Average Etch Rate Research Articles

Thermal atomic layer etching of cobalt using sulfuryl chloride for chlorination and tetramethylethylenediamine or trimethylphosphine for ligand addition

Thermal atomic layer etching (ALE) of cobalt was developed using sulfuryl chloride (SO2Cl2) for chlorination and either tetramethylethylenediamine (TMEDA) or trimethylphosphine (PMe3) for ligand addition. In situ quartz crystal microbalance (QCM) measurements were used to monitor the thermal ALE of cobalt using the SO2Cl2/TMEDA and SO2Cl2/PMe3 processes. For every SO2Cl2 exposure, there was a mass gain during chlorination. For every TMEDA or PMe3 exposure, there was a mass loss during ligand addition. The result was a net removal of cobalt during each chlorination/ligand-addition reaction cycle. Average etch rates determined from QCM measurements for the SO2Cl2/TMEDA process at 175, 200, 225, 250, 275, and 300 °C were 0.62 ± 0.41, 1.35 ± 0.64, 2.31 ± 0.91, 6.43 ± 1.31, 10.56 ± 2.94, and 7.62 ± 4.87 Å/cycle, respectively. These etch rates were corroborated using x-ray reflectivity (XRR) studies on cobalt thin films on silicon coupons. Quadrupole mass spectroscopy analysis also revealed that the cobalt etch product from TMEDA exposures on CoCl2 powder was CoCl2(TMEDA). The SO2Cl2/TMEDA process could remove the surface chloride layer formed by each SO2Cl2 exposure with one TMEDA exposure. In contrast, the SO2Cl2/PMe3 process required 20–40 individual PMe3 exposures to remove the surface chloride layer formed from each SO2Cl2 exposure at 130–200 °C. An increasing number of PMe3 exposures were needed as the temperature decreased below 130 °C. The etch rates for the SO2Cl2/PMe3 process with multiple PMe3 exposures were 2–4 Å/cycle as determined by the QCM and XRR studies. For both the SO2Cl2/TMEDA and SO2Cl2/PMe3 processes, the etch rate was determined by the amount of CoCl2 created during the SO2Cl2 exposure. Thicker surface CoCl2 layers from larger SO2Cl2 exposures resulted in higher Co etch rates that could exceed one crystalline unit cell length. Atomic force microscopy measurements determined that the cobalt surface roughness decreased after Co ALE with the SO2Cl2/TMEDA process. In contrast, the cobalt surface roughness increased after Co ALE with the SO2Cl2/PMe3 process. The chlorination and ligand-addition mechanism should be generally applicable for metal ALE for metals that form stable chlorides.

Comparative study on traditional and new etchants for radon monitoring using Poly(allyl diglycol carbonate) (PADC) and identification of (Na3H(CO3)2.2H2O) in etched products

A comparative study is conducted on Traditional Etchants (NaOH/H2O, KOH/H2O) and New Etchants viz. NaOH/Methanol, NaOH/Ethanol and NaOH/1-propanol for Poly(allyl diglycol carbonate) (PADC) used in Radon monitoring. Molar concentration of traditional etchants NaOH/H2O and KOH/H2O is varied from 1 M to 6 M and etching temperature is varied from 30 °C to 80 °C to verify optimal etching parameter. Etching is done for 8 to 9.5 h in the steps of 30 min. An average bulk etch rate (Vb) of 1.4μm/h and 1.7μm/h for NaOH/H2O and KOH/H2O is calculated respectively under optimal etching conditions. Optimal reported concentrations of 1.5 M, 1.5 M and 1 M respectively, are used to make standard NaOH/Methanol, NaOH/Ethanol and NaOH/1-propanol solutions. Track densities, track diameters, bulk etch rate (Vb), track etch rate (Vt), etching time, variation in mass and thickness for both traditional and new etchants are calculated and compared. Traditional etchants take much longer time (5 to 7 h) for etching with initial swelling due to absorption of etchant on the surface of detector due to broken molecular chains, causing increase in mass and thickness of detectors for initial 0.5 to 1 h of etching which are measured with digital balance (±0.0001 g) and digital micrometer (±0.001 mm) respectively. New etchants take short time for etching (30 min approx.). There is no initial increase in mass and thickness of detectors due to short etching time and high Vb of new etchant. Traditional etchants give high track densities with low Vb and Vt while new etchants gives low track density with high Vb and Vt. New tracks formation is rare in new etchants as compared to traditional etchants. An average radon concentration level of 21.8 kBqm−3 is calculated from detectors etched in traditional etchants while new etchants do not provide any useful information about radon track density to calculate radon concentration for the given samples. Trona i-e Na3H(CO3)2.2H2O is identified as a common product formed in the etched products of PADC etched in either of the etchants i.e. traditional etchants and new etchants, using X-ray powder diffraction (XRPD) analysis.

