XeF2 Research Articles

Aspect Ratio-Dependent Etching in Silicon using XeF2: Experimental Investigation and Comparative Analysis with Dry Etching Methods

Abstract Silicon machining plays a crucial role in shaping three-dimensional structures for Micro-Electro-Mechanical Systems (MEMS) applications. This study investigates Aspect Ratio Dependent Etching (ARDE) across various silicon etching processes, with a particular focus on Xenon Difluoride (XeF2) etching, in comparison to Reactive Ion Etching (RIE) and Deep Reactive Ion Etching (DRIE).By exploring different etching parameters, the study highlights the presence of ARDE in both plasma and non-plasma etching processes. Additionally, it is demonstrated that ARDE can be modeled by a saturating exponential function through experimental adjustment of parameters, enabling the prediction of etching profiles.

Ligand-assisted excitation of Eu(III) during interaction of Eu(FOD)3 with xenon difluoride in acetonitrile solution

The interaction of tris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium(III), (Eu(FOD)3), with xenon difluoride in acetonitrile solution was found to be accompanied by chemiluminescence (CL). The kinetic curve of CL at a ratio of reagents Eu3+/XeF2 of 1:2 has a complex form; an initial rapid decrease is replaced by an increase in CL intensity, the maximum of which is reached approximately 10 min after the start of the reaction. Next, an exponential decay of the CL intensity is observed over several tens of minutes. The CL spectrum is located in the wavelength range 400–750 nm and corresponds to the emission of an electronically excited europium (III) ion coordinated with FOD and fluoride ion. Spectral methods were used to identify the reaction products – europium (III) fluoride in the sediment and oxygen difluoride in the gas phase. A mechanism for chemiexcitation of europium(III) has been proposed, including: 1) acceptance of fluorine anion from the XeF2 molecule by europium ion with the formation of active intermediate XeF+; 2) oxidative fluorination of the ligand, which is initiated by the interaction of oxygen atom of the FOD ligand with XeF+ cation and ends with the formation of oxygen difluoride and the electronically excited product P*, presumably a fluorinated ketone; 3) non-radiative transfer of excitation energy from P* to the europium ion within its coordination sphere, followed by its radiative deactivation.

Dissociation following the Auger decay of xenon difluoride molecules.

This study investigated the dissociation after the Xe 4d Auger decay of weak-bonding XeF2 molecules by multielectron-ion coincidence spectroscopy using a magnetic bottle electron spectrometer. Fragmentations from the XeF22+ states were clarified in the Auger spectra coincident with individual ion species. It was observed that the two-hole population led by the Auger decay was not directly inherited during the fragmentation of XeF22+. Furthermore, the dissociations of XeF23+ states produced by the double Auger decay were investigated.

Facilitated fluorination and etching of 2D materials

Precise control of functionalization and etching have been required for surface modification and device fabrication of two-dimensional (2D) materials. Specifically, fluorination of graphene has been used to control the properties of graphene. Recently, xenon difluoride (XeF2) has been used for selective fluorination of graphene and etching of other 2D materials, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). However, there is a lack of studies on key factors that govern the XeF2 treatment, which results in inconsistent behaviors of fluorination and etching. Here, we report control over chemical reactions between XeF2 and 2D materials using a chemical mediator of Si. Even a small amount of Si can accelerate the dissociation of XeF2, leading to the formation of chemically reactive xenon fluoride (XeF) that enhances the fluorination and etching of 2D materials. Additionally, our findings show that defects in 2D materials serve as chemically unstable sites that facilitate the additional dissociation of XeF2, generating F single atoms that easily form covalent bonds on the surface of 2D materials. Our study suggests that Si can be utilized as a reaction mediator to regulate XeF2 treatment on 2D materials, which has important implications for the fabrication of 2D electronic devices.

