Clean Etching Research Articles

(Invited) Selective Area Growth, Etching, and Doping of GaN By MOCVD for Power Electronics

Gallium Nitride (GaN) possesses a wide band gap and excellent transport properties, making it a highly promising material for high-power and high-frequency applications. Microwave power amplification for commercial and military purposes has already benefited from GaN-based high electron mobility transistors (HEMTs). However, currently available GaN optoelectronic and electronic devices use planar junctions and heterostructures prepared by epitaxy, typically via metalorganic chemical vapor deposition (MOCVD). To enable more sophisticated device configurations, in-plane lateral junctions are necessary. The most effective method for achieving this is through selective area etching (SAE) followed by selective area growth (SAG). In this talk, we will discuss the progress made at Yale in the selective-area etching, growth, and doping of GaN.Selective-area etching of GaN is the most challenging and critical step in this process. Instead of using conventional ICP etching, we have employed tertiarybutylchloride (TBCl) as an in-situ chemical etchant of GaN in MOCVD to create smooth trenches and remove plasma etching-generated damages on the surface. With electrical and material characterizations, we have confirmed that TBCl etching is a low-damage and clean etching process. We will also describe our recent efforts to bypass the constraint of dielectric masking during in-situ etching and perform maskless selective area etching (SAE) to create low-defect lateral p-n junctions for GaN power electronics. Additionally, we will discuss non-planar selective-area growth and doping, including the principles of growth evolution, non-uniform Mg distribution, and parasitic impurities.This work is supported by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. DOE, under DE-AR0000871 as part of the PNDIODES program managed by Dr. Isik Kizilyalli.

Lithographically patterned stretchable metallic microwiring on electrospun nanofiber mats

Stretchable and flexible electronic applications require mechanically suitable electrical wiring. This article presents, for the first time, the microfabrication of photolithographically patterned microwiring on an electrospun nanofiber mat. The use of a photoresist-based technology allows for better than 10 μm pattern resolution on a good quality nanofiber mat, providing an order of magnitude smaller feature sizes than what has been published before. We demonstrate metallic wiring patterned from a 75 nm thick coating on top of the nanofiber mat. A silicone elastomer was incorporated to serve as a matrix material and form a composite substrate and an encapsulation layer on top of the microwiring. We demonstrate clean and anisotropic dry etching of the elastomer to open electrode sites that can be smaller than 10 μm in size. We speculate that these structures will be mechanically robust while being soft at the same time and provide the properties necessary for potential use in stretchable and flexible electronics.

Optimization of oxygen plasma based etching of single layered graphene through Raman and FESEM characterization

Graphene field-effect transistors (GFETs) show high electron transfer rates, high charge-carrier mobility, low electrical noise levels and flexibility of surface functionalization, which are useful to realize the sensors for various applications. The key challenge is to develop the processes to fabricate a uniform array of GFETs devices. In this paper, single-layered graphene (SLG) was synthesized on copper (Cu) substrate using chemical vapor deposition (CVD) followed by its successful transfer over silicon (Si) substrate. To realize an array of graphene-based devices/ sensors, the process of oxygen plasma based clean etching of SLG was optimized. The Raman and Field Emission Scanning Electron Microscope (FESEM) characterizations ensure the complete etching of SLG in 240 s using 200 W, RF (13.56 MHz) power. Optimized SLG etching would help to fabricate an array of GFETs as a biochemical sensor platform.

Highly Luminescent Copper Nanoclusters Stabilized by Ascorbic Acid for the Quantitative Detection of 4-Aminoazobenzene.

As one of the widely studied metal nanoclusters, the preparation of copper nanoclusters (Cu NCs) by a facile method with high fluorescence performance has been the interest of researchers. In this paper, a simple, green, clean, and time-saving chemical etching method was used to synthesize water-soluble Cu NCs using ascorbic acid (AA) as the reducing agent. The as-prepared Cu NCs showed strong green fluorescence (with a quantum yield as high as 33.6%) and high ion stability, and good antioxidant activity as well. The resultant Cu NCs were used for the detection of 4-aminoazobenzene (one of 24 kinds of prohibited textile compounds) in water with a minimum detection limit of 1.44 μM, which has good potential for fabric safety monitoring.

Moderate Bacterial Etching Allows Scalable and Clean Delamination of g-C3N4 with Enriched Unpaired Electrons for Highly Improved Photocatalytic Water Disinfection.

