LEO 912 TEM Short Manual. Prepared/copyrighted by RH Berg Danforth Plant Science Center

Transcription

1 LEO 912 TEM Short Manual Prepared/copyrighted by RH Berg Danforth Plant Science Center

2 Specimen holder [1] Never touch the holder (outside of the O-ring, double-headed arrow) because finger oils will contaminate this area and cause drift and column contamination The LEO carrier uses a screw to secure the grids [1]. PLEASE do not strip this screw, which is easily done and costs $ to replace. If the screw or grids are not stably attached then the image will suffer from instability and, worse case, they could fall off inside the microscope, incurring a major repair.

3 Permitted panel buttons * * * * * * * * * * * * * Only use the * highlighted buttons/knobs

4 The Green Screen Illum. Angle is an indicator of beam intensity (controlled by brightness buttons, see next page). M/LM toggles between the low mag mode (highlighted) and higher mag mode (not highlighted). Beam blanker parks the beam off the specimen, to prevent un-needed beam damage. Pressing Vac will display the vacuum system screen. When changing specimen, turn off the high voltage and press Close col valve. This isolates the vacuum-sensitive IGPs (by closing valves V3 & V9 ( ) from the specimen chamber, in case vacuum is broken during specimen insertion.

5 [1] [2] [3] Generating a Beam [4] Check the vacuum system: is it ready? (green Ready light on right panel [1]). Turn beam blanker on. Heat the filament up by pressing Filament [2]: it s status light will blink red during the ~four minute period during which the filament is heated and remains constantly lit when the filament is fully heated. Press the High voltage button [3]: its status light will blink red during voltage ramp-up and remains constantly lit when high voltage has been obtained. When changing grids there is no need to turn Filament off but High voltage should be turned off, before closing the column valve (see previous page). If vacuum is lost then you will hear valves close and Filament and High voltage lights will extinguish. The ready light will relight after high vacuum is again reached. Ill Aperture [4] controls brightness (condenser lens excitation): left button decreases brightness, right increases it. To prevent beam damage to the specimen or specimen drift from heating, keep brightness to a minimum, compensating for this by using longer exposure times for the camera.

6 Condenser lens aperture selection In our LEO TEM the system is typically operated in AIS, which selects the correct condenser aperture from a foil containing five apertures, selected by beam deflectors) dependent on magnification The third click selects the AIS aperture foil Center this with the x-y controls [arrows], at 4,000X

7 Objective lens aperture [1] For optimal contrast insert an objective lens aperture--the smaller the better, so use #3 (the third click in). The objective lens aperture selector [1] inserts the aperture. Center this (at 4,000x) with a section in the field of view by clicking image selector (lower left panel, next to mag buttons [2]) so that diffraction ( diff ) is activated (this images the shadow of the aperture) and center it with the x-y controls (arrows). The screen will be permanently damaged (burned) if the diffraction beam image is left on for long: be sure to switch back to image within a few seconds [2]

8 Selected Area aperture [2] [1] This is used to boost contrast in the low magnification mode ( M/LM on the green screen [1] is highlighted). This aperture is immediately below the objective aperture and on the column left [2]. Insert the second aperture (two clicks) and center with the X-Y centering knobs [arrows]. The aperture s shadow is not circular and centering is aimed at merely eliminating the unusual shadow that is seen.

9 Focusing [3] [2] [4] [1] Focusing [1] changes the objective lens current to alter its focal length. CW from focus is over-focus, CCW from focus is under-focus. With a new grid, position it in the eucentric plane (the correct height in the microscope) by first calibrating the objective lens (press [2], let it beep) then turn on the focusing aid ([3], the wobbler), and focus by moving the grid up or down using the Zcontrol on the console below the focus knob until the oscillations are at a minimum. Be sure to turn the wobbler off. At lower magnification, focus with the wobbler. With wobbler [3] on the image appears to oscillate in and out of focus. Adjust focus [2] to minimize this (and do not forget to turn it off). In wobbling, the illuminating beam is rocked between two tilted positions on opposite sides of the optical axis at high frequency, with the pivot point at the specimen plane. Focus above or below this produces two images so by adjusting focus to the specimen plane, at near true focus, the image shift is minimized. The fine-ness of focusing steps for [1] can be toggled between coarse and fine with button [4]. This is not available in low mag mode. For high magnification focus use the contrast dip (soft background) method.

