Three-dimensional molecular imaging using mass spectrometry and atomic force microscopy

We combine imaging ToF-SIMS depth profiling and wide area atomic force microscopy to analyze a test structure consisting of a 300 nm trehalose film deposited on a Si substrate and pre-structured by means of a focused 15-keV Ga⁺ ion beam. Depth profiling is performed using a 40-keV C₆₀⁺ cluster ion beam for erosion and mass spectral data acquisition. A generic protocol for depth axis calibration is described which takes into account both lateral and in-depth variations of the erosion rate. By extrapolation towards zero analyzed lateral area, an “intrinsic” depth resolution of about 8 nm is found which appears to be characteristic of the cluster-surface interaction process.     

Effects of evolving surface morphology on yield during focused ion beam milling of carbon

We investigate evolving surface morphology during focused ion beam bombardment of C and determine its effects on sputter yield over a large range of ion dose (10¹⁷-10¹⁹ ions/cm²) and incidence angles (Θ = 0-80°). Carbon bombarded by 20 keV Ga⁺ either retains a smooth sputtered surface or develops one of two rough surface morphologies (sinusoidal ripples or steps/terraces) depending on the angle of ion incidence. For conditions that lead to smooth sputter-eroded surfaces there is no change in yield with ion dose after erosion of the solid commences. However, for all conditions that lead to surface roughening we observe coarsening of morphology with increased ion dose and a concomitant decrease in yield. A decrease in yield occurs as surface ripples increase wavelength and, for large Θ, as step/terrace morphologies evolve. The yield also decreases with dose as rippled surfaces transition to have steps and terraces at Θ = 75°. Similar trends of decreasing yield are found for H₂O-assisted focused ion beam milling. The effects of changing surface morphology on yield are explained by the varying incidence angles exposed to the high-energy beam.     

The effect of C60 cluster ion beam bombardment in sputter depth profiling of organic-inorganic hybrid multiple thin films

The effects of C₆₀ cluster ion beam bombardment in sputter depth profiling of inorganic-organic hybrid multiple nm thin films were studied. The dependence of SIMS depth profiles on sputter ion species such as 500 eV Cs⁺, 10 keV C₆₀⁺, 20 keV C₆₀²⁺ and 30 keV C₆₀³⁺ was investigated to study the effect of cluster ion bombardment on depth resolution, sputtering yield, damage accumulation, and sampling depth.     

Synergetic effect between ion energy and sample temperature in the formation of distinct dot pattern on Si(1 1 0) by ion-sputter erosion

We observed a synergetic effect between ion energy and sample temperature in the formation of distinct dot pattern on Si(1 1 0) by Ar⁺ ion sputtering. The ion flux was 20 μA/cm², a value smaller than those used in preceding reports by one or two orders of magnitude. In experiments, the ion energy was from 1 to 5 keV, and the temperature from room temperature to 800 °C. A phase diagram indicating the ranges of ion energy and temperature within which distinct dot patterns can be achieved has been obtained. Data analyses and simulation results reveal that the synergetic effect is consistent with the effect of the Ehrlich-Schwoebel step-edge barrier, rather than the Bradley-Harper model.     

Formation of nanodots on oblique ion sputtered InP surfaces

Using a field emission gun based scanning electron microscopy, we report the formation of nanodots on the InP surfaces after bombardment by 100 keV Ar⁺ ions under off-normal ion incidence (30° and 60° with respect to the surface normal) condition in the fluence range of 1 × 10¹⁶ to 1 × 10¹⁸ ions cm⁻². Nanodots start forming after a threshold fluence of about 1 × 10¹⁷ ions cm⁻². It is also seen that although the average dot diameter increases with fluence the average number of dots decreases with increasing fluence. Formation of such nanostructured features is attributed due to ion-beam sputtering. X-ray photoelectron spectroscopy analysis of the ion sputtered surface clearly shows In enrichment of the sputtered InP surface. The observation of growth of nanodots on the Ar⁺-ion sputtered InP surface under the present experimental condition matches well with the recent simulation results based on an atomistic model of sputter erosion.     