Formation of high aspect ratio through-glass vias by the combination of Ultrasonic micromachining and copper electroplating

Fabrication of high aspect ratio (>5) multiple copper-filled through-holes in non-conductive materials like glass and single-crystal‑silicon is reported by using a faster (>8 μm/s) ultrasonic micromachining (USM). A customized stainless tool having multiple tips arranged in a 6 × 6 area-array manner was used with ~1 μm-size Silicon carbide (SiC) abrasive particles. Numerical simulations were performed to find a suitable horn to reduce the losses during the transmission of ultrasonic wave propagation. A tapered horn profile was chosen to transmit the vibrations uniformly from the ultrasonic head to the multiple tips. A novel technique was proposed to measure the vibration amplitude at different ultrasonic power ratings. >250 experiments were carried out to investigate the effect of ultrasonic power ratings and tool feed rate on the geometric characteristic of microholes. Average etch rates as high as 8 μm/s and 6 μm/s were obtained for glass and silicon, respectively. These average etch rates are higher than those obtained with plasma etching and electrochemical discharge machining. Through-holes were created in a 1.1 mm thick glass substrate in <3 min. The importance of post-USM cleaning by ultrasonic agitation was highlighted, which helped in removing the abrasive particles stuck inside the through-holes. Copper was deposited inside these through-holes using a bottom-up electroplating approach. These copper-filled through-holes can be used as 3-dimensional (3D) interconnects required in various microsystems packaging-related applications.

Etch mechanism of an Al2O3 hard mask in the Bosch process

The etching of high aspect ratio structures in silicon via the Bosch process is essential in modern technologies such as microelectromechanical systems (MEMS) and through‑silicon vias (TSV) fabrication. The process can be very demanding on the mask selectivity due to long etching times, and it has been shown that an Al 2 O 3 hard mask is very suitable in this regard, as it offers significantly higher selectivity compared to the conventional SiO 2 or resist masks. In this work, we employ a combination of Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and X-Ray Photoelectron Spectroscopy (XPS) depth profiling to scrutinize the Al 2 O 3 mask etching mechanism and therefore the origin of the extraordinary high selectivity. We demonstrate that by increasing the passivation step time, a thicker fluorocarbon polymer layer is formed on the Al 2 O 3 , and Al 2 O 3 is then removed with a minuscule average etch rate of ~0.01 nm/min. XPS depth profiling reveals that during Deep Reactive Ion Etching (DRIE) using the Bosch process, an AlF x layer is formed between the polymer and Al 2 O 3 . As AlF x is non-volatile, it requires sputtering to be removed. If the polymer layer is thick enough to attenuate the incoming ions such that their energy is not sufficient to lead to desorption of AlF x , such as when using a longer passivation time, the mask is not eroded. By investigating the surface after different amounts of DRIE cycles, we also obtained information about the formation rate of AlF x and the changes in the Al 2 O 3 and polymer thicknesses over the course of a DRIE process. These findings further expand the knowledge of DRIE and can help process engineers to tailor the processes accordingly. • Selectivity of an Al 2 O 3 mask in the Bosch process becomes practically infinite with longer passivation step time • XPS depth profiling reveals that an AlF x layer is formed during DRIE between Al 2 O 3 and the fluorocarbon polymer on top • This non-volatile AlF x cannot be removed by sputtering if the polymer is thick enough, i.e. with longer passivation time • The dynamics of AlF x formation during DRIE was observed with time resolved measurements

