edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 20 PRECISE FINAL SPECIMEN THINNING BY CONCENTRATED ARGON ION BEAM MILLING OF PLAN VIEW TEM SPECIMENS PREPARED IN THE XENON PLASMA FIB C.S. Bonifacio1, Y. Yu2, M.L. Ray1, M. Skowronski2, and P.E. Fischione1 1E.A. Fischione Instruments Inc., Export, Pennsylvania 2Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania cs_bonifacio@fischione.com EDFAAO (2024) 4:20-26 1537-0755/$19.00 ©ASM International® INTRODUCTION The preparation of transmission electron microscopy (TEM) specimens using a focused ion beam (FIB) system is a standard sample preparation approach because it allows site selectivity and can be accomplished quickly. Though a cross-section specimen configuration is typical, a plan view specimen configuration can also be prepared using a FIB system. There are many advantages to using plan view specimens, especially for device failure analysis. There is a higher probability of imaging a defect due to the larger field of view[1,2] and the sample yields many repeating features on a single specimen[3] (unlike a small cross-section specimen). Combining cross-section and plan view analyses yields a complete perspective of a defect in three dimensions. The choice of a plan view configuration, as opposed to a cross-section configuration, is relatively infrequent because plan view specimen preparation requires complicated sample manipulation and takes longer to accomplish. Because the plan view specimen is oriented parallel to the bulk sample surface, a large volume of material must be removed, especially beneath the region of interest (ROI). Xe plasma focused ion beam (pFIB) specimen preparation is ideal for preparing plan view TEM specimens due to its large-volume milling capabilities.[4] However, producing electron-transparent TEM specimens using a Xe pFIB system requires an adjustment of stage tilt and milling pattern placement; it deviates from the standard Ga FIB method because of the relatively large Xe beam and its wide beam tails.[5] Ga FIB preparation with post-FIB Ar ion beam milling of TEM specimens has been reported previously.[6-8] Plan view TEM specimens prepared by a Xe pFIB system followed by concentrated Ar ion beam milling constitute new work. This study used GeTe-based, mushroom-type, phase change memory (PCM) devices to investigate the extent of phase transformation from amorphous to crystalline GeTe. PCM devices are of interest due to their non-volatility, high scalability, and compatibility with back-end-of-line processing.[9] The RESET state of PCM corresponds to an amorphous dome of the chalcogenide quenched on top of the bottom electrode. The SET state is the fully crystallized functional layer; the partial SET is an intermediate crystallization state. The transition from RESET to SET can occur purely by the growth of the crystalline grains from the amorphous dome periphery toward the bottom electrode or by a combination of nucleation and growth. The GeTe-PCM crystallization process was imaged by the TEM cross-section technique in Yu and Skowronski.[10] However, a quantitative assessment of the transformation rates can only be visualized using a plan view TEM specimen. This study aimed to image the intermediate states and understand the crystallization mechanism in the transition from the amorphous RESET state to the crystalline SET state. This was achieved by leaving the device in a partial SET state; the PCM layer still contains a partially crystallized amorphous dome on top of the bottom electrode. Post-pFIB polishing of cross-section and plan view TEM specimens is essential to the preparation workflow. The GeTe layer in the PCM device is highly susceptible to ion beam damage, especially by Ga ions, and for this reason, the Xe pFIB was only used for bulk milling of the
RkJQdWJsaXNoZXIy MTYyMzk3NQ==