The effect of Ehrlich-Schwoebel step-edge barrier on the formation of self-organized Si nanodots by ion-sputter erosion

The ion flux dependence of the self-organized Si nanodots induced by 1.5 keV Ar⁺ ion sputter erosion has been studied. It shows that for the regime with ion flux >∼280 μA/cm², the currently adopted Bradley-Harper (BH) model, which is incorporated in a dynamic continuum equation holds valid. However, for ion flux <∼280 μA/cm², the measured dot size and surface roughness deviate drastically from the BH model. To interpret the data for this lower ion flux regime, the effect of the Ehrlich-Schwoebel (ES) step-edge barrier was introduced into the continuum equation. A consistency between the calculated and the experimental results was reached, furthermore, a reasonable trend was found, that is, the effective ES diffusion decreases steadily with the increasing ion flux, and at ∼280 μA/cm², it became negligibly small.     

Nano- and micro-scale patterning of Si (1 0 0) under keV ion irradiation

Evolution of Si (1 0 0) surface under 100 keV Ar⁺ ion irradiation at oblique incidence has been studied. The dynamics of surface erosion by ion beam is investigated using detailed analysis of atomic force microscopy (AFM) measurements. During an early stage of sputtering, formation of almost uniformly distributed nano-dots occurs on Si surface. However, the late stage morphology is characterized by self-organization of surface into a regular ripple pattern. Existing theories of ripple formation have been invoked to provide an insight into surface rippling.     

SEM and XPS studies of nanohole arrays on InP(1 0 0) surfaces created by coupling AAO templates and low energy Ar ion sputtering

The aim of the present study is to demonstrate the feasibility to form well-ordered nanoholes on InP(1 0 0) surfaces by low Ar⁺ ion sputtering process in UHV conditions from anodized aluminum oxide (AAO) templates. This process is a promising approach in creating ordered arrays of surface nanostructures with controllable size and morphology. To follow the Ar⁺ ion sputtering effects on the AAO/InP surfaces, X-ray photoelectron spectroscopy (XPS) was used to determine the different surface species. In_4d and P_2p core level spectra were recorded on different InP(1 0 0) surfaces after ions bombardment. XPS results showed the presence of metallic indium on both smooth InP(1 0 0) and AAO/InP(1 0 0) surfaces. Finally, we showed that this experiment led to the formation of metallic In dropplets about 10 nm in diameter on nanoholes patterned InP surface while the as-received InP(1 0 0) surface generated metallic In about 60 nm in diameter.     

Engineering of hydrophilic and plasmonic properties of Ag thin film by atom beam irradiation

Hydrophilic Ag nanostructures were synthesized by physical vapour deposition of 5 nm Ag thin films followed by irradiation with 1.5 keV Ar atoms. Optical absorbance measurements show a characteristic surface plasmon resonance absorption band in visible region. A blue-shift in absorbance from 532 to 450 nm is observed with increasing fluence from 1 × 10¹⁶ to 3 × 10¹⁶ atoms/cm². Atomic force microscopy was performed for the pristine and irradiated samples to study the surface morphology. The atom beam irradiation induced sputtering and surface diffusion lead to the formation of plasmonic surface. Rutherford backscattering spectroscopy of the pristine and irradiated film indicates that metal content in the film decreases with ion fluence, which is attributed to the sputtering of Ag by Ar atoms. The contact angle measurement demonstrates the possibility of engineering the hydrophilicity by atom beam irradiation.     

