Microfabrication of Nanoscale Diamond Tips for Atom Probe Tomography

Atom probe tomography (APT) is a technique that uses the controlled field evaporation of ions from a sample to reconstruct a nanoscale 3D model with near-atomic resolution. This technique has been used to study the atomic nanostructure of many different materials, ranging from biological materials to nanodiamond inclusions in meteorites. To prepare samples for APT, the region of interest must be mounted on a micropost and milled into a sharp point, typically using a focused ion beam (FIB). Usually, this process involves lifting out a bar with a triangular cross section and iteratively welding slices of the liftout onto microposts, and then individually sharpening each section. This is a reliable method for most thin-film and bulk samples, but challenges remain in preparing nanoscale diamond samples for APT analysis. For example, the liftout process requires relatively large trenches to be milled into the substrate, and the sharpening process also removes a significant amount of material. For particularly hard materials, such as diamond, this can be time-consuming, and the higher currents required to mill these hard materials can introduce difficulties with charging, especially in nonconductive materials like diamond. Charging also makes it difficult to mill with the precision required to fabricate APT tips. Here we present recent results using microfabricated diamond pillars with nanoscale radii of curvature and a novel FIB liftout procedure in order to improve the process and allow the simple fabrication of APT tips out of nanoscale diamond materials. In this procedure, a diamond membrane was glued to a sapphire substrate and thinned to 10 μm. Subsequently, many 10 μm pillars were dry-etched from the remaining diamond, forming conical nanoscale pillars roughly 2 μm at the base; this is the same diameter as the flat surface of the microposts used to mount APT samples. These posts can then be individually coated with a thin protective layer of platinum and lifted out by cutting near the base at a 52-degree angle to vertical. These can then be attached to the microposts by welding the gap formed by the angled cut. Following this process, minimal if any additional sharpening of the tips is required, only a light FIB polish to remove excess platinum and make minor shape adjustments. This procedure allows the efficient lift-out of well-shaped nanoscale diamond APT tips without the need to remove large amounts of material in the FIB, eliminating a major bottleneck. Additionally, this method allows the lift out of specific pillars, which may be useful for applications where there is a need to investigate individual pillars with well-defined quantum emitters. This approach also has the benefit of being able to measure entire pillars in applications for which the pillar morphology is important to the material’s properties, for example in the case of microcavities for single photon emission or diamond electrodes for bioelectronics.