Articles you may be interested inMilling of submicron channels on gold layer using double charged arsenic ion beam The use of focused ion beam ͑FIB͒ instruments for device modification and specimen preparation has become a mainstay in the microelectronics industry and in thin film characterization. The role of the FIB as a tool to rapidly prepare high quality transmission electron microscopy specimens is particularly significant. Special attention has been given to FIB milling of Cu and Si in the microelectronics arena. Although FIB applications involving Si have been extremely successful, it has been noted that Cu tends to present significant challenges to FIB milling because of effects such as the development of milling induced topographical features. We show evidence that links the occurrence of milling induced topography to the severity of redeposition. Specifically, Cu, which sputters ϳ2.5 times faster than Si, exhibits an increased susceptibility to redeposition related artifacts. In addition, the effects and the mechanism of Ga ϩ channeling in Cu is used to illustrate that Ga ϩ channeling reduces the sputtering yield, improves the quality of FIB mill cuts, and improves the surface characteristics of FIB milled Cu. Finally, a technique for improving FIB milling across grain boundaries or interfaces using ion channeling contrast is suggested.
The Focused Ion Beam (FIB) instrument has been utilized for site-specific specimen preparation for a wide range of analytical techniques due to its ability to achieve high spatial resolution imaging, milling, and deposition [1,2]. The understanding of FIB damage is important to ensure that the region being analyzed is indeed representative of the material, and is not due to a specimen preparation artifact. The interaction between the incident ions (e.g., Ga + ) and the target material during FIB operation (e.g., imaging or milling mode) may lead to surface damage and consequently limit the ability to achieve high quality high-resolution TEM images. Amorphization of a FIB milled crystalline surface may occur due to sufficient atom displacement within the collision cascade resulting in the loss of long-range order when the density of point defects reaches a critical value [3]. Redeposition of sputtered atoms has also been reported as a result of FIB milling [4]. The propensity for redeposition increases when FIB milling is performed in a confined and/or a high aspect ratio trench, or when FIB conditions are used that contribute to factors that increase the sputtering rate (e.g., using a higher beam current) [5]. Observations have shown that FIB milling with Ga at an energy of 30 keV will produce amorphization damage along a Si side-wall that is ~ 28 nm thick and up to 20 wt% Ga may be present within the damage region [6]. Previous work in our lab has shown that the side-wall damage thickness in Si varies with beam current [7]. In addition, while significant amounts of Ga were observed in the side-wall damage [8], Ga was not detectable in side-wall damage when Si was FIB milled using gas assisted etching (GAE) [9]. The following study was performed in an attempt to better understand FIB damage. In this study, three square trenches (2x2 µm 2 , 4x4 µm 2 , and 6x6 µm 2 ) were FIB milled to 1 µm in depth in a (100) Si wafer using an FEI 200TEM FIB workstation equipped with a Ga liquid metal ion source and an Omniprobe in-situ W probe. An accelerating voltage of 30 keV and a beam current of 1000 pA was used to mill the trenches. The specimen was removed from the FIB and sputter-coated with Cr to preserve the FIB milled damage layers. The specimen was put back into the FIB and the trenches were filled with CVD Pt deposition using a beam current of 100 pA. A cross-section TEM specimen was prepared across the trenches using the in-situ FIB lift-out method [10]. The specimen was observed using a Philips EM430 operating at 300 keV. A bright field (BF) TEM image of the trenches is shown as FIG. 1 (a). A BF image from the top of the wafer is shown in FIG. 1(b). A BF image from the side-wall of the middle trench is shown in FIG. 1(c). Note that the side-wall damage clearly consists of two regions with different contrast. This layer clearly indicates that the side-wall damage consists of two regions: (i) an amorphization layer and (ii) a redeposition layer. X-ray energy dispersive spectrometry (XEDS) results showed that significa...
The introduction of the focused ion beam (FIB) instrument for site-specific material removal continues to alter the course of materials characterization. However, one of the disadvantages to FIB specimen preparation is what is commonly known as "curtaining." Curtaining artifacts are most often observed in semiconductor materials where multiple patterned layers of materials having a low sputtering yield blocks a faster sputtering yield material. In a bright field TEM image, curtaining appears as mass/thickness contrast where, e.g., the Si substrate appears darker under a gate than far from the gate. This artifact can be especially problematic in electron holography of semiconductor gate structures where the phase image is dependent on specimen thickness as well as the desired dopant distribution [1,2]. To eliminate curtaining effects, and hence, local differences in specimen thickness in the region of interest (i.e. the gate region of a semiconductor device), a technique based on in-situ lift-out (INLO) was used to prepare semiconductor devices from the Si -side of the device. Backside milling by FIB INLO on an FEI single beam 200TEM FIB is described below.FIG. 1 shows an in-situ probe just touching a piece of material FIB milled free for subsequent liftout. Note that the Si substrate is located on the side opposite the probe. In FIG. 2 the probe is positioned such that is comes through the opening of a slotted Cu grid. The specimen is mounted on the side of the Cu grid using ion beam assisted Pt deposition. A low magnification FIB image of the specimen mounted on the Cu grid after the probe has been removed is shown in FIG. 3. Note that the Cu grid was cut prior to mounting it in the specimen holder such that the cut faced downward in the specimen mount. The specimen mount was then removed from the FIB instrument and the Cu grid was flipped 180 o such that the grid cut now faced upwards. The holder was placed back into the FIB such that FIB milling could now be performed from the Si side of the specimen. Gate structures in the specimen were located and material below the gate (as viewed from FIG. 4) was removed by either (i) tilting to 54 o , FIB milling the material away, tilting back to 0 o , rotating 180 o , and repeating or (ii) after the specimen was mounted as in FIG 3, the specimen was re-mounted such that the plane of the Cu grid would be perpendicular to the beam to facilitate material removal below the gate. The specimen was FIB milled from the Si side directly to a thickness of ~ 300 nm for electron holograph. Alternatively, the specimens were FIB milled to ~ 800 nm and then further thinned in an ion mill in an attempt to remove FIB damage [3]. FIG. 5 shows a phase image of a PMOS device and FIG. 6 shows a phase image of an NMOS device. Note that FIGS 4,5,6 show no evidence of curtaining. Also evident in FIGS. 5 and 6 are the signature phase contrasts from the respective p-type (dark grey) and n-type (light grey) which outline the doped junctions. More details on the phase images appear elsewhere in th...
Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
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