A 220 GHz superconducting titanium transition edge sensor array developed for cosmic microwave background experiments

In this paper, we report on the design, fabrication, and characterization of a 220 GHz superconducting transition edge sensor (TES) array developed for ground-based cosmic microwave background (CMB) experiments. Unlike conventional TES arrays adopting thermometers made of bilayer superconducting films, the TES array has thermometers made of simpler single-layer titanium (Ti) film deposited on a suspended silicon nitride (SiN x ) membrane. The thermal weak link is realized by releasing the TES’s thermal island including the Ti thermometer by dry etching with xenon difluoride (XeF2), giving a typical thermal conductance of 25 pW K−1. Its thermal-conductance mechanism is further studied by fitting the measured TES’s complex impedance with a three-block thermal model. The dark and optical noise equivalent power (NEP) of a superconducting TES are both measured. Despite existing lens reflection and dielectric loss in the superconducting microstrip line, its typical optical NEP reaches 100 aW Hz−0.5, which meets the sensitivity requirement for ground-based CMB experiments.

Effect of Refresh Time on XeF2 Gas-assisted FIB Milling of GaAs

Focused ion beam (FIB) machining can be used to fabricate gallium arsenide-based devices, which have a surface finish of several nanometers, and the FIB machining speed and surface finish can be greatly improved using xenon difluoride (XeF2) gas-assisted etching. Although the refresh time is one of the most important parameters in the gas-assisted etching process, its effect on the machining quality of the surface finish has rarely been studied. Therefore, in this work, we investigated the effect of the refresh time on the etching process, including the dissociation process of XeF2, the refresh time dependency of the sputter in yield under different incident angles, and the surface finish under different refresh times. The results revealed that a selective etching mechanism occurred at different refresh times. At an incidence angle of 0°, the sputtering yield increased with the refresh time and reached its maximum value at 500 ms; at an incidence angle of 30°, the sputtering yield reached its minimum value at a refresh time of 500 ms. For surface roughness, the incident angle played a more important role than the refresh time. The surface finish was slightly better at an incidence angle of 30° than at 0°. In addition, both F and Xe elements were detected in the processed area: Xe elements were evenly distributed throughout the processing area, while F elements tended to accumulate in the whole processing area. The results suggest that the optimum surface can be obtained when a larger refresh time is employed.

Influence of xenon difluoride on the optical properties of fluorozirconate and fluorohafnate glasses

To study the effect of xenon difluoride as a fluorinating agent on optical properties of glasses in ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF) and HBLAN (HfF4–BaF2–LaF3–AlF3–NaF) systems, their optical transmission in the range from UV to IR was investigated. The treatment of the initial fluorides with XeF2 was shown to lead to a broadening of the transmission region of the obtained glasses both in the UV and IR ranges. Moreover, the treatment of the batch with xenon difluoride leads to the removal of oxygen-containing impurities that absorb in the region of 2.8 μm.

Xenon Difluoride Dry Etching for the Microfabrication of Solid Microneedles as a Potential Strategy in Transdermal Drug Delivery (Small 27/2023)

Xenon Difluoride Dry Etching for the Microfabrication of Solid Microneedles as a Potential Strategy in Transdermal Drug Delivery (Small 27/2023)

Xenon Difluoride Dry Etching for the Microfabrication of Solid Microneedles as a Potential Strategy in Transdermal Drug Delivery.

Although hypodermic needles are a "gold standard" for transdermal drug delivery (TDD), microneedle (MN)-mediated TDD denotes an unconventional approach in which drug compounds are delivered via micron-size needles. Herein, an isotropic XeF2 dry etching process is explored to fabricate silicon-based solid MNs. A photolithographic process, including mask writing, UV exposure, and dry etching with XeF2 is employed, and the MN fabrication is successfully customized by modifying the CAD designs, photolithographic process, and etching conditions. This study enables fabrication of a very dense MNs (up to 1452 MNs cm-2 ) with height varying between 80 and 300µm. Geometrical features are also assessed using scanning electron microscopy (SEM) and 3D laser scanning microscope. Roughness of the MNs are improved from 0.71 to 0.35µm after titanium and chromium coating. Mechanical failure test is conducted using dynamic mechanical analyzer to determine displacement and stress/strain values. The coated MNs are subjected to less displacement (≈15µm) upon theapplied force. COMSOL Multiphysics analysis indicates thatMNs are safe to use in real-life applications with no fracture. This technique alsoenables the production of MNs with distinct shape and dimensions. The optimized process provides a wide range of solid MN types to be utilized for epidermis targeting.

