edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 1 4 each layer with a ZEISS mSEM. The mSEM microscope is a multi-electron beam tool allowing high speed imaging of large areas of the delayered device.[10] METHODOLOGY The process begins with a vacuum chamber equipped with a two-lens post deflection argon ion source that operates up to 5 KeV as shown in Fig. 1. The subsequent introduction of water vapor on the sample surface facilitates the uniform delayering process, overcoming the challenges associated with traditional ion sputter delayering methods of heterogenous surfaces like copper and silicon oxide. The water effectively adsorbs on the surface and slows the individual material sputter rates so they are equalized. The water dosing involves hardware improvements over traditional FIB gas injection to efficiently deliver the chemistry uniformly over large sample areas and has advantages as shown in Fig. 2. The addition of a large effuser element helps to improve the efficiency of the water delivery to the sample surface over simply injecting gas chemistry.[11] Delayering quality is controlled by several factors including effective chemical delivery, ion beam shape, ion current density, and charge mitigation. The argon ion beam spot size for full chip delayer is on the order of 200 μm diameter. Reaching the next layer of the 10 nm device is the Metal 0 (M0) interconnect layer. It is constructed of a dense tungsten routing structure. The high atomic number of tungsten (W) makes stopping on the layer very effective with water dosing and Ar+ milling. The W material has multiple primary UV intensity peaks that are easily measured.[12] In combination with uniform delayering, dual photomultiplier tubes (PMTs) as photon detectors are used to collect the UV photons from the silicon oxide (SiO2) at the 250 nm wavelength and the metal layers (Cu and W) at 325 nm as shown in Fig. 3.[13] Aluminum interconnects can also be monitored by measuring a strong UV emission peak at the 395 nm wavelength as shown in Fig. 4. RESULTS Figure 5 shows a 64 μm x 50 μm selected area image capture of the 10 nm IC surface at the M0 layer using the Fig. 2 A schematic demonstrating the design of the gas doser system with an effuser (Pe) in comparison to a traditional gas injector in a FIB system. The effuser results in a more efficient delivery to the sample and results in a lower background chamber pressure. Fig. 3 UV plots to monitor the sputter rates of the silicon oxide and metal interconnect showing the silicon peak first as the STI layer is removed and the emergence of the tungsten V0 layer. These plots are generated by two separate PMT detectors with narrow bandwidth filters at 250 nm for silicon, and 325 nm for copper/tungsten. Fig. 4 An ion induced UV spectrum from Ar+ sputtered aluminum surface showing two peaks with the primary peak at the 395 nm wavelength.
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