Delamination treatment is crucial in promoting the activity of bulk graphitic carbon nitride (g-C3N4). However, most of the currently used methods of exfoliating bulk g-C3N4 to achieve g-C3N4 thin layers suffer from low yield and environmental pollution. Herein, we developed a facile bacterial etching approach for the preparation of high-quality g-C3N4 nanosheets by exfoliating bulk g-C3N4 under room temperature. Morphology and physicochemical characterizations show that the bacteria-treated g-C3N4 (BT-CN) samples, especially BT-CN-2d, have a lamina-like two-dimensional (2D) in-plane porous structure, a significantly enlarged specific surface area (82.61 m2 g-1), and a remarkable narrow band gap (2.11 eV). X-ray photoelectron spectroscopy and electron paramagnetic resonance spectra confirm the dramatic enrichment of unpaired electron in the BT-CN-2d g-C3N4 nanosheets. EIS spectra and photocurrent tests indicate the fast electron transportation. As a result, the representative BT-CN-2d g-C3N4 photocatalyst shows an optimal visible light-driven photocatalytic performance in water disinfection (fourfold higher than bulk g-C3N4), as well as good cycle stability. This moderate and clean bacterial etching process can be realized in tens of gram scale in the laboratory and should be readily extended to kilogram scale. The present work provides fundamental knowledge about the scalable production of high-quality g-C3N4 by bioengineering method, offering extendable availability for designing and fabricating other functional 2D materials.

Patterning of silver nanowire for grid formation by using ultrasonic assisted clean chemical etching for the application of high transparent electrode

We propose a novel, fast, clean, and dimensionally very accurate ultrasonic assisted technique for silver nanowire (AgNW) etching by using Ni etchant for the purpose of fabricating a grid type structure under static and ultrasonic conditions. A photoactive material pattern on an AgNW network was prepared by using a photolithography process, followed by chemical etching of unwanted AgNWs under static and ultrasonic conditions. The ultrasonic assisted etching process facilitates complete etching of the AgNW pattern (initial condition was 22.4 Ω resistance with 94% light transmittance) within 20–25 s, with almost 35.4% dimensional accuracy relative to the longer static etching condition (7.5 min). A combined process was followed for accurate dimensions because the initial short static etching process weakened the unwanted nanowire structure, while the secondary ultrasonic chemical etching process helps to remove all the AgNW residue from pattern area with faster etching.

Fabrication of precisely aligned microwire and microchannel structures: Toward heat stimulation of guided neurites in neuronal cultures

Microwire arrays are a powerful tool for the exertion of localized thermal stress on cellular networks. Combining microwire arrays with a set of orthogonal axon-guiding microchannels on-chip allows for the positioning of neurites, as well as control over their polarity. In this paper, we present a new fabrication approach, based on standard clean room fabrication and sacrificial layer etching, for the integration of microwire arrays into neurite guiding structures. The system permits the application of strong temperature gradients, enabling localized thermal stimulation inside microchannels.

Block Textured a-Si:H Solar Cell

A series of etching experiments on light trapping structure have been carried out by glass etching. The block structure provides long light traveling path and a constant distance between the cathode and anode electrodes regardless of the block height, which results in higher efficiency of the block textured solar cell. In terms of etching profile of the glass substrate, the addition of NH4F resulted in the smooth and clean etching profile, and the steep slope of the block was obtained by optimizing the composition of etching solution. For a higher HF concentration, a more graded slope was obtained and the addition of HNO3and NH4F provided steep slope and clean etching profile. The effects of the block textured glass were verified by a comparison of the solar cell efficiency. For the textured solar cell, the surface was much rougher than that of the plain glass, which also contributes to the improvement of the efficiency. We accomplished block shaped light trapping structure for the first time by wet etching of the glass substrate, which enables the high efficiency thin film solar cell with the aid of the good step coverage deposition.

The interaction of 193 nm excimer laser radiation with single-crystal zinc oxide: Neutral atomic zinc and oxygen emission

We report mass-resolved time-of-flight measurements of neutral particles from the (101¯0) surface of single-crystal ZnO during pulsed 193-nm irradiation at laser fluences below the threshold for avalanche breakdown. The major species emitted are atomic Zn and O. We examine the emissions of atomic Zn as a function of laser fluence and laser exposure. Defects at the ZnO surface appear necessary for the detection of these emissions. Our results suggest that the production of defects is necessary to explain intense sustained emissions at higher fluence. Rapid, clean surface etching and high atomic zinc kinetic energies seen at higher laser fluences are also discussed.