Fabrication of densely distributed Si nanorods by Ar-ion bombardment

For the direct fabrication of densely distributed one dimensional nanostructures on Si substrates, Si (1 0 0) surfaces were bombarded by obliquely incident 3 keV Ar⁺ ions with a simultaneous supply of Mo seed atoms at various temperatures ranging from room temperature to 400 °C. The surface sputtered at room-temperature with Mo seeding was characterized by the nanocones pointing in the direction of the incident ion beam. In addition, they possessed a so-called “web” at their acute-angle side. This web decreased in size with an increase in the sputtering temperature. Thus, the projections fabricated at elevated temperatures were featured by the nanorod-like structure rather than conical structure. With increasing the sputtering temperature, projections decreased in base diameter (from ∼90 nm at 200 °C to ∼50 nm at 400 °C) while they increased in both length (from ∼160 nm at 200 °C to ∼240 nm at 400 °C) and numerical density (from ∼5 × 10⁷ mm⁻² at 200 °C to ∼1.2 × 10⁸ mm⁻² at 400 °C). The controlled fabrication of such densely distributed one dimensional nanoprojections on Si using ion beam technique, we believe, would open up a variety of applications such as nanoelectronics and optoelectronics devices.     

Formation of two ripple modes on Si by ion erosion with simultaneous Fe incorporation

This report focuses on the self organized nanostructure formation on Si (0 0 1) by erosion with low energy Kr⁺ ions with simultaneous incorporation of metallic atoms, in particular Fe. The incorporation of Fe is thought to play an important role in the formation of some features. In the experimental set-up used here the Fe atoms come from the sputtering of a cylindrical stainless steel target situated between the source and the sample holder. It is demonstrated how the Fe flux can be regulated by operational parameters of the ion source. It is shown that two different ripple modes, one perpendicular to the ion beam projection on the surface and the other parallel, were formed at near normal incidence (α = 20°) with ion energy between 300 eV and 2000 eV and a fluence of 6.7 × 10¹⁸ cm⁻². The perpendicular mode ripples dominated the topography when E_ion = 2000 eV, while the parallel mode ripples were the main features observed when E_ion = 300 eV. The correlation of Fe concentration with ion sources parameters and resulting topography is analyzed. It is demonstrated that a certain Fe concentration is necessary for the formation of ripples that are oriented perpendicular to the ion beam and that the Fe concentration alone does not determine the evolving topography.     

Study of S ion-assisted sulfurization of n-GaAs (1 0 0) surface

The chemical structure and site location of sulfur atoms on n-GaAs (1 0 0) surface treated by bombardment of S⁺ ions over their energy range from 10 to 100 eV have been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. The formation of Ga-S and As-S species on the S⁺ ion bombarded n-GaAs surface is observed. An apparent donor doping effect is observed for the n-GaAs by the 100 eV S⁺ ion bombardment. It is found that the S⁺ ions with higher energy are more effective in the formation of Ga-S species, which assists the n-GaAs (1 0 0) surface in reconstruction into an ordered (1 × 1) structure upon subsequent annealing. The treatment is further extended to repair Ar⁺ ion damaged n-GaAs (1 0 0) surface. It is found that after a n-GaAs (1 0 0) sample is damaged by 150 eV Ar⁺ ion bombardment, and followed by 50 eV S⁺ ion treatment and subsequent annealing process, finally an (1 × 1) ordering GaAs (1 0 0) surface with low surface states is obtained.     

Element-specific hard X-ray micro-magnetometry of magnetic modifications in Co–Pt dots fabricated by ion etching

We developed a micro-magnetometry with a 2.5 μm spatial resolution based on micro X-ray magnetic circular dichroism (XMCD) technique in order to study magnetic properties of dot arrays for bit-patterned media. This micro-magnetometer was applied to the magnetic characterization of Co–Pt dot arrays fabricated by ion beam etching. As the dot size became small, the intensity of XMCD drastically decreased for dots fabricated by Ga-focused ion beam. This suggested that the dot edges were damaged magnetically by implantation of Ga ions. The damaged width of the dot edge was estimated to be about 13 nm from the decrease in XMCD intensities. This damaged edge width agreed with the ion-implanted area estimated by Monte-Carlo simulation. The less-damaged effect of Ar ion etching was verified by the XMCD measurement of Co–Pt dots with diameter of 20 and 70 nm. It was concluded that ions with inertness, lower energy and smaller atomic number should be used to fabricate dot arrays with an areal density of 1 Tbit/in².     

