Nanoscale Height Research Articles

Momentum Fourier ptychographic topography

Reflective Fourier Ptychographic Microscopy (FPM) has demonstrated the potential as a technology for semiconductor metrology and inspection applications, offering both a wide field-of-view (FOV) and high resolution with nanoscale height reconstruction accuracy. However, to enhance resolution, the wavelength of the illumination source has been shortened to the ultraviolet (UV) range, leading to a significant decrease in height calculation accuracy due to low signal-to-noise issues. Additionally, the time-consuming aspect persists as a challenge. To address these issues, we introduce Momentum Fourier Ptychographic Topography (MFPT). MFPT can overcome the problem of getting trapped in local minima due to noise, and it does not require an iterative process of total variant regularization, allowing for significant computational time savings. Using UV illumination with MFPT, we measure a United States Air Force (USAF) 1951 target and two step-height samples, providing evidence that MFPT is robust to noise and accurately reconstructs nanoscale height. As a result, utilizing UV illumination with MFPT allows us to achieve a resolution of 173 nm, and we can rapidly and accurately calculate the height of step-height samples with nominal heights of 30 nm and 70 nm.

Optimization of visible photoluminescence emission from Ni-Zn ferrite thin films

Ni-Zn ferrite films with different thicknesses were prepared by the spray method, aiming to study the relationship between the annealing effect in an oxygen rich environment and the structural, optical properties and photoluminescence emission. X-ray diffraction (XRD) analysis used with Rietveld refinement showed that all prepared samples had a single spinel phase structure. Likewise, the Fourier transform infrared (FTIR) spectra confirmed the phase formation of Ni-Zn ferrites by appearing in both of the two characteristic absorption bands which are related to the tetrahedral and octahedral sites. For annealed thin film samples of Ni-Zn ferrite, the atomic force microscope (AFM) surface morphology exhibits pinning structure on the surface in nanoscale height, whereas for un-annealed samples, there are hills and valleys cover a broad region. The different electronic transitions were estimated from the UV-visible transmission spectrum. Strong photoluminescence (PL) intensity in the visible range was observed under the excitation of UV radiation. The intensity of the PL signal was strongest at a film thickness of 750 nm then decreased for higher thicknesses. This could be interpreted by using proposed energy level structures based on the transmission spectrum of the investigated samples. The strong PL intensity introduces the samples as a direct optical detector for UV radiation.

Effect of nanoscale roughness on four different atomic force microscopy probes in aqueous solutions using adhesion force measurement

Despite substantial theoretical approaches explaining the effect of nanoscale roughness (NR) on particle deposition, experiments are rarely performed. Fabricating the sophisticated rough surfaces and obtaining the experimental data for interaction between the colloids and surfaces are crucial to understand colloidal adsorption onto the rough surfaces. We investigated the adhesion of different particle types of atomic force microscope (AFM) probes (different shapes, i.e., sphere and plateau; diameters, i.e., 2 μm–15 μm) onto smooth and rough fabricated surfaces. We successfully fabricated well-organized rough surfaces on Silicon (Si) using colloidal lithography and metal-assisted chemical etching using a cylindrical shape of constant nanoscale height and diameter. The properties of roughness (i.e., height, diameter, and fraction) were quantitatively analyzed, and the corresponding. The force-distance curves measured with the AFM revealed that, for the four types of probes, contact area varied with the surface roughness, and their adhesion forces differed accordingly. Expanding the contact area tended to increase the repulsive force possibly because of the hydration force. This study effectively devised a simplified experimental system that explains the influence of NR on particle deposition using previous theoretical data. The findings should be considered in designing NR for subsequent colloidal deposition.

Pursuit of hidden rules behind the irregularity of nano capillary lithography by hybrid intelligence

Nature finds a way to leverage nanotextures to achieve desired functions. Recent advances in nanotechnologies endow fascinating multi-functionalities to nanotextures by modulating the nanopixel’s height. But nanoscale height control is a daunting task involving chemical and/or physical processes. As a facile, cost-effective, and potentially scalable remedy, the nanoscale capillary force lithography (CFL) receives notable attention. The key enabler is optical pre-modification of photopolymer’s characteristics via ultraviolet (UV) exposure. Still, the underlying physics of the nanoscale CFL is not well understood, and unexplained phenomena such as the “forbidden gap” in the nano capillary rise (unreachable height) abound. Due to the lack of large data, small length scales, and the absence of first principles, direct adoptions of machine learning or analytical approaches have been difficult. This paper proposes a hybrid intelligence approach in which both artificial and human intelligence coherently work together to unravel the hidden rules with small data. Our results show promising performance in identifying transparent, physics-retained rules of air diffusivity, dynamic viscosity, and surface tension, which collectively appear to explain the forbidden gap in the nanoscale CFL. This paper promotes synergistic collaborations of humans and AI for advancing nanotechnology and beyond.

Fourier ptychographic topography.

