Etching Threshold Research Articles

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing.

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

Laser etching of 2D materials with single-layer precision up to ten layers

Patterned 2D materials with layer-controlled thickness and precise lateral resolution are of great potential for many applications. Laser etching is a promising technique for large-scale patterning of 2D materials, but better control in film thickness is strongly desired. Here, we explore the dynamic characteristics in the laser etching process in which a local temperature lock phenomenon is observed as laser power reaches the etching threshold. A layer-by-layer etching strategy is then developed based on the temporal evolution of the local temperature as measured by in-situ Raman spectroscopy. Employing such a method in a typical layered material MoS2, we demonstrate thickness control up to ten layers in layer-by-layer laser etching. The local temperature lock during laser etching is explained by thermodynamic simulation of temperature distribution within the 2D material under laser illumination. The influence of substrate thermal conductivity on the thickness of the controlled laser etching has been revealed. The results are valuable for potential applications of 2D material devices built with complex layered structures.

Electrochemical etching of p-type GaN using a tunnel junction for efficient hole injection

In this work electrochemical etching (ECE) of p-type GaN under constant bias without an external light source is demonstrated for the first time. A tunnel junction (TJ) was used as a cap to ensure stable and controllable hole injection to the p-type semiconductor. GaN and In0.02Ga0.98N layers with two different Mg dopant concentrations (5.6∙1018 and 2∙1019cm−3, respectively) were investigated. Structuring of the samples into double-step stripes allowed examination of the etching mechanism. The etch rate and threshold etching voltage were determined. Finally, a model of the band structure of the etched samples is presented. Based on experimental results and theoretical considerations this work provides a method for controllable ECE of p-type GaN using TJ for efficient hole injection.

Analysis and research on the processing threshold of femtosecond laser

In this work, the intensity distribution formula of the focused femtosecond (fs) laser beam is derived. MATLAB was used to establish the light intensity distribution model of the focused fs laser under the processing condition, and the light intensity of focused fs laser under different laser power was simulated. Then the fs laser processing platform was built. The glass was etched by fs laser, and the SU-8 photoresist was processed by fs laser two-photon polymerization (2PP). By adjusting the change of laser power, the etching and polymerization lines of different sizes are obtained. Based on the simulation results and experimental results, the estimated etching threshold of the glass using the fs laser is 5.096 6×1016 W/m2, and the estimated processing threshold of SU-8 photoresist by 2PP using the fs laser is 1.194 2×1014 W/m2. This work is helpful to further analyze the applications of fs laser processing. It provides guidance and reference for different kinds of fs laser processing methods.

Surface micromachining on a polymethylmethacrylate substrate using visible laser-induced backside wet etching with a KMnO4 solution as an absorber

In this paper, the authors report a method for continuous trench micromachining on polymethylmethacrylate (PMMA). Visible laser-induced backside wet etching (v-LIBWE) on PMMA using a potassium permanganate (KMnO4)-based absorber liquid was studied. PMMA is widely used in microfluidic devices for chemical and biological applications. Conventional micromachining of channels in the PMMA substrate using CO2 lasers achieves the smallest feature of approximately 85 μm. In this study, a continuous 12 μm-wide trench etching on PMMA was achievable by v-LIBWE using a 532 nm nanosecond pulsed laser. The etching threshold is ∼10 J/cm2, which corresponds to the average power of 58 mW for the repetition rate of 10 kHz. The authors also determined that the lowest scanning speed of 2 mm/s was necessary for the v-LIBWE of PMMA to generate the crack-free surface. Our study provides a new perspective and a convenient approach for the micromachining of the polymer substrate using v-LIBWE.