Transparent wafer-scale self-standing fluorinated graphene films

The fluorination of self-standing graphene oxide (GO) films under mild conditions at 70 °C for 1h using a mixture of elemental fluorine and nitrogen gases (0.35 F2/0.65 N2 v/v) results in transparent, highly fluorinated (F/C = 1 as seen by XPS) graphene oxide (FGO) films which maintain initial densely packed structure and mechanical strength. Most of the oxygen atoms in the GO film were replaced by fluorine after fluorination, and two types of carbon-fluorine bonds coexist, i.e. primarily weakened C⋯F bonds and also pure covalent C–F bonds as evidenced by FTIR, XPS, NMR, and cyclic voltammetry when FGO is used as electrode in Li/electrolyte (PC:EC:3DMC/LiTFSI 1 M)/FGO cell similar to a lithium battery. To further understand the fluorination mechanism and to distinguish the reaction steps, xenon difluoride XeF2 was used as fluorinating agent for GO and in situ EPR and FTIR analyses suggest hydroxyl groups as preferential reactive sites. These self-standing transparent FGO films exhibit unusual electrochemical properties in lithium battery and can be used as hydrophobic lubricant membranes because of the low and stable coefficient of friction evidenced in the present work.

Novel first-principles insights into graphene fluorination

Fluorination of graphene sheets with xenon difluoride leads to the formation of the widest bandgap Gr derivative, namely, fluorographene. Accurate experimental observations distinguish two stages of mechanism in the fluorination procedure: the half-fluorination stage, wherein one side of the Gr sheet is rapidly fluorinated, and the full-fluorination stage, involving much slower fluorination of the opposite side of the sheet [R. J. Kashtiban et al., Nat. Commun. 5, 5902 (2014)]. Here, we perform comprehensive density functional calculations to illustrate accurate microscopic insights into the much slower rate of the full-fluorination stage compared with the half-fluorination one. The calculated minimum energy paths for the half- and full-fluorination processes demonstrate much enhanced fluorine adsorption after the half-fluorination stage, which sounds inconsistent with the experimental picture. This ambiguity is explained in terms of significant chemical activation of the graphene sheet after half-fluorination, which remarkably facilitates the formation of chemical contaminants in the system and, thus, substantially slows down the full-fluorination procedure. After considering the binding energy and durability of the relevant chemical species, including hydrogen, oxygen, and nitrogen molecules and xenon atom, it is argued that oxygen-fluorine ligands are the most likely chemical contaminants opposing the complete fluorination of a graphene sheet. Then, we propose an oxygen desorption mechanism to carefully explain the much enhanced rate of the full-fluorination procedure at elevated temperatures. The potential photocatalytic application of the pristine and defected samples in water splitting and carbon dioxide reduction reactions is also discussed.

Controlled introduction of defects into single-walled carbon nanotubes via a fluorination–defluorination strategy using xenon difluoride and their alkaline oxygen reduction reaction catalytic activity

The defect edges in carbon nanomaterials have attracted attention as catalytic active sites for the oxygen reduction reaction (ORR) of the cathode in electrolyte fuel cells, and the defect control in carbon nanomaterials is becoming increasingly important. This study evaluates a fluorination–defluorination strategy for the controlled introduction of defects into single-walled carbon nanotubes (SWCNTs) involving the fluorination of SWCNTs using xenon difluoride (XeF2) and their subsequent defluorination through thermal annealing. We synthesized fluorinated SWCNTs with different fluorine contents using gaseous XeF2 and annealed the fluorinated SWCNTs at 1000 °C for 3 h under nitrogen gas flow. Structural analyses revealed that SWCNTs derived from fluorinated SWCNTs with low fluorine contents primarily had single point defects. In contrast, SWCNTs derived from fluorinated SWCNTs with high fluorine contents had vacancy defects with edges. According to the ORR catalyst evaluation in alkaline aqueous solution, SWCNTs with edge defects, rather than point defects, can improve the efficiency of ORR catalytic activity. The proposed fluorination–defluorination strategy using gaseous XeF2 is expected to enable the controlled introduction of defects in different types of carbon materials.