Thermal Atomic Layer Etching of Nickel Using Sequential Chlorination and Ligand-Addition Reactions

The thermal atomic layer etching (ALE) of nickel was demonstrated using sequential chlorination and ligand-addition reactions. Nickel chlorination was achieved using SO2Cl2 (sulfuryl chloride) as the chlorine reactant. PMe3 (trimethylphosphine) was employed as the ligand-addition reactant. Sequential exposures of SO2Cl2 and PMe3 led to Ni thermal ALE. This procedure was inspired by the covalent bond classification (CBC) method that categorizes the most common compounds of various metals. Based on the CBC method, the surface reactions during thermal Ni ALE were performed to form NiX2L2 products, where Cl is the X ligand and PMe3 is the L ligand. Using this strategy, thermal Ni ALE was observed at temperatures from 75–200 °C. The etch rates were determined from in situ quartz crystal microbalance (QCM) measurements. The average etch rates determined from the mass changes were 0.14 ± 0.13, 0.57 ± 0.36, 0.67 ± 0.45, 1.30 ± 0.68, and 3.07 ± 1.56 Å/cycle for the temperatures 75, 100, 125, 150, and 175 °C, respectively. The QCM investigations revealed that there was a mass increase on every SO2Cl2 exposure and a mass loss on every PMe3 exposure, resulting in a net mass loss. The amount of chlorination for a given SO2Cl2 exposure increased with increasing temperature. The amount of mass lost on each PMe3 exposure also increased with increasing temperature. The etch rates were also confirmed using ex situ X-ray reflectivity measurements on Ni films on silicon wafers. The etch rates varied from 0.39 ± 0.10 Å/cycle at 125 °C to 2.16 ± 0.47 Å/cycle at 200 °C. Mass spectrometry analysis revealed that the volatile etch product was NiCl2(PMe3)2 as expected from the CBC method. In addition, scanning electron microscopy revealed that the nickel surface morphology had negligible changes after ALE. Atomic force microscopy analysis showed that thin nickel films remained smooth during initial etching and may experience some slight roughening with progressive etching. Using the CBC method to create novel thermal ALE procedures can be generalized for the thermal ALE of many different metals.

Depth Profile Analysis of the Modified Layer of Poly(vinyltrimethylsilane) Films Treated by Direct-Current Discharge

Previously, we found that modification of the membrane surface from polyvinyltrimethylsilane (PVTMS) by treatment with low-temperature plasma induced by low pressure DC discharge leads to a significant increase in gas separation characteristics. To understand the mechanism of this phenomenon, in this article XPS combined with precision etching 10 keV beam of Ar2500+ clusters was used for depth profiling of PVTMS spin-coated films before and after DC discharge treatment. The etching craters depths were measured by stylus surface profiler. The average etching rate of the untreated PVTMS film by Ar2500+ clusters was defined (230 nm/min). It was found that the low temperature plasma treatment of PVTMS leads to a sharp increase in the oxygen concentration on a surface with a simultaneous decrease in the carbon content. The experimental data obtained indicate also that the treatment of PVTMS film by plasma leads not only to a change in the chemical structure of the surface, but also to the formation of a gradient subsurface layer with a thickness of about 50 nm.

Fabrication of ridge optical waveguide in thin film lithium niobate by proton exchange and wet etching

In this paper, a low-loss ridge waveguide based on the thin film lithium niobate (TFLN) is successfully fabricated using a proton-exchanged wet etching (PEWE) technique, which can result in a much better vertical sidewall angle and improve the surface roughness to reduce the scattering loss. The average etching rate is obviously improved by the structural defects and phase transition induced by our modified PE process, and the finally optimized etching rate is as fast as 13.5 nm/min. The propagation loss of the fabricated straight ridge waveguide is measured to be 4.3 dB/cm by using an 850 nm single mode laser as the light source. Meanwhile, a Y-branch waveguide is also measured with a uniform light splitting ratio of nearly 1:1. Furthermore, this work offers the route for the manufacturing of low-cost and high-efficiency photonic integration platform.