Laser-induced frontside etching of silicon using 1550-nm nanosecond laser pulses

High-quality, low-defect laser machining of dielectric materials remains challenging due to the high energy densities required for inducing the physical processes of laser ablation. Therefore, experiments were performed to demonstrate the laser-induced frontside etching (LIFE) of silicon by nanosecond laser pulses at a wavelength of 1550 nm, utilizing a thin chromium layer as the absorber. Etching was found at a fluence range from 200 to 700 mJ/cm², where the etched surfaces were smooth in the center (5 to 80 nm rms). The etching process was accompanied by incubation as well as saturation effects. At a fluence of 280 mJ/cm², an averaged etching rate of about 0.05 nm/pulse was determined for a high number of laser pulses (N > 1000). At the edges where the fluence was below the etching threshold of about 200 mJ/cm², chromium film melting and the formation of different kinds of laser-induced periodic surface structures (LIPSS) were observed.The LIFE was accompanied by different surface modifications. The optical absorption of silicon likely increased due to the incorporation of chromium into the silicon and the formed defects near the surface. Further, LIPSS were observed at fluences near the threshold of laser etching. In particular, the formation of regular linear ripples with a period of approximately 1.52 µm was observed. The morphological characteristics of the ripples suggest that the mechanism was related to surface scattering at surface features or small ripples and the hydrodynamic instabilities of the molten silicon. In addition, outside the etching track, periodic but nonlinear LIPSS were observed in the chromium film. These hexagonal arranged dot-like patterns had a period of around 440 nm. It is suggested that the hexagonal dot-like patterns resulted from the multi-pulse irradiation and the sequential stress relaxation in the chromium film below the melting point of chromium during spot movement.

Experimental investigation of femtosecond laser micro/nano processing of quartz material

Quartz material is widely used in the automotive industry, aerospace, optical and electronic fields and so on. But quartz is a hard and brittle material, which makes it very difficult to process. Femtosecond laser is applied to micro/nano processing of quartz material due to its excellent performance. This paper uses different energy density femtosecond lasers to process quartz materials and analyzes the surface morphology. The experimental results show that the etching threshold of quartz material is 4.63 J/cm2. The etching depth of the single pulse and the femtosecond laser beam waist radius are 0.005 μm and 8.7 μm, respectively. Besides, there are some granular characteristics on the side wall of etching groove that affect surface quality. The larger the laser power, the more deposits there are.

Theoretical model for ablation of thick aluminum film on polyimide substrate by laser etching

The ablation mechanism of thick aluminum film on polyimide by a 1064 nm Nd:YAG laser etching was investigated. By establishing a theoretical model based on interaction of laser and material, the effects of laser parameters including laser power, equivalent laser pulse number, and scanning velocity of laser beam were studied. The results show that when a laser beam with fluence higher than the etching threshold of aluminum irradiated onto the target surface, the etched depth for a single laser pulse is a definite value; the equivalent laser pulse number can be derived when a polyimide plate coated with aluminum film was selected as an object of study. Namely, an optimized etched result can be obtained which corresponds to certain laser fluences and beam scanning velocities. By substituting the calculated results of laser processing parameters into a finite element simulation, the etched results of aluminum film on polyimide could be achieved. The simulated and experimental results present a good coincidence, and the results can be used to set practical laser processing parameters.

Growth study and characterization of KDP crystals grown by 2D translation method

Transparent potassium dihydrogen phosphate (KDP) crystals that are grown from different translational velocities and orientation configurations were obtained during growth through a novel motion mode called 2D translation method. The grown crystals were investigated by microhardness, dielectric, chemical etching, thermal analysis and laser damage threshold studies. A higher mechanical hardness is observed in crystals grown by the 2D translation method than that grown by the rotating-crystal method. Low dielectric loss and less dislocation density show that the 2D translation method grown crystals contain minimum defects. The thermal stability and damage threshold of the 2D translation method grown crystals are higher than those of the crystal grown using the rotating-crystal method. Moreover, the properties of various crystals grown by the 2D translation method under different conditions are compared, and results show that the grown crystals' quality is best when the translation velocity is 0.04 m/s.

Effect of phosphorous on the growth, optical, mechanical and thermal properties of L-alanine crystals