Auger electron angular distributions following excitation or ionization from the Xe 3d and F 1s levels in xenon difluoride.

Linearly polarized synchrotron radiation has been used to record polarization dependent, non-resonant Auger electron spectra of XeF2, encompassing the bands due to the xenon M45N1N45, M45N23N45, M45N45N45 and M45N45V and fluorine KVV transitions. Resonantly excited Auger spectra have been measured at photon energies coinciding with the Xe 3d5/2 → σ* and the overlapped Xe 3d3/2/F 1s → σ* excitations in XeF2. The non-resonant and resonantly excited spectra have enabled the Auger electron angular distributions, as characterized by the βA parameter, to be determined for the M45N45N45 transitions. In the photon energy range over which the Auger electron angular distributions were measured, theoretical results indicate that transitions into the εf continuum channel dominate the Xe 3d photoionization in XeF2. In this limit, the theoretical value of the atomic alignment parameter (A20) characterizing the core ionized state becomes constant. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the βA parameters extracted from our non-resonant Auger spectra. For a particular Auger transition, the electron kinetic energy measured in the resonantly excited spectrum is higher than that in the directly ionized spectrum, due to the screening provided by the electron promoted into the σ* orbital. The interpretation of the F KVV Auger band in XeF2 has been discussed in relation to previously published one-site populations of the doubly charged ions (XeF22+). The experimental results show that the ionization energies of the doubly charged states predominantly populated in the decay of a vacancy in the F 1s orbital in XeF2 tend to be higher than those populated in the decay of a vacancy in the Xe 4d level in XeF2.

Luminescence and optical absorption of fluorozirconate glass doped with K2MnF6

Fluorozirconate glass doped with K2MnF6 was produced to obtain a red-emitting phosphor, and K2MnF6 was synthesized according to a specially developed procedure using xenon difluoride. The luminescence spectrum exhibited a band at 550 nm attributed to Mn2+, and the absorption spectra belonged to Mn2+ and Mn3+ ions, which were formed due to the reduction of Mn4+ ions.

Surface micromachining of chip-edge silicon microcantilevers using xenon difluoride etching of silicon-on-insulator

We demonstrate a straightforward surface micromachining process for the rapid prototyping of thin ‘chip-edge’ silicon microcantilevers protruding from the edge of a silicon-on-insulator (SOI) chip. The process uses a single photolithographic mask—with xenon difluoride used to both pattern the silicon microcantilevers and release them by etching part of the underlying silicon wafer. During the release step, the silicon microcantilevers are protected from the xenon difluoride by a combination of photoresist and buried silicon dioxide. The use of common microfabrication materials (SOI and positive photoresist) and chemicals (buffered hydrofluoric acid and xenon difluoride), along with a maximum process temperature of 100 °C, makes for a generic, soft micromachining process which is—in principle—compatible with preserving the integrity of any pre-patterned circuitry present on the silicon microcantilever top surface. Doppler vibrometry measurements of the silicon microcantilevers reveal a well-defined resonant frequency and a quality factor comparable with that of similar silicon microcantilevers fabricated using other means. Our enabling technological process allows the rapid prototyping of chip-edge silicon microcantilevers—potentially integrating sensitive circuitry for novel probe technologies—by avoiding the relatively cumbersome, expensive, and potentially circuit-damaging front-to-back processing/deep etching combination.

Sacrificial grid release technology: a versatile release concept for MEMS structures

Micro-electro-mechanical-systems (MEMS) structures with different in-plane dimensions often need to be released simultaneously from the bulk of the wafer and a single dry etching or wet etching technique cannot fulfill all release requirements. In this paper we present a universally applicable solution to release MEMS structures with different surface areas in a controlled and uniform way, which combines isotropic etching of a sacrificial silicon support structure by xenon difluoride with a predefined etch surface made by deep reactive ion etching. Two applications of this Sacrificial Grid Release Technology are presented, in which MEMS devices are released in silicon-on-insulator wafers. The demonstrated applications involve the release of microstructures with in-plane dimensions ranging from tens of micrometers to a few millimeters. The sacrificial silicon structure provides mechanical support which allows freedom in process flow design for fragile MEMS structures. The release technique can also be used to separate the chips from the wafer.