Role of energy-band offset in photo-electrochemical etching mechanism of p-GaN heterostructures

For developing p-GaN gate-based enhancement-mode AlGaN/GaN high-electron-mobility transistors, the removal of the p-GaN layer around the gate region is demonstrated by photo-electrochemical etching. The etching behavior of p-GaN/AlGaN/GaN heterostructures along with p-GaN/u-GaN reference samples is investigated in acidic H2SO4/H3PO4 electrolyte solution. The strong oxidant sulfate radical SO4−∗ formed by H2SO4 stimulates the etching process by either extracting excess electrons from the valence band or providing supplementary holes in the valence band. By optimizing the etching parameters, an average etching rate of 0.27 nm/min with a surface roughness of 2.3 nm is achieved for removing p-GaN from the AlGaN/GaN structure. The etching dynamics of the heterostructure are attributed to the valence-band offset ΔEv caused by band discontinuity at the p-GaN/AlGaN interface. Under positive anodic bias, the etching process is also controlled by local charge transport between p-GaN and the two-dimensional electron gas channel due to a reduced barrier height.

Through-holes micromachining of alumina using a combined pulse-feed approach in ECDM

ABSTRACT Micromachining of alumina has gained a lot of interest due to unique combinations of electrical resistivity and good thermal conductivity, thus being a potential substrate for microsystems packaging in high-power electronics. Creating multiple through-holes simultaneously in alumina is difficult due to its hard and brittle nature. In this article, the formation of multiple through-holes in a 425 µm thick alumina substrate is demonstrated for the first time using the combined pulse-feed approach of the ECDM process with multi-tip array tool to improve productivity. Initial experiments are conducted with a single-tip electrode to understand the effect of duty factor, pulse frequency, and feed rate. A combined pulse-feed approach is adopted to create deep microholes (>600 µm). Later, multiple through-holes are formed using 3 × 3 multi-tip array tool to achieve an average etch rate of 24 µm/min using the combined pulse-feed approach. A duty factor of 70% and a pulse frequency of 20 kHz are suggested for stable electrochemical discharge. A low feed rate (<1 µm/s) is recommended to avoid physical contact between the tool and workpiece. These multiple through-holes can be filled with electroplated copper to form 3D interconnects for upcoming radio-frequency MEMS applications.

Combined reactive plasma jet-laser etching method for technical optical glass containing metal oxides

The machining of optical glass materials with beam technologies enables novel fabrication processes for innovative optical elements with freeform surfaces. However, the complex material composition of optical glass such as N-BK7® is challenging for non-abrasive, pure chemical machining. N-BK7 can be machined utilizing reactive atmospheric plasma jet (RAPJ) etching. However, the etching mechanism results in process-inherent surface composition changes that limit potential applications of RAPJ-machining. A residues layer is formed which causes a temporally changing etching behavior and an increased roughness. Therefore, a process sequence of utilizing RAPJ etching and laser cleaning is proposed and demonstrated that enables to overcome the stated limitations. In this cyclic sequence, N-BK7 optical glass is etched first with a fluorine-containing reactive atmospheric plasma jet resulting in an etched but also residues-modified surface. Thereafter, the residues are removed by pulsed UV laser irradiation without ablation of the glass. This combined laser-enhanced reactive plasma jet etching allows etching of optical glass with a higher average etching rate and smoother machined surfaces.

