Flexible metrology: from memory to medicine

Quality control of surface texture is possible on an atomic scale

BEING able to accurately control the quality of a material’s surface as well as its shape is crucial for a wide range of research and manufacturing processes. In some situations for example, a very smooth surface with a mirror-like finish is required to determine the usefulness of a given material as a substrate, but in other cases we need to achieve the correct amount and/or type of material roughness. However, control of surface roughness on an atomic scale is not an easy task; it requires the application of sophisticated experimental methods to measure surface height, roughness, pittings, profiles, layer thickness, and step heights. A number of methods exist to enable such analysis, but very few of them are as flexible as the Olympus LEXT measuring confocal Laser Scanning Microscope (mcLSM) system.

The Olympus LEXT OLS4000 is designed for ultra-precise measurements and observations with the highest achievable levels of reliability. No sample preparation is required, so nanoparticles, functionalised nanoparticles, doped nanocrystals and after a period of growth, memory bars, can be placed directly onto the microscope stage. Highly-precise 3D measurements are possible in real-time and the LEXT system features a much greater resolution than any other conventional optical device. Furthermore, several different observation methods can be applied to ensure faster, more accurate specimen analysis. LEXT utilises low wavelength optical methodology with a 405nm laser diode in combination with confocal scanning, to exceed the resolution limits of standard optical imaging systems. By developing an optical system that minimises the aberrations associated with a short wavelength and at the same moment maximises the transmission at 405nm, the Olympus LEXT achieves both high image quality and signal response.

The advanced XY scanner used in the LEXT system makes the scanning process faster and the results more reproducible compared to conventional scanner technologies. The twin confocal pinholes ensure that even steep slopes (up to 85O) can be visualised and measured.

The result is a world-leading plane resolution that clearly recognises individual lines spaced only 0.12µm apart and 0.01µm in height. Therefore, the LEXT system enables ultra-precise measurements of the micro fabrication surfaces. All of these advantages come together with a remarkable reproducibility of 3σ n-1 = 0.02µm (XY direction) and 3σ n-1 = 0.05 + 0.002Lµm (height; L= measurement height).

The use of three dimensional structures for the realisation of ‘non-volatile-memories’ (NVM – data storage that retains information without the need for power) requires bit storage and selection elements to be stacked up vertically. Silicon appears unsuitable for this application as the high thermal budget, required for both deposition and dopant activation, makes its integration with the bit storage elements extremely difficult or even impossible.

One of the newer NVM architectures is based on a cross-bar memory concept. This enables an increase in memory density as the cross-bar concept allows vertical multiple stacking and the subsequent retention of a small footprint. Furthermore, the cost per bit of such architecture is significantly lower than the conventional CMOS based ones since fewer photolithographic masks are used.

Prof Marek Godlewski and his group are working on an EU funded project with the acronym VERSATILE, to select materials suitable for cross-bar memory. One of their tasks is to integrate junctions made of II-VI (ZnO) and organic/polymeric semiconductors into these cross-bar type memories. This will enable the production of a scalable NVM with both bit storage and selection elements that can be vertically stacked on top of each other. The junction material must have a low thermal budget in order to be compatible with the most promising bit storage element technologies, such as chalcogenides. The objectives can be summarised as follows:

• Making Schottky and p-n type heterojunctions able to withstand high currents and with a high Ion/Ion(1/2) able to operate at low voltage.
• Demonstrating the integration of cross-bar architecture, made of junctions based on II-VI and/or polymeric semiconductors and a storage element, up to a 100x100 nodes.
• Demonstrating the scalability of vertically stacked array junctions for memory application by means of electron beam processing.

As with most electronic devices, the various layers of material involved in cross-bars are built-up through deposition. In this project, extremely low deposition temperature (90-200ºC) atomic layer deposition (ALD) methodology was selected, to avoid damaging the organic layers. As a result, close attention has been paid to the layer deposition process, including how the layers are formed, their composition and roughness. The Olympus LEXT system stands out as an excellent tool for this task, eliminating the need for sample preparation, chamber pump-down, or any restrictions in the step height measurement. The LEXT was also used to evaluate the thickness of the deposited layers and the overall 3D nature of the surface. Furthermore, it provided highly accurate and reproducible results for roughness, height, width, depth, area and volume. This was achieved without the need to draw a probe across the surface as the system is able to generate images and results for the entire area, not just a small section.

