ANL AAEM Project.

The ANL Advanced AEM Project

Nestor J. Zaluzec

Materials Science Division
Kinetics and Irradiation Effects Group
Argonne National Laboratory
Argonne, Illinois 60439 , USA

Commercial instrumentation on the market today, although labeled as "analytical electron microscopes", are in fact transmission or scanning transmission electron microscopes with analytical attachments. While the manufacturers always optimize their instruments, they inevitably do so at the expense of analytical performance, and instead optimize for the best possible image resolution. To overcome thses limitations Argonne National Laboratory (ANL) began a development project to specify and acquire a true advanced analytical electron microscope (AAEM). The instrument specifications were devised with the primary goal of attaining the best possible analytical sensitivity, resolution and versatility consistent for state of the art materials research and still provide moderate imaging capabilites. Early in the conceptual design it was determined that the instrument was sufficiently involved that it could not be built by the research staff at ANL alone, and the decision was made to enter a development contract with a commercial manufacturer. After spending 3 years raising funds, 2 years visiting manufacturers to discuss and detail the specifications, and 1 year in bidding and negotiating a contract, the project was finally awarded to VG Scientific Instruments, (now VG Microscopes Ltd.) of East Grinstead, England.

The general specifications of the instrument which were finally developed are listed in Table 1 below. Basically, the system consists of a CFEG with gun lens, a triple condenser, objective and quadruple projector (Figures 1-2). While high resolution imaging of the specimen is not important, imaging of the probe is considered essential, thus the four projectors allow sufficient magnification to do so under all conditions. All apertures (VOA, Cond, Obj, Diff, EELS) are electrically isolated and have 8 position stops for electron dosimetry, together with a faraday cup in the probe forming system. A conventional electrostatic beam blanking system is integral and is located between C2 and C3 lenses. One of the most important features in the system design was the development of an objective lens configuration which satisfied the ANL requirements. Figures 3 and 4 sketch out the geometrical configuration of this area, which includes 7 experimental ports for analytical equipment development, and 3 for electrical connections. The pole piece gap is sufficient to allow complete inversion of the stage, although in practice due to the need for gearing the motor drives the actual rotation about the primary tilt axis is limited to +/ 85 degrees. Initially 3 types of specimen stages are being constructed: ambient double tilt Be, LN2 double tilt Be, and a single tilt 1000 C heating stage. Image detection is accomplished by using conventional NTSC video in the CTEM and BF/ADF imaging in STEM, using one of four operator-selected YAG screens, the signals from which are flash digitized (8, 16, 32 bit) and routed to two independent frame stores (2K x 2K x 8 bit). A comprehensive specimen preparation chamber is directly interfaced to the column (Figure 2) which allows complete extensive cleaning, characterization and preparation of the specimen surface (see Table 1). A "parking lot" for 12 specimens is also provided for in the specimen prep chamber. Two 400 l/s and two 60 l/s Ion Getter, four Titanium Subliminators, 1 Turbomolecular comprise the evacuation pumps for the system.

Consistent with the goal of maximizing the analytical sensitivity is minimizing spectral artifacts. Careful scrutiny has therefore been given to all parts of the probe forming system, with special attention to the various beam defining apertures. For example, multi-layer low/high Z material combinations have been employed in both beam and non-beam defining apertures and at all critical surfaces to minimize potential sources of uncollimated hard x rays which give rise to the hole count phenomenon. The windowless XEDS system has been optimized to maximize the subtending solid angle and allow retraction along a direct line of sight path to the specimen. This allows the instrument to achieve a continuously variable solid angle up to a maximum of 0.3 sR. A conventional hemispherical Auger spectrometer with extraction optics is interfaced to the center of the objective lens and both serial and parallel EELS detection capabilities will be present. In addition, not shown in Figure 3, on the incident beam side of the objective lens between C2 and C3 is a drift space for secondary and Auger electron spectrometers utilizing parallelizer optics installed within the objective prefield. The entire system is microprocessor controlled using a 386 level PC as system controller with a second 286 system for image handling using the two frame stores. The system may be controlled either directly by the operator using conventional multi-function dials and switches or using the PC and a mouse directed interactive graphical user interface.

The high voltage generator has already demonstrated combined ripple and noise of 150 mV at 300 kV (0.5 ppm!) under a full load bench test. With the column operating at 300 kV Gold 0.2 nm STEM lattice images have been achieved. The expected gun brightness is ~4x109A/cm2/sR at 300 kV and the nominal image resolutions in both TEM and STEM should be < 3 pt/pt BF, and < 2 in HADF-STEM. Figure 5 plots a calculated contrast transfer function for the Objective Lens at 300 and 100 kV, while Figure 6 plots the calculated probe current/size relationships at 300 kV.

This project was supported by U.S. DoE , contract BES-MS W-31-109-Eng-38. Many people contributed enormously to the success of this project among these are all the staff in the ANL and at VG Microscopes Ltd.

Figure 1a AAEM Column during various stages of assembly

Figure 1b AAEM Column Schematic Diagram of Optics

Figure 1c Final Assembly

Figure 2: Plan View of System showing location of Preparation Chamber relative to Column

Figure 3: Plan View of Objective Lens Area Showing Experimental Port Configuration.

Figure 4: Cross Section through Ports 1/2 and Ports 3/4 of Figure 1. Ports 5/6 have the same geometry as Ports 1/2, Port 7 is similar to Port 4. Ports 8-10 are for electrical connections.

Figure 5: Calculated Contrast Transfer Function for the ANL AEM at 300 and 100 kV; Scherzer Defocus, Cs=4.4 mm, CFEG Source

Figure 6: Calculated Probe Size/Current Relationship for the ANL AEM at 300 kV

Table 1

Summary of Instrumental Capabilities of the AAEM

- A Cold Field Emission Electron Source

- Ultrahigh vacuum (UHV) environment

- Electron Optics capable of :

- Side Entry Goniometer Stages

- Analytical SubSystems on the E/O Column

- Specimen Preparation Chamber

-Computer Control

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Nestor J. Zaluzec /