Isotropic Wet Etching and Improving Surface Flatness of Ge for Etchback Ge-on-Insulator Fabrication

1. Introduction Ge has been received much attention as the next generation semiconductor materials due to its attractive characteristics such as high carrier mobility[1] and narrow bandgap corresponding near infrared wavelength[2]. In order to utilize its characteristics for the applications, Ge-on-Insulator (GOI) structure is necessary. Several fabrication methods for GOI have been suggested such as wafer bonding and mechanical thinning[3], Smart-CutTM[4,5], and SiGe condensation[6]. It is well known Smart-CutTM is widely used for commercial Si-on-Insulator (SOI) fabrication. However, Ge is more sensitive to damages caused by ion implantation and difficult to recover damages completely. Therefore, the best way for GOI fabrication have not been developed.In early stage of SOI R&D, many approaches were proposed, too[7]. Wafer bonding and etchback is one of the simplest technique, and the advantage of this method is any damages due to ion implantation or mechanical stress are not induced to the top semiconductor layer. Therefore, we focused on etchback technique for GOI fabrication. It is necessary to develop appropriate Ge etching method with moderate etching rate and keeping or improving surface flatness. In this study, we aim to develop appropriate etching method of Ge for etchback GOI fabrication. 2. Experimental and results We used single side mirror polished Ge wafer with a thickness of 500 μm. Figure 1 shows the experimental procedure in this study. The original polished top surface was covered by photoresist to avoid etching from this side. Firstly, we confirmed HF + HNO3 mixture solution for Ge etching because it is widely known as etching solution for Si. Figure 2 shows optical microscope images for the back side (non-polished side) of (100)-oriented Ge wafer (a) before etching and (b) after etching by HF + HNO3 for 7 minutes. By etching, surface uniformity drastically improved. So, wet etching can be used for Ge thinning and planarization. However, this solution reacts handle Si substrate of GOI, too. As an alternative etching solution, we selected HF + H2O2 mixture[8]. Figure 3 shows the etching result of (100)-oriented Ge etched by HF + H2O2 + H2O (7:7:6) solution. Although Ge was etched similar to Fig. 2(b), surface uniformity was inferior to HF + HNO3 etching. To improve surface uniformity, CH3COOH was added as diluent instead of H2O because politic amount of CH3COOH into HF + HNO3 solution leads isotropic etching and mirror plane on Si surface[9]. Figures 4 shows the etching results of (100)-oriented Ge by HF + H2O2 + CH3COOH (1:1:1) solution. Surface uniformity improved than Fig. 3 and there is no orientation dependence (data not shown). Therefore, isotropic etching occurs on Ge surface. The average etching rate in the first 20 minutes calculated from the lost masses was 7.9 μm/min. It should be noted any agitation was not carried out during etching and longer etching time leads decreasing of etching rate. Possible etching reaction is expressed as[10]:2CH3COOH + 2H2O2 → 2CH3COOOH + 2H2O (1)Ge + 2CH3COOOH + 2e - → Ge2+ + 2CH3COO- + 2OH- (2)As further investigation of surface uniformity, atomic force microscope (AFM) observation was carried out. Figure 5 shows AFM images for backside of (100)-oriented Ge etched for 110 minutes. The RMS of 30×30 μm2 area is 0.48 nm. Compared with the RMS of original backside (> 5 μm), the surface flatness is improved drastically. In the conference, electrical characteristics of etched surface will be presented. 3. Conclusions For etchback GOI fabrication, we study isotropic etching for Ge. HF + H2O2 + CH3COOH mixture solution can etch Ge isotropic and improving surface uniformity. This solution has a potential as etching solution for etchback to make GOI structure. Acknowledgements This work was supported by JSPS KAKENHI Grant Numbers 19K15028 and 19H05616. Appendix Original concentrations of the chemicals in this study are HF: 49 wt%, HNO3: 69 wt%, H2O2: 30 wt%, and CH3COOH: 99.7 wt%. All compounding ratios in this article are volume rate.

Isotropic silicon etch characteristics in a purely inductively coupled SF6 plasma

The characteristics of isotropic etching of silicon in a purely inductively coupled SF6 plasma are quantitatively studied. Since the etch results are strongly dependent on mask features, the authors investigated both large area and narrow trench etch characteristics. Circles of diameter 500 μm were used as a proxy for unpatterned surfaces and etched for different durations to establish the material etch rate and surface roughness. The average etch rate using the chosen recipe was found to be 2.27 μm/min. Arrays of narrow trenches ranging from 8 to 28 μm were also etched to analyze the effect of trench size on etch rate and degree of anisotropy. The etch rate of the trenches was found to strongly decrease with decreasing trench width. The results demonstrate that isotropic SF6 etch can be readily used as a replacement for more exotic silicon vapor phase etch chemistries such as XeF2.