Thus the LEXT microscope can be used in routine microscopy to provide fast, highly-accurate layers metrology data. As a result of large-scale investigations with samples grown under a variety of conditions, the different growth parameters of the layers were vastly improved. Therefore, an extremely important advantage of the system is its ability to quickly and accurately analyse data such as roughness, thickness and gradient.

Figures 1 - 6 show typical LEXT images and analysis processes with end user report taken during the 3D cross-bars project.

Modern medical innovations involve a large amount of cross-over between biological and physical sciences. For example, nanoparticles, functionalised nanoparticles and doped nanocrystals have a lot of visualisation applications not only in physics and chemistry, but also in biology and medicine. Several advanced imaging techniques have been developed for these investigations, allowing the accumulation of a vast amount of knowledge in the above-mentioned areas. The LEXT system, even though developed for other tasks, has proven to have a role in this field as well.

Comprehensive and intensive investigations of rare earth (RE) and transition metals (TM) doped nanomaterials have lead to the discovery of a wide range of interesting phenomena. Optoelectronics, medicine and biology are the main application areas of such nanocrystals, due to their advantageous optical (luminescence) properties and the prospective development of new, more efficient light emitting devices.

Approximately 22% of generated power from developed countries is consumed for indoor and outdoor illumination. Furthermore, due to the low efficiency of incandescent lamps, a large part of this energy is wasted. As such, the development of a new generation of highly-efficient light emission devices is extremely important. Light–emitting diodes (LEDs) and compact fluorescent lamps (CFLs) can be used as alternatives to incandescent lamps and quartz–halogen bulbs. However, to optimise the efficiency of these light sources, a new generation of more efficient phosphors using semiconductor nanoparticles needs to be developed.

Nanoparticles are also involved in a visualisation technique based on the light emission of RE doped nanoparticles. For example displays, displaying matrices and e–ink displays have been created to visualise information in modern electronic devices. Furthermore, nanoparticles doped with RE are good candidates for photoluminescence markers for medical as well as biological applications due to their physical properties. These include a small size, comparable with that of human tissue, bacteria and viruses, along with high luminescence efficiency. Nanoparticles have been used for these new applications due to a high quantum yield of dopants (intra-shell emission in case of RE and TM ions) photoluminescence (PL).

Precise control of nanoparticle diameter, which should be well defined, is extremely important and is commonly in the region of 10 - 20 nm, below the XY resolution of the LEXT (120 x 120nm). The LEXT though still has an important role here and can provide a lot of useful information. For example, before measurements are taken, nanoparticles are commonly diluted in alcohol. After the alcohol has evaporated, large (about 150nm) agglomerates of doped nanoparticles are formed, which can be easily resolved using the LEXT system. During their measurements under UV laser excitation (405nm), not only could agglomeration size be studied, but the luminescence of the nanoparticles could also be easily observed without the use of any spectroscopic equipment. As such, under 405nm UV excitation, using CCD camera detection, an intra-shell luminescence of Mn2+ in small II-VI nanoparticles (ZnS, CdS) could be observed. Therefore, the Olympus LEXT microscope was used not only for typical structural investigations but also for qualitative optical investigations. Using the LEXT system, it was possible to gain an accurate overview of the shape and luminescence properties of the particles due to the clever combination of bright-field and confocal Laser Scanning microscopy.

Figures 7 - 9 show typical LEXT images and analyses taken during nanoparticle investigations.

The Olympus LEXT system is a highly flexible metrology instrument that enables quick and easy analysis of a broad range of samples. The stage is designed to accept large items and there is no need for any sample preparation. Furthermore, the samples are not damaged or affected by the measurement procedures and the LEXT system provides highly-precise and repeatable results, even when taken at the highest resolutions.