediate image is then transferred by the imaging column lenses via at least two further intermediate images to the image converter (channel plate). Finally a CCD camera acquires the image from the screen (3). In the X-PEEM mode, the illumination column is no more long used. The intermediate image of the sample surface in the back focal plane, is created from the photoelectron emitted after the absorption of a photon at a given absorption edge.
Fig. 2: Sketch of the French X-PEEM microscope.
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