Estimation of diffusion coefficient by photoemission electron microscopy in ion-implanted nanostructures

We have fabricated parallel stripes of nanostructures in an n-type Si substrate by implanting 30 keV Ga⁺ ions from a focused ion beam (FIB) source. Two sets of implantation were carried out. In one case, during implantation the substrate was held at room temperature and in the other case at 400 °C. Photoemission electron microscopy (PEEM) was carried out on these samples. The implanted parallel stripes, each with a nominal dimension of 4000 nm × 100 nm, appear as bright regions in the PEEM image. Line scans of the intensities from the PEEM image were recorded along and across these stripes. The intensity profile at the edges of a line scan is broader for the implantation carried out at 400 °C compared to room temperature. From the analysis of this intensity profile, the lateral diffusion coefficient of Ga in silicon was estimated assuming that the PEEM intensity is proportional to Ga concentration. The diffusion coefficient at 400 °C has been estimated to be ∼1.3 × 10⁻¹⁵ m²/s. Across the stripes an asymmetric diffusion profile has been observed, which has been related to the sequence of implantation of these stripes and the associated defect distribution due to lateral straggling of the implanted ions.     

Structural and electrical analysis of S ion bombarded p-InP(1 0 0)

The chemical state of sulfur and surface structure on low-energy S⁺ ion-treated p-InP(1 0 0) surface have been investigated by high-resolution X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). S⁺ ion energy over the range of 10-100 eV was used to study the effect of ion energy on surface damage and the process of sulfur passivation on p-InP(1 0 0) by S⁺ ion beam bombardment. It was found that sulfur species formed on the S⁺ ion-treated surface. The S⁺ ions with energy above 50 eV were more effective in formation of In-S species, which assisted the InP surface in reconstruction into an ordered (1 × 1) structure upon annealing. After taking into account physical damage due to the process of ion bombardment, we found that 50 eV was the optimal ion energy to form In-S species in the sulfur passivation of p-InP(1 0 0). The subsequent annealing process removed donor states that were introduced during the ion bombardment of p-InP(1 0 0). Results of theoretical simulations by Transport of Ions in Materials (TRIM) are in accordance with those of experiments.     

Self-assembling of Mn12 molecular nanomagnets on FIB-patterned Au dot matrix

We have developed a novel strategy to build arrays of magnetic nanodots on the 100 nm scale, which exploits the potentialities of both bottom-up and top-down approaches, by self-assembling sulfur-functionalized Mn12 single molecule magnets (SMMs) on patterned Au dot matrices nanofabricated by FIB (focus ion beam). In this way, we demonstrate the capability to assemble SMMs in ordered arrays, where the magnetic information can be easily addressed, being the single bit represented by a 2D distribution of few hundred Mn12 clusters, grafted on top of each 100 × 100 nm² Au dot. Moreover, the chosen Mn12 functionalization is expected to favour a preferential orientation of the grafted molecule with the easy magnetization axis normal to the surface.     

Carbon sputtering yield measurements at grazing incidence

In this investigation, carbon sputtering yields were measured experimentally at varying angles of incidence under Xe⁺ bombardment. The measurements were obtained by etching a coated quartz crystal microbalance (QCM) with a low energy ion beam. The material properties of the carbon targets were characterized with a scanning electron microscope (SEM) and Raman spectroscopy. C sputtering yields measured under Ar⁺ and Xe⁺ bombardment at normal incidence displayed satisfactory agreement with previously published data over an energy range of 200 eV-1 keV. For Xe⁺ ions, the dependence of the yields on angle of incidence θ was determined for 0° ≤ θ ≤ 80°. Over this range, an increase in C sputtering yield by a factor of 4.8 was observed, with the peak in yield occurring at 70°. This is a much higher variation compared to Xe⁺ → Mo yields under similar conditions, a difference that may be attributed to higher scattering of the incident particles transverse to the beam direction than in the case of Xe⁺ → C. In addition, the variation of the yields with θ was not strongly energy dependent. Trapping of Xe in the surface was observed, in contrast to observations using the QCM technique with metallic target materials. Finally, target surface roughness was characterized using atomic force microscope measurements to distinguish between the effects of local and overall angle of incidence of the target.     