A novel method to fabricate on-chip ultra-high-Q microtoroid resonators

Fabricating an optical microtoroid with ultra-high quality factor is an indispensable procedure both for scientific research and engineering. Here, HF/HNO3 wet etching is specially tailored to form silicon pillars for ultra-high quality factor microtoroids instead of xenon difluoride (XeF2) or reactive ion etching (RIE) methods. This is a low-cost, easily maintained and time-saving method. The method uses a very simple etching device and is robust to temperature variations and humidity changes at room temperature(20–25 °C). Anisotropy in isotropic HF/HNO3 etching, which may limit the quality factor, is minimized in the experiment. Quality factor dependence on silicon orientation is experimentally investigated and theoretically explained with group theory, which shows that [111] oriented silicon wafers are superior to [100] and [110] in the wet etching method. The maximum quality factor observed at 1550 nm waveband in the experiment is 1.05 × 108. We believe this technique will have great influence both on laboratory and future production use.

Solid-phase chemiluminescent reaction of Eu2(SO4)3 with XeF2 as a possible source of the Eu(IV) highest oxidation state

The highly exothermic solid-phase interaction of anhydrous europium (III) sulfate polycrystalline powder with xenon difluoride was found to be a potential source of the highest oxidation state (HOS) of europium, Eu4+. The reaction is accompanied by chemiluminescence (CL) in the visible spectral region and proceeds spontaneously at room temperature for ​~ ​20 ​h. Several photon emitters are established and identified, namely, atomic Xe and single-ionized xenon Xe+, emitting in the wavelength range 420–700 ​nm, as well as the electronically excited ion Eu3+ (590–700 ​nm). Absorption with a maximum of ~340 ​nm is registered in the diffuse reflectance spectrum of the reaction mixture during first few minutes of the reaction. This signal is tentatively assigned to the HOS of europium (+4). The reaction mechanism is proposed which includes a) Eu3+-assisted decomposition of xenon difluoride leading to atomic fluorine as well as highly reactive intermediates XeF+ and Xe+; b) oxidation of Eu3+ ion to the HOS by the latter; c) generation of excited species in the reduction stages followed by CL: Xe2+(Xe+) → Xe0 (blue-green emission) and Eu4+ → Eu3+ (red emission), sulfate environment is considered as the reductant; d) the F• ​+ ​SO4•- interaction and partial replacement of sulfate ligands with fluoride and fluorosulfate anions in the coordination sphere of the europium ion. Theoretical calculations using composite methods suggest the proposed mechanism and demonstrate the thermodynamic probability of ozone formation in the studied reaction, which was detected by the characteristic UV absorption band.

Implementation of a two-qubit state by an auxiliary qubit on the three-spin system

The method for preparation of a two-qubit state on two spins-1/2 that mutually interact through an auxiliary spin is proposed. The essence of the method is that, initially, the three spins evolve under the action of an external magnetic field during a predefined period of time. Then, the auxiliary spin is measured by a monochromatic electromagnetic radiation that allows obtaining a certain state of the remaining spins. We study the entanglement of this state and obtain the condition for achieving the maximally entangled state. The implementation of the method on the physical system of nuclear spins of xenon difluoride is described. As a results, the conditions which allow preparing the maximally entangled state on this system are obtained.

High-efficiency and stable silicon heterojunction solar cells with lightly fluorinated single-wall carbon nanotube films

High-quality single-wall carbon nanotube (SWCNT) films were lightly fluorinated by treatment with xenon difluoride at room temperature, which led to the formation of ionic C–F bonds on tube walls and controllable p-type doping of the nanotubes. The fluorinated SWCNT films showed improved electronic conductivity and a higher work function. In addition, the fluorination process increased the areal density of SWCNT films and decreased their surface roughness, leading to better interface contact between them and silicon. As a result, a heterojunction solar cell constructed using the lightly fluorinated SWCNT film has a high power conversion efficiency of 13.6% and excellent stability.