Single crystals of L-alanine were grown from aqueous solution with a small amount of orthophosphoric acid (H 3 PO 4 ). The single crystal X-ray diffraction (XRD) analysis was employed to identify the cell parameters. The grown crystal was characterized by Energy Dispersive X-ray (EDX) spectrum. The UV-visible transmittance was recorded. The presence of a functional group was confirmed by FTIR analysis. The TGA/DTA studies showed the thermal properties of the crystals. The mechanical strength was analyzed by measuring microhardness of the grown compounds. The second harmonic generation of grown crystals was confirmed by the Kurtz-Perry method using an Nd:YAG laser. Full Text: PDF References D.S. Chemla, J. Zyss, (Eds.), Nonlinear Optical Properties of Organic Molecules and Crystals (Academic Press, Orlando, New York, 1987). C. Razzetti, M. Ardoino, L. Zanotti, M. Zha, C. Paorici, "Solution Growth and Characterisation of L-alanine Single Crystals", Cryst. Res. Technol. 37, 456 (2002). CrossRef E. Ramachandran, S. Natarajan, "Synthesis of L-valine crystals", Cryst. Res. Technol. 44, 641 (2009). CrossRef K. Srinivasan, "Growth of non-linear optical ?-glycine single crystals and their characterization", J. Arumugam, Optic. Mater. 30, 40 (2007). CrossRef Sd. Z. Ali Ahamed, G.R. Dillip, L. Manoj, P. Raghavaiah, B. Deva Prasad Raju, "Growth and characterization of new NLO material: L-serine sodium nitrate", Phot. Letters Poland 2(4), 183 (2010). CrossRef S. Dhanuskodi, J. Ramajothi, "Crystal growth, thermal and optical studies of L-histidine tetrafluoroborate: A semiorganic NLO material", Cryst. Res. Tech. 39, 592 (2004). CrossRef R. Ittyachan, P. Sagayaraj, "Growth and characterization of a new promising NLO L-histidine bromide crystal", J. Cryst. Growth 249, 557 (2003). CrossRef M. Lydia Caroline, R. Sankar, R.M. Indirani, S. Vasudevan, "Growth, optical, thermal and dielectric studies of an amino acid organic nonlinear optical material: l-Alanine", Mater. Chem. Phys. 114, 490 (2009). CrossRef G. Ravi, S. Anbukumar, P. Ramasamy, "On the growth and characterization of mixed crystals of the triglycine sulphate family", J. Cryst. Growth 133, 212 (1993). CrossRef N. Nakatani, M. Yoshio, "Crystal Form of Phosphoric-Acid-Doped Triglycine Sulfate", Jpn. J. Appl. Phys. 35, L 508 (1996). CrossRef N. Nakatani, M. Yoshio, "Doping of Phosphoric Acid into Ferroelectric Triglycine Sulfate Crystal", Jpn. J. Appl. Phys. 35, 5752 (1996). CrossRef A. Saxena, V. Gupta, K. Sreenivas, "Characterization of phosphoric acid doped TGS single crystals", J. Cryst. Growth 263, 192 (2004). CrossRef M. Anbuchezhiyan, S. Ponnusamy, C. Muthamizhchelvan, C.C. Kanakam, S.P. Singh, P.K. Pal, P.K. Datta, "Etching, micro hardness and laser damage threshold studies of a nonlinear optical material L-valine", Eur. Phys. J. Appl. Phys. 58, 10201 (2012). CrossRef K. Seethalakshmi, S. Perumal, "Studies on growth, structure and optical studies of undoped and KBr doped Lalanine single crystals", Recent Research Sc. Techn. 4(6), 13 (2012). DirectLink Ferdousi Akhtar, Jiban Podder, "A Study on Growth, Structural, Optical and Electrical Characterization of L-alanine Single Crystal for Optoelectronic Devices", Research J. Physics 6(2), 31 (2012). CrossRef K. Sangwal, B. Surowska, "Study of indentation size effect and microhardness of SrLaAlO 4 and SrLaGaO 4 single crystals", Mater. Res. Innovat. 7, 91 (2003). DirectLink S.K. Kurtz, T.T. Perry, "A Powder Technique for the Evaluation of Nonlinear Optical Materials", J. Appl. Phys. 39, 3798 (1968). CrossRef

Xe- and U-tracks in apatite and muscovite near the etching threshold

Ion irradiation of a wedge-shaped Durango apatite backed by a mica detector allows investigating ion track ranges and etching properties at different points along the tracks. Transmission profiles obtained by irradiation with 2×106cm−2 11.1MeV/amu 132Xe and 2×106cm−2 11.1MeV/amu 238U parallel to the apatite c-axis correspond to ranges calculated with SRIM (Xe: 76.3μm; U: 81.1μm). However, the measured profiles show much greater etchable track-length variations than the calculated longitudinal straggles. The probable cause is that the length deficit exhibits significant variation from track to track. The measured length deficit in muscovite is in agreement with most existing data. In contrast, the length deficit in apatite appears to be close to zero, which is in conflict with all earlier estimates. This probably results from the etching properties of the apatite basal face, which permit surface-assisted sub-threshold etching of track sections in the nuclear stopping regime. These sections are not accessible from the opposite direction, i.e. by etching towards the endpoint of the tracks or in the direction of the ion beam. This conclusion is supported by the fact that linear dislocations are revealed in apatite basal faces and by the observation of imperfect etch pits that are separated from the etched ion track channel by a section that appears unetched under the microscope.

