No abstract

Microscopists demand better performance from their electron microscopes with every new instrument. With the advancement of new instrument technologies, better images, higher resolution, more precise analysis, and faster throughput are all benefits that are expected of expensive purchases. Still, in many cases, a well-known problem detrimentally affects the quality of results: specimen contamination.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

The scanning electron microscope (SEM) was developed for imaging applications and later, with the introduction of the Si(Li) energy dispersive spectrometry (EDS) detector, simultaneous imaging and xray microanalysis became possible. However, long working distances were needed to use the EDS detector because the SEM was initially optimized for imaging only where the best spatial resolution is generally obtained at short working distance. This problem is still topical today and unfortunately x-ray microanalysis is never performed in the best imaging conditions, i.e., not with the smallest probe size. With the introduction of the Bruker QUANTAX Annular QUAD silicon drift detector (SDD) system, scanning electron microscopy is facing a revolution. Since this detector is inserted below the pole piece. A low working distance can be used and an improved spatial resolution becomes possible. Also, since the count rate can be as high as 2 000 000 cps, many x-ray will be acquired in a short period of time, allowing to lower significantly the detection limit of elements and as well the minimum size of different phase features. However, the effect of the detector geometry and position on the quantification microanalysis is unknown. Figure 1 compares the typical solid angle for SDD on the side of the chamber (with different sensor area) with an annular SDD below the pole piece. For the detector distance typical for each detector position, the solid angle for the annular detector was 10 higher. An optimum detector distance was observed at 1.5 mm, where the solid angle was 1.35 sr. The maximum solid angle for a side SDD of 150 mm 2 sensor area was only 0.09 sr with a detector distance of 40 mm. For quantification microanalysis where the absorption of the x-ray is important, the value of the takeoff angle is important. Lower value increases the absorption in the sample. Also the correction model, suppose a fix value of takeoff angle. However, Figure 2 shows that the minimum and maximum of the takeoff angle on the sensor change with the detector distance. For a side detector with 80 mm 2 area, the takeoff angle change from -7 to 7 degree around the center value of 30 degree. For an annular detector, the large takeoff angle (> 70 degree) was obtained for detector distance larger than 5 mm, which minimize the absorption effect and should give more accurate quantification results for strongly absorbed sample, for example light element analysis. At the optimum detector distance the mean takeoff angle was 33 degree with a minimum of 24 and maximum of 50 degree. The effect of this large takeoff angle variation on the correction model is currently studied. The advantage of the larger solid angle is show in Figure 3. The detection limit CD min was 10 smaller for an annular SDD. With adapted correction model, the annular SDD with is larger solid angle will clearly revolution the quantification microanalysis by moving from point analysis to quantitative micrograph with simultaneous electron imaging.

This paper presents a nanomanipulation system for operation inside scanning electron microscopes (SEM). The system is small in size, capable of being mounted onto and demounted from an SEM through the specimen exchange chamber without breaking the high vacuum of the SEM. This advance eliminates frequent opening of the high-vacuum chamber, thus, incurs less contamination to the SEM, avoids lengthy pumping, and significantly eases the exchange of endeffectors (e.g., nano probes and grippers). The system consists of two independent 3-DOF Cartesian nanomanipulators based on piezo motors and piezo actuators. High-resolution optical encoders are integrated into the nanomanipulators to provide position feedback for closed-loop control. A look-then-move control system and a contact detection algorithm are implemented for horizontal and vertical nanopositioning. The system design, system characterization details, and system performance are described.