Chemical Vapor Deposition and Etching of High-Quality Monolayer Hexagonal Boron Nitride Films

The growth of large-area hexagonal boron nitride (h-BN) monolayers on catalytic metal substrates is a topic of scientific and technological interest. We have used real-time microscopy during the growth process to study h-BN chemical vapor deposition (CVD) from borazine on Ru(0001) single crystals and thin films. At low borazine pressures, individual h-BN domains nucleate sparsely, grow to macroscopic dimensions, and coalescence to form a closed monolayer film. A quantitative analysis shows borazine adsorption and dissociation predominantly on Ru, with the h-BN covered areas being at least 100 times less reactive. We establish strong effects of hydrogen added to the CVD precursor gas in controlling the in-plane expansion and morphology of the growing h-BN domains. High-temperature exposure of h-BN/Ru to pure hydrogen causes the controlled edge detachment of B and N and can be used as a clean etching process for h-BN on metals.

Femtosecond laser induced micropatterning of graphene film

Micropatterning of CVD synthesized large area graphene film is demonstrated with femtosecond laser cutting process. Homogenous microribbon or other patterned structure can be fabricated without using any resist or other material containing the graphene surface within a very short duration. Once the suitable laser beam doses are determined, sharp edge profile and clean etching are obtained. Scanning electron microscopic study shows that the patterned microribbon is having 5 μm width and mm in length. The width of the patterned microribbon can be controlled with control of laser energy and preprogramming of laser ablation process. Raman study at the edge of the microribbon shows increase in D peak and appearance of D + G mode, signifying edge defects. The defect can be explained from the breaking of sp 2 carbon hybridization with oxidation due to laser etching. The Raman study shows no amorphous carbon formation with laser cutting of the graphene film. The presented process shows a simple way to make patterned microribbon on large area graphene sheet which can be extremely necessary for microelectronics fabrication.

Scanning electrochemical microscopy as an etching tool for ITO patterning

This work presents a new patterning technique of Indium Tin Oxide (ITO) thin films using the microelectrode of a Scanning Electrochemical Microscope (SECM). With the development of new display technologies, patterned indium tin oxide (ITO) films have enormous growth potential and can find applications in any device that requires transparent and conductive electrodes. To optimize the performances of such devices, a clean patterning technique providing straight sidewalls and control of surface chemistry and physical properties is needed. We report here a simple and one-step patterning technique of ITO using scanning electrochemical microscopy as a soft etching tool. This electrochemical wet-lithographic technique provides a fast and low cost patterning process and is performed in an aqueous acid electrolytic solution. This maskless technique leads to a clean etching with straight walls and no redeposition of the ablated part. The subjacent substrate (glass or polymer) is not damaged by this technique and the electrical and/or optical properties of the remaining ITO film are preserved.

Precision material modification and patterning with He ions

The authors report on the use of a helium ion microscope as a potential technique for precise nanopatterning. Combined with an automated pattern generation system, they demonstrate controlled etching and patterning of materials, giving precise command over the geometery of the modified nanostructure. After the determination of suitable doses, sharp edge profiles and clean etching of areas in materials were observed. In this article they present examples of patterning on SiO2 and graphene, which is particularly relevant. This technique could be an avenue for precise material modification for future graphene based device fabrication. The technique has the potential to revolutionize the way that very thin, one-atomic layer materials are modified in a controlled and predictable way.

Precision cutting and patterning of graphene with helium ions

We report nanoscale patterning of graphene using a helium ion microscope configured forlithography. Helium ion lithography is a direct-write lithography process, comparableto conventional focused ion beam patterning, with no resist or other materialcontacting the sample surface. In the present application, graphene samples onSi/SiO2 substrates are cut using helium ions, with computer controlled alignment,patterning, and exposure. Once suitable beam doses are determined, sharp edgeprofiles and clean etching are obtained, with little evident damage or doping to thesample. This technique provides fast lithography compatible with graphene, with∼15 nm feature sizes.

Ablation mechanism of PTFE under 157 nm irradiation

We report time- and mass-resolved measurements on neutral particles emitted from polytetrafluoroethylene (PTFE) during exposure to 157-nm laser radiation at fluences where etching is observed. By comparing the time-of-flight signals over a range of masses, we conclude that (CF2) N fragments for N=1–6 are emitted directly from the surface in substantial quantities. In contrast, the monomer (N=2) is the principal product during irradiation at 248 nm, where thermal decomposition is important. The time-of-flight signals of all the (CF2) N fragments show fast components with kinetic energies on the order of an electron volt. These high kinetic energies are consistent with photochemical scission of the polymer backbone, where a fraction of the excitation energy is delivered to the fragment as kinetic energy. Although clean etching is observed under these conditions, the great majority of the mass removed from the target appears as much larger fragments that do not reach our detector. The nature of this material and its affect on the velocity distribution of the observed (CF2) N fragments is discussed.

