Christian Penzkofer

Manufacturing and advanced characterization of sub-25nm diameter CD-AFM probes with sub-10nm tip edges radius

Atomic force microscopy (AFM) is increasingly used in the semiconductor industry as a versatile monitoring tool for highly critical lithography and etching process steps. Applications range from the inspection of the surface roughness of new materials, over accurate depth measurements to the determination of critical dimension structures. The aim to address the rapidly growing demands on measurement uncertainty and throughput more and more shifts the focus of attention to the AFM tip, which represents the crucial link between AFM tool and the sample to be monitored. Consequently, in order to reach the AFM tool’s full potential, the performance of the AFM tip has to be considered as a determining parameter. Currently available AFM tips made from silicon are generally limited by their diameter, radius, and sharpness, considerably restricting the AFM measurement capabilities on sub-30nm spaces. In addition to that, there’s lack of adequate characterization structures to accurately characterize sub-25nm tip diameters. Here, we present and discuss a recently introduced AFM tip design (T-shape like design) with precise tip diameters down to 15nm and tip radii down to 5nm fabricated from amorphous, high density diamond-like carbon (HDC/DLC) using electron beam induced processing (EBIP). In addition to that advanced design, we propose a new characterizer structure, which allows for accurate characterization and design control of sub-25nm tip diameters and sub-10nm tip edges radii. We demonstrate the potential advantages of combining a small tip shape design, i.e. tip diameter and tip edge radius, and an advanced tip characterizer for the semiconductor industry by the measurement of advanced lithography patterns.

Controlled fabrication of advanced functional structures on the nanoscale by means of electron beam-induced processing

The controlled deposition of materials by means of electron beam induced processing (EBIP) is a well-established patterning method, which allows for the fabrication of nanostructures with high spatial resolution in a highly precise and flexible manner. Applications range from the production of ultrathin coatings and nanoscaled conductivity probes to super sharp atomic force microscopy (AFM) tips, to name but a few. The latter are typically deposited at the very end of silicon or silicon-nitride tips, which are fabricated with MEMS technologies. EBIP therefore provides the unique ability to converge MEMS to NEMS in a highly controllable way, and thus represents an encouraging opportunity to refine or even develop further MEMS-based features with advanced functionality and applicability. In this paper, we will present and discuss exemplary application solutions, where we successfully applied EBIP to overcome dimensional and/or functional limitations. We therefore show the fabrication stability and accuracy of “T-like-shaped” AFM tips made from high density, diamond-like carbon (HDC/DLC) for the investigation of undercut structures on the base of CDR30-EBD tips. Such aggressive CD-AFM tip dimensions are mandatory to fulfill ITRS requirements for the inspection of sub-28nm nodes, but are unattainable with state-of-art Si-based MEMS technologies today. In addition to that, we demonstrate the ability of EBIP to realize field enhancement in sensor applications and the fabrication of cold field emitters (CFE). For example: applying the EBIP approach allows for the production of CFEs, which are characterized by considerably enhanced imaging resolution compared to standard thermal field emitters and stable operation properties at room temperature without the need for periodic cathode flashing – unlike typical CFEs. Based on these examples, we outline the strong capabilities of the EBIP approach to further downscale functional structures in order to meet future demands in the semiconductor industry, and demonstrate its promising potential for the development of advanced functionalities in the field of NEMS.

Overcoming silicon limitations: new 3D-AFM carbon tips with constantly high-resolution for sub-28nm node semiconductor requirements

The demands on atomic force microscopy (AFM) as a reference technique for precisely determining surface properties and structural designs of multiple patterns in the semiconductor industry are steadily increasing. With the aim to meet ITRS requirements and simultaneously improve the accuracy of AFM-based critical dimension (CD) measurements at constant resolution, the AFM tip more and more becomes a factor crucially determining the AFM performance. In this context, AFM tip limitations are given by lack of sharpness with too large tip radii/diameter, insufficient wear resistance, and high total cost, which does not conform to production environment needs. One technical approach to overcome these tip limitations is provided by electron beam induced processing (EBIP), which allows for manufacturing AFM tips of desired sharpness, shape, and mechanical stability. Here, we present T-shape-like 3D-AFM tips made of bulk amorphous, high density diamond-like carbon (HDC/DLC), and compare their performance and wear resistance to standard silicon tips. We show the advantages of this approach for the semiconductor industry, in particular on AFM3D technology in order to answer to sub-28 nm nodes requirements, and present tips with 15 nm diameter at 10 nm vertical edge height.

Nanoemitter: ultra-high-resolution electron source for CD metrology

Nanoemitters (NEs) are a promising replacement for electron sources in producing field emission CD-SEMs and CDTEMs. So far, NEs have been fabricated by, e.g. carbon nanotubes or nanowhiskers of conductive materials. Here, we present a new method to manufacture NEs using electron beam induced processing (EBIP) - a method well established in the nanofabrication of super sharp probes for scanning probe microscopy - and show their unique performance. NEs manufactured by EBIP combine a high density, diamond-like carbon core (HDC/DLC) with high aspect ratio and tip sharpness, and a highly conductive coating. The EBIP process allows for the batch-fabrication of NEs at larger scales with desired sharpness, shape, mechanical stability and conductivity. NEs, which can easily be mounted into existing SEM/TEM assemblies, have been operated for > 5.000 h without any sign of degradation at a comparatively constant beam current of 3 μA, wherein maximum current oscillations of 10% occurred, while current oscillations were less than 3% over a time span of several minutes. Due to the cold operating temperature and small tip radius, the resolution improved up to 30% compared to a standard Schottky thermal field emitter. The improvement is significant in the low voltage range below 5 kV.

Ultrashort cantilever probes for high-speed atomic force microscopy

Since its invention in 1986, atomic force microscopy (AFM) has become the most widely used scanning-probe imaging technique.1 The microscope ‘maps’ the topography of the sample by scanning its surface with a small tip integrated near the free end of a flexible cantilever. In contrast to other microscope techniques, it is possible to image the surface of many materials—including electrically isolating ones, such as ceramics and glasses—with a resolution in both the vertical and horizontal direction of below 1nm. Figure 1 shows a typical example of an AFM measurement in vacuum, depicting a cerium oxide surface in true atomic resolution. The inherent mechanical characteristics of AFM requires serial acquisition of data, which limits the speed for obtaining high-resolution images. Using commercially available microscopes, a typical image of 256 256 pixels resolution can be obtained in 1–10min. This is significantly slower than the time resolution required, for example, to resolve biological processes such as the movement of motor proteins in cells.2 Enhancing imaging speed demands improved scanner technology and control electronics. Specifically, several limiting factors with respect to the cantilever probe must be addressed to reduce resolution time. First, the measurement bandwidth of the local interaction between the probe tip and sample, as well as the velocity at which the tip moves, must be increased. Additionally, the tip must be able to follow the sample’s topography at greater speed.3 Ideally, cantilever probes will function at resonance frequencies in the megahertz region with low force constants (approximately a few nano-Newtons per nanometer). Here, we report our progress in fabricating cantilever probes for highspeed imaging in AFM.4 Figure 1. Atomic force micrograph of a cerium oxide surface with true atomic resolution, imaged using a conventional atomic force microscope in noncontact mode.