10 Energy filter entrance aperture [1] [2] This is opposite from the top of the electron spectrometer [1], on the column right [2]. The first aperture is selected and centered with the x-y knobs. There is no need to remove this aperture.

11 Energy filter slit aperture [1] This is the next aperture down from the energy filter entrance aperture, but is on the column left [1]. It images electrons elastically scattered at small angles (elastic brightfield imaging, AKA zero loss imaging because inelastically scattered electrons are blocked by the aperture). This aperture produces a major improvement in image contrast, but at the cost of reduced image brightness. Insert the second aperture and center, first with the smaller right hand knob and then with the other, if needed [arrows, note the left is the smallest/outermost of the three]. The aperture shadow is not a circle unless it is nearly centered and will not always center at the screen center. Centering alters the size of the shadow. Adjust it so that the shadow becomes circular and illumination is as small as possible without cropping the image captured by the camera. If the area of illumination is too large then a (a sometimes subtle) dark spot will appear within the illuminated area and this is undesirable (distorts the image in that area). As magnification is changed often this aperture will need to be adjusted as it tends to close (screen becomes black) with magnification increase. The slit width of this aperture can be adjusted and this affects the spectral resolution of electron energy loss spectra (EELS).

12 Choice of magnification and camera operation Use the specimen carrier mechanical selector (on the left of the column) to release the clamp and select the first grid (look on the screen and also there will be a bit of steel showing on the holder on the column right) or the second. The LEO has a low mag mode M/LM mode on the green screen, used to select a decent section and region therein, and to image large areas. 80x-2,000x. Use the second SA (selected area) aperture for contrast (this aperture selector is immediately below the objective selector, and on the left). Go to the hi mag mode by first inserting the objective aperture, then touching M/LM so that it is not highlighted. Center the objective aperture. Select a mag that images the area of interest and also provides the desired resolution. The CCD camera is sufficient size (2k x 2k pixels) to produce a large image, but images a relatively small field of the image seen on the phosphor screen. To increase the field of capture make a montage ( MIA ) with the caveat that you will likely have to massage the data to get the montage put together decently. Minimize specimen heating/contamination by blanking the beam when it is not needed (this uses beam deflectors to point it away from the specimen). To preserve filament life turn the beam off if it is not being used for a while). Exposure time is controlled using the camera control menu (open this under the SIS Image drop down menu).

13 Shut down When you are through with the microscope, remove your grids and store the specimen holder inside the microscope. Confirm that high vacuum is reached (green Ready lamp). Be sure to turn off filament heating and high voltage. Confirm that this happens by waiting the ~4 minutes for the heating lamp to stop blinking and switch off. Both red lights should be off. Turn down the green screen brightness. Turn off the sodium vapor room light.

14 Signs of Trouble The most common problem you are likely to have is centering apertures. Come get me to help you if you are having problems do this rather than move the aperture so far out of alignment that it is difficult to find or that vacuum is broken. Find or call staff ( , Howard Berg on call 24/7) if you experience any problems. Specimen grid holder screw does not tighten. It may be stripped. Do not insert the carrier into the microscope if the grids are not firmly secured and if the screw is loose. Ready light is not lit green: there is not adequate high vacuum for generating the electron beam. This could happen when inserting or removing apertures and the vacuum loss in this case is usually minimal high vacuum could return in a few minutes, signaled by the green light. Then ramp up high voltage and filament heating. More fundamental measures must be taken if the vacuum is much poorer.