In materials science, atom probe tomography (APT) plays a crucial role for the investigation of the three-dimensional elemental composition of materials at near-atomic resolution. Since its beginning, rooted in the principles of field ion microscopy, APT has witnessed an evolution in both measurement equipment and specimen preparation methodologies. Initially confined to the study of electrochemically etched metal wire specimens, the field has expanded to incorporate a diverse array of specimen holders and carriers, such as microtip coupons and half-grid type holders, with focused ion beam (FIB) lift-out methods emerging as a standard practice. These advancements resulted in a variety of approaches for achieving ready-to-run field emitter type specimens, yet the intricate and time-intensive nature of these procedures continues to pose significant challenges, representing a bottleneck in APT research.
Recent developments in femtosecond (fs) laser ablation technology have shown promising potential to mitigate these complexities, streamlining both electrochemical and ion beam assisted specimen preparation processes [1, 2]. Remarkably, the integration of fs-laser techniques can eliminate the need for lift-outs entirely, offering a more efficient pathway to specimen preparation [3]. This presentation will provide a comprehensive overview of different approaches for the utilization of fs-laser ablation followed by final specimen preparation via focused ion beam (FIB) techniques to create ready-to-run specimens. These innovative strategies facilitate the rapid pre-preparation of specimens in an assortment of geometries — ranging from microtip arrays and half-grids to whole specimen holders, suitable for correlative microscopy investigations, directly prepared from the specimen materials — without necessitating lift-outs, thereby significantly reducing preparation time and complexity [3]. At the same time, utilizing the material of interest to create whole specimen carriers of different geometries, tailored to specifically meet the requirements of the research problem in question. This bypasses the necessity of expensive consumables, FIB utilization and personnel hours are significantly reduced and thus, the herein presented approaches hold a significant potential for cost saving.
Three exemplary types of fs-laser processed specimen types are shown in Fig. 1a to c. The processed microtip coupon shown in Fig. 1a essentially represents a replica of a commercially available standard microtip coupon and a detail of a pre-prepared post with a TiN coating on top of it. A large number of posts of the material to be investigated is readily available directly after fs-laser processing and the efforts to prepare a ready-to-run specimen using FIB are quite moderate. This approach can be applied to virtually any material, a Pt-weld or similar as required for lift-out specimens, and frequently considered as a weak point, is not needed and in case of specimen fracture further specimens can be prepared with moderate effort. Besides the easy access to a large number of specimen posts, this methodology is predestined to be coupled with correlative techniques for predefinition of sites from which posts should be processed. Examples utilizing energy dispersive X-ray spectroscopy, electron backscatter diffraction and FIB time-of-flight secondary ion mass spectroscopy implemented on the basis of cross-correlative site specific sample preparation using image overlay techniques will be presented and discussed. This cross correlative approach allows to investigate e.g. precipitates, specific individual grain orientations and grain boundaries, even in larger quantity, in quite effective manner. Fig. 1b shows a prepared half-grid type specimen which is quite similar to frequently utilized lift-out half grids as used for correlative microscopy investigations based on sample holders such as the one presented in ref. [4]. Similar to the prepared microtip coupon, this type is directly made from the material of interest and can be utilized for correlative microscopy investigations requiring e.g. transmission Kikuchi diffraction or pre- or post-evaporation transmission electron microscopy experiments. For thin films and coatings, this type has the further advantage that it can be applied in in-plane or cross-plane geometry, providing specimens which can either be aligned across or along an interface. While such grids still need a specimen holder as the one presented in ref. [4], fs-laser processing holds the potential to create whole holders including the specimens at the very apex of the holder as presented in Fig. 1c. Such holders provide the same functionality as half-grid type specimens, yet the necessity of handling the half-grid which are frequently a source for failure is avoided and the whole holder is much more comfortable to handle than a single individual half-grid.
In addition to the presentation of different specimen geometries, a discussion on different processing strategies, different sample stages and laser beam incidence directions, as well as possibilities of final preparation via alternative routes such as broad ion beam milling or electrochemical etching will complement this talk to provide a thorough image of the current status and future directions of fs-laser preparation for APT related research. This will clearly illustrate that the advancement of fs-laser processing signifies a major leap forward in APT specimen preparation, as it embodies a progressive step towards increased productivity heralding a faster, more adaptable, and sustainable methodology.
Different types of fs-laser pre-prepared APT specimens ready for final FIB preparation, overviews are provided in the upper row and details in the bottom row. Microtip coupon providing large number of specimens on one carrier (a) a half-grid type holder allowing for correlative microscopy investigations (b) and whole sample holder ready to be used in a TEM omitting the need to delicately handle and manipulate half-grids (c).