A refined Ehrlich-Schwoebel effect on the modification of Si surface nanostructures by post ion milling

A modeling work has been conducted on a phenomenon called post ion milling (PIM), a post-treatment of Ar⁺ ion sputtering to modify nanostructures on solid surface. It was found by experiments that for PIM with a sufficiently low ion flux, both the average dot size and the surface roughness of Si nanodot arrays on Si(1 0 0) decline steadily against milling time. However, the usually adopted Kuramoto-Sivashinsky (KS) model involving the Bradley-Harper (BH) theory failed to explain the experimental results, nor the KS model that combines both the BH and Ehrlich-Schwoebel (ES) effects. We reexamined the ES term in the KS equation, and derived new terms reflecting the ES contribution. With such a modification, the KS model involving both the BH and the refined ES effects finally gave a qualitative explanation to the PIM result.     

Ion implantation induced by Cu ablation at high laser fluence

High energy laser plasma-produced Cu ions have been implanted in silicon substrates placed at different distances and angles with respect to the normal to the surface of the ablated target. The implanted samples have been produced using the iodine high power Prague Asterix Laser System (PALS) using 438 nm wavelength irradiating in vacuum a Cu target. The high laser pulse energy (up to 230 J) and the short pulse duration (400 ps) produced a non-equilibrium plasma expanding mainly along the normal to the Cu target surface. Time-of-flight (TOF) technique was employed, through an electrostatic ion energy analyzer (IEA) placed along the target normal, in order to measure the ion energy, the ion charge state, the energy distribution and the charge state distribution. Ions had a Boltzmann energy distributions with an energy increasing with the charge state. At a laser fluence of the order of 6 × 10⁶ J/cm², the maximum ion energy was about 600 keV and the maximum charge state was about 27+.In order to investigate the implantation processes, Cu depth profiles have been performed with Rutherford backscattering spectrometry (RBS) of 1.5 MeV helium ions, Auger electron spectroscopy (AES) with 3 keV electron beam and 1 keV Ar sputtering ions in combination with scanning electron microscopy (SEM). Surface analysis results indicate that Cu ions are implanted within the first surface layers and that the ion penetration ranges are in agreement with the ion energy measured with IEA analysis.     

Desorption of chemisorbed Carbon on Mo(1 0 0) by noble gas ion sputtering: Validation of ground test measurements of ion engine lifetimes

We report desorption cross section measurements for one monolayer of chemisorbed carbon on a Mo(1 0 0) surface induced by sputtering with noble gas ions (Ne⁺, Ar⁺, Xe⁺) at different incident angles, ion energies, and substrate temperatures. Desorption cross sections were determined by using low-energy ion scattering (LEIS) to monitor the increase of the signal from the Mo substrate. A monolayer of p(1 × 1) carbon adatoms on the Mo(1 0 0) surface was created by dosing ethylene (C₂H₄) to the substrate at 800 K, and characterized by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). We find that the carbon desorption cross section increases with increasing mass and energy of the impinging ions, and there is a maximum value for the desorption cross section at an incident angle for the ions of 30° from the surface plane. The desorption cross section also increases up to a substrate temperature of 300 °C. Values for the carbon desorption cross section for carbon adatoms on Mo(1 0 0) by 400-eV Xe⁺ ion sputtering are about 2 × 10⁻¹⁵ cm², which is one order of magnitude higher than those for bulk carbon samples. This information is particularly important for evaluation of ion-engine lifetimes from ground-test measurements in which contaminant carbon is deposited on Mo accelerator grids, potentially altering the sputtering rate of the Mo. Our measurements show that monolayer amounts of carbon on Mo have desorption cross sections that are two orders of magnitude higher than estimates of what would be required to reduce the Mo erosion rate, and thus ground-test measurements can be used with confidence to predict ion-engine wear in space, from this perspective.