Formation of nanostructures on the surface of KTP single crystals by argon cluster ion beam

In this work, the surface of potassium titanyl phosphate (KTP) single crystals has been processed by argon cluster ions having low energy per atom E/Nmean = 12.5 eV/atom. The formation of periodic nanostructures has been studied using atomic force microscopy (AFM) and the power spectral density (PSD) functions. To evaluate the processing efficiency, the dependence of the etching rate of KTP on the incident angles of cluster ions has been investigated. It is shown that the average etching rate 〈Vetch〉 varies between 0.46 and 1.1 nm/min with an incident angle change from 0° to 70°.

Surface morphology evolution of a polycrystalline diamond by inductively coupled plasma reactive ion etching (ICP-RIE)

The needle-like surface morphology evolution in oxygen plasma in combination with a secondary gas (Cl2, CHF3 or CF4) by inductively coupled plasma reactive ion etching (ICP-RIE) on a free-standing polycrystalline diamond was investigated. The addition of CF4 can produce trans-polyacetylene (t-PA), which is similar to the result when the pure O2 etching takes place, and generate compact needle-tip particles. However, the t-PA disappears with the introduction of Cl or H ions. The optimised etching parameters for the needle-like structure formation are as following: Cl2/O2 ratio 20% and RF-power (RFP) 100 W, where more compact and even nano-needles are realised with an average etching rate of 2 μm/min. The Cl2/O2 plasma etching results indicate that the time-dependent etching mechanism of diamond nano-needles results from (1 1 1) crystal plane selective etching and preferential graphitisation at the twin-plane boundary and dislocation area.

Study on the impact of Fe3O4@Au magnetic nanoparticles in metal-assisted chemical etching

Metal-assisted chemical etch (MACE) has received more and more attention due to its low cost and simple process. Ag, Au, Pt and other noble metals can be used as catalyst in the MACE of silicon. In this paper, MACE on silicon in a hydrofluoric acid (HF) and hydrogen peroxide (H2O2) mixture with different HF-to-H2O2 molar ratio R (R = [HF]/([HF] + [H2O2])) with Fe3O4@Au core–shell magnetic nanoparticles (MNPs) as catalyst under an external magnetic field was investigated. Boron doped p-type (100) silicon wafers with resistivity of 20∼30 Ω · cm and 0.01∼0.03 Ω · cm were used as substrates, respectively. We observed that an average etching rate of 5.17 μm min−1 under an applied magnetic field while the average etching rate was 4 μm min−1 without magnetic field. The factors were investigated that affect the average etched depth in silicon substrate under magnetic field by using MACE. The relationship between the average etching depth and etching time, resistivity of silicon substrate, ratio R, and the concentration of Fe3O4@Au magnetic nanoparticles was also studied. This result can be applied to increase the etching rate of silicon to fabricate a variety of devices, where high aspect ratio silicon structure is demanded.

Electrical contact uniformity and surface oxidation of ternary chalcogenide alloys

Uniform metal contacts are critical for advanced thermoelectric devices. The uniformity of the contact resistance for gold, tungsten, and SrRuO3 electrodes on polycrystalline ternary Bi2Te3-based alloys for different types of surface cleaning procedures was characterized. The presence of a nanometer-thick native oxide layer on the Bi2Te3 surface leads to large and non-uniform contact resistance. Surface treatments included solvent cleans and chemical and dry etching prior to metallization of the Bi2Te3. Only etching the surface led to a significant improvement in contact resistance uniformity. None of the tested contacts reacted with the underlying Bi2Te3 substrate. Etching resulted in the removal of the native oxide on the Bi2Te3 surface, which was characterized using X-ray photoelectron spectroscopy (XPS). The average thickness, chemistry, and dry etch rate of the native oxide was further characterized using XPS. The non-uniformity in contact resistance suggests that the native oxide grows non-uniformly on polycrystalline bismuth telluride surfaces.

