Nanotomography

Details Nanotomography Nanotomography

2001-06-13

2003-04-15

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Surface and cutting edge testing

Roughness

C250S306000, C250S307000

Reexamination Certificate

active

06546788

ABSTRACT:

FIELD OF INVENTION
The subject matter of the present invention is a novel device and a novel method that permit the determination at high spatial resolution of the spatial structure of a sample to be examined. A plurality of different material characteristics can simultaneously be investigated, which are substantially merely limited by the specific basic method of microscopy chosen. In principle the method according to the invention allows spatial resolutions in the atomic range (100 picometer).
BACKGROUND OF THE INVENTION
Meanwhile, a plurality of methods have become available in microscopy. The most widely known methods are methods of optical microscopy, the resolving power of which is on principle limited to a range of some hundred nanometers. Considerably higher resolving power is attained in electron microscopy that exists in various types. A resolution within the range of some nanometers can be routinely obtained by means of electron microscopy. In the course of the last 15 years, further methods of microscopy were added of which the most outstanding are the methods of so-called scanning probe microscopy which permit atomic resolution in using specific probes and which may be sensitive to a plurality of material properties. Sharp metallic tips, sharp tips on a cantilever (a thin beam to be curved by external forces) or stretched tips of optical waveguides may serve as probes for example. These probes are generally run at a very close distance (some nanometers and less) across the sample (the sample is scanned) and certain material parameters are investigated at the same time. Electric currents between the surface of the sample and the tip, a mechanical distortion or torsion of the cantilever, the number of protons intercepted by the waveguide or even the attenuation of mechanical oscillations of the probe for example may serve as control parameters for controlling the spacing between the sample and the tip.
All these methods of microscopy, which have only been mentioned by way of example, have in common that they only allow the production of two-dimensional images of the objects to be examined. For anyone interested in the spatial structure of samples to be examined, the number of available methods of investigation is strongly limited.
A greater number of methods of determining structures do not investigate the spatial structure of the samples directly but in a roundabout way by making use of so-called scattering techniques (e.g. X ray scattering or neutron scattering). These scattering methods, however, are to be essentially used on objects that have a regular structure. Accordingly, scattering methods are particularly suited to determine crystal structures, but they are not to be used, or only with severe limitations, on samples having an irregular structure.
The majority of the methods that permit one to determine at high spatial resolution the spatial structure of an irregular sample is based on the fact that the volume of the sample is divided into a sequence of layers, a map of the properties of interest being built for each layer. Then, the spatial distribution of the properties of interest is reconstructed from a heap of such maps by means of appropriate mathematical methods. Various methods operate on this principle and differ in the manner of imaging the investigated properties (the contrast method) and in the way of dividing the volume of the sample in individual layers and of building the map of the properties of interest for each layer.
The best known methods to the present day are the X ray tomography and the magnetic resonance imaging technique. In both methods, but one single thin layer of the volume of the sample is nondestructively detected by an appropriate measuring technique and a map of the properties of interest is built in this layer from a large number of projections of the layer taken from various angles by means of mathematical methods. Both methods permit one to attain a spatial resolution of some micrometers. Confocal microscopy falls into this same class and permits one to build up an image of the focal plane with very little depth of definition. However, in confocal microscopy, the spatial resolution is limited to some hundred nanometers by the wavelength of the utilized light and optically transparent specimens only can be examined.
In the historically first methods of spatial reconstruction, the sample was mechanically divided into a series of individual thin layers by means of a microtome. A sequence of maps was then built from these thin sections using optical microscopy, which permitted one to reconstruct the spatial shape of the investigated objects. Today, some thin layers, of some nanometers thin, may be produced by means of an ultramicrotome and may be studied with the scanning electron microscope at a high lateral resolution (in the x-y-plane) (several nanometers are attainable). Resolution in depth (in z direction) of this method however is limited by the thickness of the thin sections employed. Said thickness is in turn limited to some several nanometers by the mechanical stability of the investigated material. A major drawback to this method is the high need for time and staff since reliable manufacturing, manipulation and investigation of the extremely thin serial sections require much experience and occur in great parts manually. Additionally, sections thus thin cannot be made from all the materials so that metals, ceramics, semiconductors and many other significant raw materials cannot be studied by using this method.
The X ray tomography and the magnetic resonance imaging technique extensively use mathematical methods to reconstruct the distribution of the property investigated in one single examined layer from a large number of projections taken from various angles. Similar methods may also be employed in electron microscopy. In particular cases, a resolution in the range of nanometers is attainable. In the thin sections, of some several nanometers thick, which are required for this purpose, the maximum time of action of the electrons that one single thin section is able to bear before it is destroyed by the beam of electrons is limited, the limited overall time of action of the electrons having to be distributed over the individual projections as a result thereof, which entails severe limitation of the image quality of the individual projections. For this reason, for the purpose of reconstructing the image, model assumptions on the symmetry of the sample must be made in order to be capable of attaining a resolution ranging in nanometers. Therefore, irregular structures in the sample cannot be determined with such a high spatial resolution.
The problem of making the sections and of the mechanical stability of the thin sections is circumvented by methods in which the sample is ablated layer by layer, the distribution of the properties of interest being determined after each ablation on the bare surface. The technique of the dynamic secondary-ion mass spectroscopy (SIMS) operates according to this method. The lateral resolution (in the x-y-plane) is, however, limited by the diameter of the beam of ions that generates the secondary ions and that, at best, is approximately 50 nm large. In the dynamic SIMS, the attainable resolution in depth (in z-direction) is limited by the depth of penetration of the beam of ions into the material (approximately 10 nm and more) and by the roughness of the sample surface prior to and more particularly during ablation. In many cases and specifically with heterogeneous samples, the material is unevenly ablated since the rate of ablation is a property that depends on the material, which causes the surface of the sample to become rough in the course of the investigation. As a result, the detected property (e.g. the concentration of a certain element) originates concurrently from various depths which drastically impairs resolution in depth of the dynamic SIMS. For this reason, many interesting (since heterogeneous) samples cannot be examined using this method, since no procedure of even ablation

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