Surface reaction pathways of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione on clean and pre-oxidized Ni(110) surfaces

Ni contamination of Si surfaces is an increasing problem for very large scale integrated manufacturers seeking sub-0.25 μm features. Dry etching methods are considered important, potential processes for removal of this contaminant. We and others have recently proposed a chemical vapor etching (CVE) treatment using oxygen and hexafluoropentanedione that could be used to remove Ni. In this article, we have used temperature programmed desorption to examine the surface kinetic pathways of Ni removal by CVE. Ni was successfully dry etched from the pre-oxidized surface by the desorption of Ni(CF3COCHCOCF3)2 between 280 and 480 K with a maximum desorption rate around 390 K. However, above 500 K, a variety of decomposition products appeared and etching ceased. Analysis of the clean (i.e., fully reduced surface) showed these products to be a result of the scission of bonds attached to the β-carbon: C–CF3, C–CH and C=O. Furthermore, stabilization of radical species like CF3 on the clean surface resulted in some unexpected decomposition products observed desorbing between 360 and 540 K. Otherwise, some Ni-fluoride desorption was observed on both surfaces above 800 K and Auger revealed carbon on both surfaces after flashing to 900 K. From these results, it is clear that we need to understand the β-scission process better before we can determine how clean etching can be achieved.

Surface dopant concentration measurement using the Surface Charge Profiler (SCP) method: characterization of hydrogen and metallic contamination in silicon

Boron deactivation by pairing with contaminants and boron compensation are studied in silicon wafers from various IC manufacturing processes using the Surface Charge Profiler (SCP) method. Dissociation energies of pairs formed due to noble metals contamination in HF chemistries are the same independently of the noble metal and correspond to the one of B–H pairs. This suggests that metals are not directly responsible for the doping change but enhance hydrogen contamination into the silicon during HF cleaning. Hydrogenation during SC1 clean (ammonium hydroxide–peroxide mixture), RCA clean (RCA=SC1+SC2 where SC2 is hydrochloric acid–peroxide mixture), and plasma etching is responsible for boron deactivation through B–H formation. Copper contamination introduced into Si during thermal oxidation compensates initial boron doping by formation of stable complexes.

Pulsed Femtosecond Excimer Laser-Induced Chemically Clean Etching of Diamond

AbstractChemically clean etching of thermodynamically metastable diamond using a 500 femtosecond (fs), 248 nm KrF excimer laser is reported. The experimental results, characterized by micro-Raman spectroscopy indicate that unlike nanosecond (ns) pulsed laser processing of diamond, fs laser irradiation of the surface does not generate any graphite or amorphous carbon residues. Microstructural analysis of the fs pulsed laser etched surface indicates streaks.

Ablative etching of nitrocellulose with infra-red and ultra-violet laser radiation

The ablative etching of nitrocellulose membranes with CO 2 ( λ = 10.6 μm), Nd : YAG ( λ = 1.06 μm) and XeCl ( λ = 308 nm) lasers has been studied. From etch rate measurements the threshold fluence was estimated and the energy deposition at threshold fluence was calculated for the three wavelengths. The electron microscope examination of the infra-red laser ablated areas reveals a fairly clean etching under CO 2 laser irradiation, confirming that melting of the crater surroundings cannot be considered as a criterion for the thermal mechanism. It is proposed that the CO 2 laser, which also possesses some operational advantages, compared with excimer lasers, can be effectively used for the clean ablation of the nitrocellulose film dielectric.

Laser-photoetching characteristics of polymers with dopants

The photoetching behavior of poly(methylmethacrylate), poly(dimethylglutarimide) and chlorinated poly(methylstyrene) doped with pyrene and 4-aminobenzoylhydrazide excited by 308 nm excimer-laser pulses has been studied. Some common laser-etching characteristics including the reduction of the threshold fluence for ablation, the enhancement of etching efficiency and the existence of optimal conditions regarding the laser fluence and dopant concentration for generating clean and smooth etching patterns are identified. The photoetching mechanism and the potential application of the doping technique to material processing are discussed.