edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 22 POST-PFIB ARGON ION MILLING The PicoMill TEM specimen preparation system from Fischione Instruments, which has a 600 nm diameter Ar ion beam, was used to thin the TEM specimens to electron transparency post-pFIB milling. During milling, the specimen was positioned so that the Ar ion beam first hit the bottom part of the specimen.[11-14] This milling method requires a high specimen tilt angle (15 to 20°); the milling direction is from the bottom of the specimen up to the ROI (“bottom-up milling”). The high milling angle increases specimen surface visibility, which allows accurate placement of the milling box for targeting (Fig. 3a). The ROI remains intact throughout the Ar ion milling process, which prevents unwanted sputtering of the FIB cap (Fig. 3a inset, light blue layer), typically Pt, in the ROI. The bottom-up milling method allows for the Ar ion beam to reach the bottom of the specimen first and then the ROI (Fig. 3b, side view), which uses the Ar ion beam and its beam tails to mill the specimen uniformly. The size of the milling box can be adjusted to the width of the specimen and the height of the ROI or the height of the whole specimen. A user-defined 10 x 3 µm (width x height) milling box was placed near the middle of the specimen; the concentrated beam of argon ions moved in a serpentine raster pattern within the box. Decreasing energies (900 to 700 eV) were employed to remove the Pt layer and further thin the specimen. ELECTRON MICROSCOPY IMAGING AND ANALYSIS TEM and precession electron diffraction (PED) patterns were acquired at 200 kV to differentiate the amorphous and crystalline areas on the cross-section TEM specimen using a beam precession system from NanoMegas. Details of the PED analysis are described in Yu and Skowronski.[10] High-resolution TEM images and electron dispersive x-ray spectroscopy (EDS) data in scanning transmission electron microscopy (STEM) mode of the cross-section specimen were acquired at 300 kV accelerating voltage. Fast Fourier transform (FFT) analyses were performed to distinguish crystalline areas on the plan view TEM specimen. Using the same plan view specimen, high-resolution-STEM images were obtained using an aberration-corrected TEM, the Themis from Thermo Fisher Scientific, operated at 200 kV with the elemental distribution verified using EDS by means of the Super-X EDX system attached to the microscope. RESULTS AND DISCUSSION DEVIATION IN CONVENTIONAL Ga FIB SPECIMEN THINNING PRACTICES Specimen thinning using the pFIB required placing the milling box further away from the edge of the specimen to prevent over-milling the top of the specimen. This contrasts with Ga FIB milling where the milling box is placed close to the surface that requires thinning. The beam sizes of Xe and Ga sources can explain the difference in the placement of the milling boxes. The full width half maximum (FWHM) and depth of Xe beam burns on Si in cross-section were 0.48 µm and 1.34 µm at 300 pA and 1.87 µm and 18.28 µm at 60 nA.[5] For the same ion energy of 30 kV, the Xe beam in the pFIB is wide at low currents and sharp and narrow at higher currents. Beam currents in the 1 nA to 50 pA range are typically applied during the Ga specimen thinning, while for Xe pFIB milling, a slightly higher beam current ranges from 4 nA to 100 pA was employed for a narrow Xe beam. Thinning of the specimen still occurred due to the Xe beam tails, which extend beyond the bounds of the milling pattern.[5] In this case, specimens prepared by Xe pFIB are slightly thicker than those prepared by Ga FIB. This is not a setback because controlled specimen thinning for electron transparency can be performed by Ar ion beam milling. PARTIAL SET PROCESS STUDY USING THE CROSS-SECTION TEM SPECIMEN The elemental composition and the changes at the partial SET state of the GeTe layer of the PCM device after Xe pFIB fol- Fig. 3 High-tilt bottom-up Ar ion milling method. The ion beam image (a) illus- trates the visibility of the specimen’s surface with the specimen tilted at 15°, making targeting easier by accurately placing the milling box. Inset illustrates the specimen configuration consisting of the specimen (dark blue) and FIB cap layer (light blue). The schematic of the specimen’s side view (b) illustrates the Ar ion beam path: from the bottom of the speci- men, through the region of interest, and last, toward the FIB cap layer. (a) (b) (continued on page 24)
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