The ever-increasing complexity of materials and architectures in nanoelectronics devices has driven the demand for new high-resolution imaging methods. Specifically, for three-dimensional (3D) analysis of confined volumes, atomic force microscopy (AFM) has been recently explored as a method for tomographic sensing. Here, we report on the innovative design of a dedicated microscopy solution for volumetric nanoscale analyses that achieves tomographic AFM by using a novel multi-probe sensing architecture. First, we describe the development of a custom scan head that is based on an exchangeable multi-probe hardware. Second, we demonstrate the use of our machine for tip-induced material removal in thick SiO2. Finally, we perform a tomographic reconstruction of nanosized poly-Si vertical channels, considered here as a prototypical system for vertical memory cells.
The Rapid Probe Microscope (RPM), exists as an integrated solution for photomask repair, with its application extended to include wafer metrology in 2016 [1]. The RPM can acquire non-destructive, high resolution, sub-nm detail in all 3 dimensions, overcoming some of the limitations of conventional AFM. In addition, it is flexible and can be configured to run either in air or in vacuum. The RPM includes the innovative use of an interferometric detection system to simultaneously measure both the height and the deflection of the cantilever, while the probe is controlled through photo thermal actuation. This combination delivers an accurate, very fast, direct measurement of the height of the probe and the corresponding structure of the sample surface. The X,Y probe scanner movement is also monitored by an interferometer. This guarantees both the linearity and XY position of the probe tip, delivering a corresponding sub-nm metrology of the wafer structure.
Current methods to characterise specific properties of polymeric nanocomposites (PNCs), such as particle loading and
dispersion profile, rely on a number of techniques that require special sample preparation and treatment, are very
expensive, require long measurement times and quite often produce ambiguous results that are difficult to evaluate and
interpret. In addition, given their complexity, they are not entirely suited for in-situ industrial environments. This paper
presents alternative techniques based on optical diffraction and diffusion mechanisms combined with signal processing
that can successfully discriminate between different particle loadings and levels of dispersion. The techniques discussed
in this paper are Fourier-domain optical coherence tomography in the infra-red, Fraunhofer wavefront correlation in the
visible red and oscillatory photon correlation spectroscopy in the visible green parts of the spectrum. Most importantly,
they are non-invasive, are compact, fast and efficient, can potentially analyse large areas of the material and therefore
suited for a wide variety of research and industrial situations.
In this paper a novel method for determining refractive indices of a multi-layered samples using low coherence
interferometry (LCI), developed at the National Physical Laboratory, UK, is introduced. Conventional Optical
Coherence Tomography (OCT) utilises a lateral scanning optical probe beam to construct a depth resolved image of the
sample under investigation. All interfaces are detected in optical path length, resulting in an image depending on the
refractive index of all prior layers. This inherent ambiguity in optical and geometric path length reduces OCT images to
purely qualitative ones. We have demonstrated that by optically probing the sample at multiple angles we can determine
bulk refractive index of layers throughout plane parallel samples. This method improves upon current approaches of
extracting refractive index parameters from multi-layered samples as no prior geometrical information is required of the
sample and the phase index for each layer is obtained as opposed to the group index. Consequently the refractive index
result for each layer is independent of the refractive index of surrounding layers. This technique also improves on
conventional measurements, as it is less susceptible to error due to surface defects. This technique is easily implemented,
and can easily be modified to obtain in situ measurements. Investigating a silica test piece and comparing the refractive
index obtained by that of standard critical angle refractometry has validated the robustness of the technique.
Two significant figures of merit for optical coherence tomography (OCT) systems are the axial and transverse resolutions.
Transverse resolution has been defined using the Rayleigh Criterion or from Gaussian beam optics. The axial resolution
is generally defined in terms of the coherence length of a Gaussian shaped source. Whilst these definitions provide a
useful mathematical reference they are somewhat abstracted from the three dimensional resolution that is encountered
under practical imaging conditions. Therefore, we have developed a three-dimensional resolution target and measurement
methodology that can be used to calibrate the three-dimensional resolution of OCT systems.
The flow of culture medium through a mechanically stimulated
cell-seeded tissue scaffold is a factor influencing not only
the transport of essential nutrients and waste product removal but also impacting on the degradation kinetics of the scaffold.
Being able to map spatial and temporal changes in fluid flow behaviour is key to the development of improved bioreactors
and tissue scaffold designs, especially for the new generation of multiple tissue reactors. In this paper we demonstrate
the excellent metrological benefits of fast Doppler optical coherence tomography for time-lapse characterisation of tissue
scaffolds placed in a dynamic flow environment.
A novel dual angle optical coherence tomography (OCT) method is developed that has been termed stereoscopic OCT,
highlighting the similarities between this technique and stereoscopic ranging. OCT images are obtained at two angles of
incidence with respect to the surface of a layered phantom. From these measurements, optical path lengths are determined
for each layer that are used to calculate the refractive index and physical thickness of each layer directly from Snell's law.
This method may prove to be useful for characterising the bulk optical properties of biological material in vivo, that are
presently not well known or understood.
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