The local electrode atom probe (LEAP ® ) [1] geometry enables analyses of multiple microtip specimens fabricated from a planar surface, Fig. 1. However, the preparation of polymeric, biological and particulate specimens for such analyses remains a challenging task. One general approach to solving this problem is the use of mold-replication techniques. We report here the first successful efforts at forming microtip specimens from polymers.In addition to shape, a significant issue with analyzing organic material in atom probe tomography is electrical conductivity. A potential solution is to embed the organic material in a matrix of conductive material. Although most polymers are not electrically conductive, many intrinsically conductive polymers (ICP) are used in various industries for applications such as antistatic surfaces on television screens, flexible electrode materials and polymer light-emitting diodes. In this study, a conductive polyurethane dispersion (CPUD2 [2]) was chosen for its low viscosity and ability to form a thin, uniform coating on a surface. To form the specific specimen shapes required for LEAP, polydimethylsiloxane, a silicone rubber, was used to prepare a mold due to its ability to replicate fine features and to withstand the 50˚C temperature needed to cure CPUD2. A micro-centrifuge tube was used to contain the silicone mold. Initial experiment molds were made from several sizes and types of needles. The polymer filled these molds well, but the needles of polymer were undesirably long (~1-2cm) and flexible, Fig. 2. The tip radii were relatively large (~3-10µm), which necessitated long milling times in a focused-ion-beam (FIB) tool. In addition, this process was time intensive and only created a few good specimens.In the interest of making many identical specimens in parallel, a silicon substrate with a nine-by-nine array of atom-probe-sharp tips [3] was chosen to create a new mold. The microtip coupons, attached to copper stubs were slowly dipped into the silicone and held in place with alligator clips, Fig. 3. These were set aside to cure for twenty-four hours before removing the silicon substrate, Fig. 4. The CPUD2 was poured into the mold, capped and microcentrifuged for 30 minutes at room temperature and 3 RCF (relative centrifugal force). After curing, the CPUD2 replica was removed, trimmed and epoxied to a copper stub, Fig. 5. The extraction of the specimen from the mold must be done slowly and in a linear fashion. The resulting microtips' radii were ~ 2-3µm. With slight FIB milling using annular mill patterns [4], they were ready for attempted LEAP analysis, Fig. 6. This specimen molding technique has the potential to be a relatively quick, simple and repeatable process for creating multiple specimens, enable the encapsulation of particulate specimens within a conductive matrix [5] and enable atom probe analysis of these soft materials.
The Local Electrode Atom Probe (LEAP®) is an innovative three-dimensional atom probe microscope developed at Imago Scientific Instruments that provides 3-D atomic-scale imaging with quantum-level detection capability [1,2]. The LEAP microscope functions by taking specimens apart one atom at a time, and thence one atomic layer after another: Individual specimen atoms are ionized from the surface of a needle-shaped specimen by a rapidly pulsed electric field, and are then accelerated to a position-sensitive detector. The location where each ionized atom excites this detector directly maps to its original specimen position by projection microscopy, while time-offlight measurement determines elemental identity. The LEAP microscope provides 3-D images with a nominal magnification of one million times with ~0.5 nm lateral resolution and 0.2 nm axial resolution, in combination with single atom analysis of elemental composition.316L stainless steel alloy is widely used in medicine for applications that include implanted spinal fixation devices, bone screws, cardiovascular and neurological stents, and as critical components of minimally invasive surgical devices. These applications are made possible due to suitable physical and mechanical properties, good corrosion resistance in biological environments, reasonable biocompatibility, and good manufacturability. As medical technology advances towards microsurgical and minimally invasive techniques, there is a drive to produce ever-smaller devices that demand higher material performance and hence enhanced nano and micro-scale control of material structure. The nano-structure and composition of the material surface, typically an oxide, is especially critical since biological responses and corrosion occur at the material-environment interface. Thus, there is an increasing need to understand the 3-D structure and composition of metallic biomaterials at the atomic scale. Three-dimensional atom probe microscopy can uniquely provide such atomic-level structural information.In the present study, two different materials were analyzed. The first were medical device 316L stainless alloy sheets chemically modified at Medtronic, while the second were model specimens obtained as 0.38 mm diameter 1/8 hard 316L wires from a commodity supplier. To determine the bulk structure of the Medtronic sheet specimens, segments were cut from the interior volume and polished smooth. These were then mounted in holders, electropolished with 10% perchloric acid in acetic acid at 8-20 V DC to a nominal end radius of 20-50 nm, in order to provide the necessary needle-shape for atom probe analysis. The bulk atomic composition and structure of this material was then analyzed with the LEAP microscope (Fig. 1). All anticipated elements were located in the specimen including Fe, Cr, Ni, C, P, Si, Mo, and Cu, with a uniform elemental distribution and without precipitates or grain boundaries in the analyzed regions. The 316L wires were also electropolished as described above. In the course of electropolishing and ...
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