Topography measurement is essential for surface characterization, semiconductor metrology, and inspection applications. To date, performing high-throughput and accurate topography remains challenging due to the trade-off between field-of-view (FOV) and spatial resolution. Here we demonstrate a novel topography technique based on the reflection-mode Fourier ptychographic microscopy, termed Fourier ptychograhpic topography (FPT). We show that FPT provides both a wide FOV and high resolution, and achieves nanoscale height reconstruction accuracy. Our FPT prototype is based on a custom-built computational microscope consisting of programmable brightfield and darkfield LED arrays. The topography reconstruction is performed by a sequential Gauss-Newton-based Fourier ptychographic phase retrieval algorithm augmented with total variation regularization. We achieve a synthetic numerical aperture (NA) of 0.84 and a diffraction-limited resolution of 750 nm, increasing the native objective NA (0.28) by 3×, across a 1.2 × 1.2 mm2 FOV. We experimentally demonstrate the FPT on a variety of reflective samples with different patterned structures. The reconstructed resolution is validated on both amplitude and phase resolution test features. The accuracy of the reconstructed surface profile is benchmarked against high-resolution optical profilometry measurements. In addition, we show that the FPT provides robust surface profile reconstructions even on complex patterns with fine features that cannot be reliably measured by the standard optical profilometer. The spatial and temporal noise of our FPT system is characterized to be 0.529 nm and 0.027 nm, respectively.

Bottom-Up Generated Height Gauges for Silicon-Based Nanometrology.

Steady progress in integrated circuit design has forced basic metrology to adopt silicon lattice parameter as a secondary realization of the SI meter that lacks convenient physical gauges for precise surface measurements at a nanoscale. To employ this fundamental shift in nanoscience and nanotechnology, we propose a set of self-organized silicon surface morphologies as a gauge for height measurements within the whole nanoscale (0.3-100 nm) range. Using 2 nm sharp atomic force microscopy (AFM) probes, we have measured the roughness of wide (up to 230 μm in diameter) singular terraces and the height of monatomic steps on the step-bunched and amphitheater-like Si(111) surfaces. For both types of self-organized surface morphology, the root-mean-square terrace roughness exceeds 70 pm but has a little effect on step height measurements having 10 pm accuracy for AFM technique in air. We implement a step-free 230-μm-wide singular terrace as a reference mirror in an optical interferometer to reduce the systematic error of height measurements from >5 nm to about 0.12 nm, which allows visualizing 136-pm-high monatomic steps on the Si(001) surface. Then, using a "pit-patterned" extremely wide terrace with dense but counted monatomic steps in a pit wall, we have optically measured mean Si(111) interplanar spacing (313.8 ± 0.4 pm) that agrees well with the most precise metrological data (313.56 pm). This opens up avenues for the creation of silicon-based height gauges using bottom-up approaches and advances optical interferometry among techniques for metrology-grade nanoscale height measurements.

Divided-aperture subtraction-differential confocal method with nanoscale axial resolution

In this paper, a divided-aperture subtraction-differential confocal method (DASDCM) is proposed to meet the requirements of nanoscale noncontact height measurements for precision machining, materials science, and biology. The DASDCM divides the spot on the detection focal plane into two groups of circular detection areas, which are symmetrical to the optical axis and consist of two concentric detection pinholes with different sizes in each group. Then, the DASDCM uses a subtraction of the intensity signals received from the two detection pinholes in each group to suppress the interference of the nonconjugated information on the intensity signal; it also uses the differential subtraction of two obtained circular detection signals to obtain a sensitive axial response curve. Thereby, the DASDCM greatly improves the axial resolution while considering the signal-to-noise ratio and axial dynamic range of the system and can realize surface height measurement without axial scanning by using the linear range of the axial response curve. Theoretical analysis and preliminary experiments show that DASDCM has an axial resolution of 2nm with a laser wavelength of λ=632.8 nm and a numerical aperture of NA=0.8. It provides an effective technique for nanoscale height detection with high axial resolution.

Spatial Analysis of Nanofluidic-Embedded Biosensors for Wash-Free Single-Nucleotide Difference Discrimination.

In this work, we demonstrate that the analysis of spatially resolved nanofluidic-embedded biosensors permits the fast and direct discrimination of single-nucleotide difference (SND) within oligonucleotide sequences in a single step interaction. We design a sensor with a linear dimension much larger than the channel depth in order to ensure that the reaction over the whole sensor is limited by the convection rate. Thus, the targets are fully collected, inducing a nonuniform spatial hybridization profile. We also use the nanoscale height of the channel, which enables us to minimize the amount of labeled molecules flowing over the sensor and hence to reduce the fluorescence background, to carry out real-time hybridization detection by fluorescence microscopy. Taken together, these design rules allow us to show that the spatial hybridization profile depends on the duplex affinity, and we speculate that the on and off-rate constants can be inferred during target injection, which is not possible in local analysis where the dissociation step through rinsing must be conducted. We finally manage to discriminate a GT mismatch on a microRNA sequence by optimizing the interaction temperature and the probe design after a few minutes of interaction in a single step protocol. This work may be applied to any biosensing transduction scheme with spatial resolution, e.g., surface plasmon resonance imaging, integrated into nanofluidic channels for applications where high oligonucleotide sequence selectivity and short analysis times are required.