Plasma–surface interaction at atmospheric pressure: A case study of polystyrene etching and surface modification by Ar/O2 plasma jet

In this paper, the authors studied atmospheric pressure plasma–surface interactions using a well-characterized radio-frequency Ar/O2 plasma jet with polystyrene (PS) polymer films in controlled gas environments as a model system. A number of plasma processing parameters, such as the treatment distance, environmental gas composition, and substrate temperature, were investigated by evaluating both the changes in the thickness and the surface chemical composition of PS after treatment. The authors found that the polymer average etch rate decayed exponentially with the nozzle–surface distance, whereas the surface oxygen composition increased to a maximum and then decreased. Both the exponential decay constant and the oxidation maximum depended on the composition of the gaseous environment which introduced changes in the density of reactive species. The authors previously reported a linear relationship between measured average etch rates and estimated atomic O flux based on measured gas phase atomic O density. In this work, the authors provided additional insights into the kinetics of surface reaction processes. The authors measured the substrate temperature dependence of the PS etch rate and found that the apparent activation energy (Ea) of the PS etching reaction was in the range of 0.10–0.13 eV. Higher values were obtained with a greater nozzle-to-surface distance. This relatively low Ea value suggests that additional energetic plasma species might be involved in the etching reactions, which is also consistent with the different behavior of etching and surface oxidation modification reactions at the polymer surface as the treatment distance is varied.

XeF2 etching of epitaxial Nb2N for lift-off or micromachining of III-N materials and devices

This paper presents characterization of the effects of XeF2 vapor phase etching conditions on the lateral etch rate and etch uniformity of a sacrificial, epitaxial Nb2N layer grown between a III-N high-electron-mobility transistor heterostructure and a 6H-SiC substrate. To achieve uniform and repeatable lateral Nb2N removal, an etch temperature of 100 °C or higher was required, providing average etch rates ranging from 10 to 40 μm/min. A net compressive stress and positive strain gradient in the released III-N material were inferred from the buckling of clamped-clamped beams and the convex curvature of cantilever structures, respectively. XeF2 etching of epitaxial Nb2N sacrificial layers in III-N material structures allows for a highly selective, completely dry release process that is compatible with common micromachining and epitaxial lift-off techniques.

Investigation of etching method for fabricating deep through holes on ultra-high resistivity silicon**Project supported by the National Natural Science Foundation of China (Nos. 61574108, 61574112, 61504099)

In this paper, the etching characteristics of the ultra-high resistivity silicon (UHRS) by using the Bosch process were investigated. The experimental results indicated that the sulfur hexafluoride flux, the temperature of the substrate, the platen power and the etching intermittence had important influence on the etching rate and the etching morphology of the UHRS. The profiles and morphologies of sidewall were characterized with scanning electron microscopy (SEM). By using an improved three-stage Bosch process, 380-μm deep through holes were fabricated on the UHRS with the average etching rate of about 3.14 μm/min. Meanwhile, the fabrication mechanism of deep through holes on the UHRS by using the three-stage Bosch process was illustrated on the basis of the experimental results.

Controlled Fabrication of High-Aspect-Ratio Microstructures in Silicon at Etching Rates Beyond State-of-the-Art Microstructuring Technologies

