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edfas.org 19 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 24 NO . 2 CHALLENGES IN FIB-BASED SAMPLE PREPARATION The traditional FIB transmission electron microscopy (TEM) sample- preparationmethods result incurtaining (waterfall) effect which is problem- atic for SCM and topographic analysis. Additionally, the surface that is FIB milled is amorphizedduring theprocess. The approximate depth of this amor- phous layer ranges from 10 to 30 nm, depending on the beam accelerating voltage. This amorphous layer impedes carriermovement at the sample surface, resulting in partial or even total loss of the SCM signal. To overcome these issues, the team implemented an SCM sample-preparation method using an inverted TEM sample-preparation method (FIB-SEM) to remove the curtaining effect in front-end-of-the-line (FEOL) structures. Improved electrical conductivity is also accomplished by platinum wire deposition. A platinum “wire” connects the silicon chip to the TEMgrid to improve electrical contact between the sample and TEMgrid. A low energy with shallow angle non-gallium-based ion milling techniquewas used to reduce the depthof the amorphous layer introducedby a final low-voltage (5 keV) FIB cleanup. Inverted sample preparationwith low-voltage, ionmilling for SCM applications has never been reported and mul- tiple obstacles had to be overcome such as: (i) adequate waterfall or curtaining elimination, (ii) amorphous layer removal, (iii) adequate electrical contact to the sample through the sample holder TEM grid, and (iv) process repeatability. The solutions developed to successfully address these challenges are discussed here, which will open up new areas of inverted sample preparation and low-voltagewith shallow-angle ionmilling for site-specific, defect localization. SAMPLE PREPARATION APPROACH Samples with 14 nm FinFET devices were prepared using: • Mechanical polishing with an iterative technique for site-specific, defect localization • TEM inverted sample preparation in a dual beam FIB followed by a low-energy argon (Ar) ion milling. Figure 1a shows a schematic of a three-dimensional FinFET device where the black arrow indicates the direction of polishing. The samples for both mechani- cal and inverted TEM sample preparation were prepared to expose the fins of failing device in the direction as shown in Fig. 1a. The desired perspective for analyzing the fins is shown in the TEM image in Fig. 1b. The sample as received was at the FEOL contact level. Referencemarkswere placedon the sample as guideposts that wouldbecome visible in the field of viewon the cross- section face when failing device is cross-sectioned. The markswere constructedusinga FIB. Cross-sectionA-A’ was the target (Fig. 2). At this location, the three PFET fins that comprise P4 (as shown in Fig. 2 layout) would be exposed. The first set of samples were prepared by using mechanical polishing followed by referencemarkingwith FIB-SEM for controlled-material removal. A thin (~300 Å) layer of chromium (Cr) was deposited on the top surface of the sample to help facilitate good electrical contact to the region of interest. A thin cover glass was epoxied on top of the Cr layer using optically transparent epoxy. The procedure described in the last paragraph is done for structural support and to minimize rounding of the cross-section face during polishing. The sample was then Fig. 1 (a) Schematic of a three-dimensional FinFET device (arrow indicates the direction of polishing). (b) Cross-sectional TEM image of FinFET devices. (a) (b) Fig. 2 Device layout showing the location of the desired cross-section: A-A’.