Intrinsic inhomogeneous barrier height at the n-TiO2/p-Si hole-blocking junction

Abstract Using Kelvin probe force microscopy (KPFM) and temperature-dependent current-voltage characteristics, we study the charge transport across an n-TiO2/p-Si heterojunction. In particular, the KPFM result shows a variation in the work function at the TiO2 surface. On the other hand, temperature-dependent current-voltage characteristics depict a non-ideal hole-blocking behaviour of the same. In addition, the measured barrier height is found to decrease with temperature and does not follow the thermionic emission theory, strongly suggesting an inhomogeneous nature of the barrier. The observed barrier inhomogeneity is attributed to the nanoscale height modulation, arising due to the growth dynamics of TiO2 and corroborates well with the KPFM map. The presented results will open a new avenue to understand the charge transport in TiO2-based nanoscale devices.

Bottle microresonator broadband and low-repetition-rate frequency comb generator.

We propose a new type of broadband and low repetition rate (RR) frequency comb generator that has the shape of an elongated and nanoscale-shallow optical bottle microresonator created at the surface of an optical fiber. The free spectral range (FSR) of the broadband azimuthal eigenfrequency series of this resonator is the exact multiple of the FSR of the dense and narrowband axial series. The effective radius variation of the microresonator is close to a parabola with a nanoscale height that is greater or equal to λ/2πn0. (Here λ is the characteristic radiation wavelength and n0 is the refractive index of the microresonator material.) Overall, the microresonator possesses a broadband, small FSR and accurately equidistant spectrum convenient for the generation of a broadband and low RR optical frequency comb. It is shown that this comb can be generated by pumping with a cw laser, with a radiation frequency that matches a single axial eigenfrequency of the microresonator or, alternatively, by pumping with a mode-locked laser, which generates a narrowband low RR comb matching a series of equidistant axial eigenfrequencies situated between adjacent azimuthal eigenfrequencies.

Adhesion enhancement of titanium nitride coating on aluminum casting alloy by intrinsic microstructures

Aluminum casting alloys have excellent castability, high strength and good corrosion resistance. However, the presence of silicon in these alloys prevents surface finishing with conventional methods such as anodizing. Hard coating with titanium nitride can provide wear and corrosion resistances, as well as the aesthetic finish. A critical factor for a durable hard coating is its bonding with the underlying substrate. In this study, a titanium nitride layer was coated on LM25 casting alloy and a reference high purity aluminum substrate using Ion Assisted Deposition. Characterization of the coating and the critical interface was carried out by a range of complementing techniques, including SIMS, XPS, TEM, SEM/EDS and nano-indentation. It was observed that the coating on the aluminum alloy is stronger compared to that on the pure aluminum counterpart. Silicon particles in the alloy offers the reinforcement though mechanical interlocking at microscopic level, even with nano-scale height difference. This reinforcement overcomes the adverse effect caused by surface segregation of magnesium in aluminum casting alloys.

Nanoscale size-selective deposition of nanowires by micrometer scale hydrophilic patterns.

Controlling the post-growth assembly of nanowires is an important challenge in the development of functional bottom-up devices. Although various methods have been developed for the controlled assembly of nanowires, it is still a challenging issue to align selectively heterogeneous nanowires at desired spatial positions on the substrate. Here we report a size selective deposition and sequential alignment of nanowires by utilizing micrometer scale hydrophilic/hydrophobic patterned substrate. Nanowires dispersed within oil were preferentially deposited only at a water/oil interface onto the hydrophilic patterns. The diameter size of deposited nanowires was strongly limited by the width of hydrophilic patterns, exhibiting the nanoscale size selectivity of nanowires deposited onto micrometer scale hydrophilic patterns. Such size selectivity was due to the nanoscale height variation of a water layer formed onto the micrometer scale hydrophilic patterns. We successfully demonstrated the sequential alignment of different sized nanowires on the same substrate by applying this size selective phenomenon.

Real-time monitoring of cell viability by its nanoscale height change with oxygen as endogenous indicator

A method for real-time evaluation of cell viability was developed by using oxygen as an endogenous indicator in scanning electrochemical microscopy to monitor the nanoscale height change of a single cell in a physiological environment with a novel Pt nanodisk electrode and a newly designed step-approaching strategy.

High sensitivity of surface plasmon of weakly-distorted metallic surfaces

Effects of nano-scale surface geometry on surface plasmon are studied by the coordinate-transformation differential method to numerically calculate surface plasmon modes on a weakly disordered metallic surface. An air-silver surface profile with a subwavelength period and a nano-scale height at wavelength of 650 nm are chosen and it is found that the Bloch wave numbers and the surface plasmon modes are highly sensitive with distortions of only a few nanometers for periods much less than wavelength. On the contrary, distortions of long periods which are comparable to wavelength have little impact. Three typical surface profiles exhibit surface plasmon modes of wide variations.