After R. Feynman’s inspiring speech in 1959 “There is plenty of room at the bottom”, silicon microstructuring technologies have been increasingly developed, thus pushing silicon toward novel research topics and market opportunities beyond Moore’s law, a trend well known as More Than Moore. In spite of this, emerging applications in different fields, among which through silicon vias for 3D chip stacking and 3D capacitors with dramatically enhanced value for unit area in microelectronics, and lab-on-chips for point-of-care clinical diagnostics and microneedles for transdermal drug delivery and biosensing in nanomedicine, represent new challenges that silicon microstructuring technologies are facing today. These applications require fabrication of structures with high-aspect-ratio values that are in most cases beyond those attainable by commercial technologies (aspect ratio AR<40). On the other hand, state-of-the-art technologies able to control microfabrication at higher aspect-ratios (AR>100) are restricted to etching rates below 2 μm/min when the aspect-ratio increases over 20. Here we report on the controlled electrochemical etching of deep (up to 200μm) microstructures in silicon with high-aspect-ratio, from 5 to 100, at high etching rates, from 10 to 3 μm/min respectively, at room temperature. This sets a novel record for the high speed fabrication of microstructures with high-aspect-ratio in silicon, and allows silicon microfabrication technology to enter a region in the parameter space etching-rate Vs aspect-ratio that was formerly prohibited for both commercial and research technologies. Addition of an inert oxidant molecule, namely H2O2, to a standard aqueous hydrofluoric (HF) acid electrolyte is used to reduce the valence of the dissolution process to 1 under anodic biasing, thus making the electrochemical etching more effective, on the one hand, and to catalyze the etching rate by opening a more efficient silicon dissolution path with respect to the well-known Gerischer mechanism, thus increasing etching speed at both shorter and higher depths, on the other hand. Etching experiments of both regular macropores (with spatial period 3.5µm) and complex microstructures were carried out by back-side illumination electrochemical etching of pre-patterned n-type silicon (3–8Ω cm resistivity, (100) oriented) in HF–H2O electrolytes ([HF]=5% by vol.) containing different concentrations of H2O2, from 0 (control electrolyte) to 25% (by vol.). All the experiments were conducted under anodic biasing of the silicon electrode at 1.2V, for a maximum etching time of 65 minutes. Experimental data highlight that for a given set of etching parameters, the etching speed monotonically increases with the H2O2 concentration, with respect to the control electrolyte, in spite of the fact that the HF concentration is kept constant over the different experiments [1]. For instance, at [H2O2]=25% ordered macropore arrays with depth of about 50µm and AR=25 were etched in 10 minutes, at an average etching rate of 5 µm/min; when the etching time is increased to 60 minutes, macropore arrays with depth of about 180 µm and AR=100 are etched at an average etching rate of 3 µm/min. The etching speed is 2.8 and 2 times larger, respectively, than that of the control electrolyte (without H2O2). The increase in etching rate does not effect control on accuracy of the microfabrication, with respect to control electrolyte[2]. For a given H2O2, experimental data show that etching speed can be further increased by increasing the HF concentration[3]. For instance, for [H2O2]=15% and etching time of 10 minutes, regular macropore array with depth of 40 (4μm/min) and 70μm (7μm/min) are etched increasing the HF concentration from 5 to 8%. Remarkably, the dissolution valence, i.e. the number of charge carriers required for the dissolution of a single silicon atom, decreased as the H2O2 concentration in the electrolyte increased, thus suggesting that H2O2 was able to promote a change in the stoichiometry of silicon dissolution process by consuming conduction band electrons. The dissolution valence in the presence of [H2O2] = 25% had an average value of 1.12 (standard deviation sd=0.24), whereas had an average value of 2.97 (sd=0.40) in the control electrolyte in good agreement with literature data. We propose that a parallel etching, with respect to the well-known Gerischer mechanism [4], introduced by the addition of H2O2, is triggered by the capture of a conduction band electron released during fluoride-induced etching. This parallel pathway is responsible both for the rate enhancement and the reduction in effective valence. [1] C. Cozzi, G. Polito, K. Kolasinski, G. Barillaro, Advanced Functional Materials (2016) (DOI:10.1002/adfm.201604310). [2] M. Bassu, S. Surdo, L. M. Strambini, G. Barillaro, Advanced Functional Materials, 22, 1222, 2012. [3] V. Lehmann, Electrochemistry of Silicon, Wiley-VCH, Weinheim, Germany, 2002. [4] H. Gerischer, P. Allongue, V. C. Kieling, Phys. Chem. 97, 753, 1993.