Photo-assisted etching of silicon in chlorine- and bromine-containing plasmas

Cl2, Br2, HBr, Br2/Cl2, and HBr/Cl2 feed gases diluted in Ar (50%–50% by volume) were used to study etching of p-type Si(100) in a rf inductively coupled, Faraday-shielded plasma, with a focus on the photo-assisted etching component. Etching rates were measured as a function of ion energy. Etching at ion energies below the threshold for ion-assisted etching was observed in all cases, with Br2/Ar and HBr/Cl2/Ar plasmas having the lowest and highest sub-threshold etching rates, respectively. Sub-threshold etching rates scaled with the product of surface halogen coverage (measured by X-ray photoelectron spectroscopy) and Ar emission intensity (7504 Å). Etching rates measured under MgF2, quartz, and opaque windows showed that sub-threshold etching is due to photon-stimulated processes on the surface, with vacuum ultraviolet photons being much more effective than longer wavelengths. Scanning electron and atomic force microscopy revealed that photo-etched surfaces were very rough, quite likely due to the inability of the photo-assisted process to remove contaminants from the surface. Photo-assisted etching in Cl2/Ar plasmas resulted in the formation of 4-sided pyramidal features with bases that formed an angle of 45° with respect to ⟨110⟩ cleavage planes, suggesting that photo-assisted etching can be sensitive to crystal orientation.

Threshold and criterion for ion track etching in SiO2 layers grown on Si

SiO2 layers thermally grown on Si wafers were irradiated with swift heavy ions in the energy range of (20–710) MeV. Subsequent chemical etching in 4% HF for 6 min produced conical pores with diameters from ∼20 to ∼80 nm in the SiO2 layers. We have calculated radii and lifetime of the molten regions in the SiO2 layers and compared them with the pore diameters and diameter dispersions estimated from scanning electron microscopy and atomic force microscopy. It is shown that the existence of a molten region and its radius can serve as a valid criterion for track “etchability”. In the same etching conditions the etched track diameter and the etching velocity in the track region are proportional to the radius and the lifetime of the molten region.

Etching of fused silica fiber by metallic laser-induced backside wet etching technique

The tip of multimode fused silica fiber (core diameter: 550μm) was etched by metallic laser-induced backside wet etching (M-LIBWE) method. Frequency doubled, Q-switched Nd:YAG laser (λ=532nm; τFWHM=8ns) was used as laser source. The laser beam was coupled into the fiber by a fused silica lens with a focal length of 1500mm. The other tip of the fiber was dipped into liquid gallium metallic absorber. The etching threshold fluence was measured to be 475mJ/cm2, while the highest fluence, which resulted etching without breaking the fiber, was 1060mJ/cm2. The progress of etching was followed by optical microscopy, and the etch rate was measured to be between 20 and 37nm/pulse depending on the applied laser energy. The surface morphologies of the etched tips were studied by scanning electron microscopy. A possible application of the structured fibers was also tested.

Laser induced backside wet and dry etching of solar glass by short pulse ytterbium fiber laser irradiation

Microstructures fabricated on the surface of solar glass have the potential to improve the performance of solar cells. In this paper, in order to overcome the high transmittance to 1064 nm center wavelength fiber laser irradiation and realize high efficiency process on transparent glass substrates, different absorber materials, including alumina powder, alumina ceramic wafers, and copper sulphate solutions, were applied for dry and wet etching under the irradiation of 1064 nm pulsed fiber laser respectively. The laser fluence was varied from 7 to 10 J/cm2 with a pulse repetition rate of 20 kHz. The morphology of trenches etched by means of laser induced backside dry etching (LIBDE) and by laser induced backside wet etching was measured using a scanning electronic scope, and compared from the aspects of etch depth and width, as well as the roughness. On the basis of this comparison, a higher etch rate can easily be obtained by dry etching, while lower roughness is a feature of wet etching. The mechanism of LIBDE of solar glass was investigated by demonstrating the procedure of dry etching using alumina ceramic wafer. Moreover, the etch threshold fluence was estimated to be 7.47 J/cm2 by extrapolation. Both types of laser induced backside etching techniques, wet and dry, show the evidence of effective microprocessing on solar glass.

Etching, micro hardness and laser damage threshold studies of a nonlinear optical material L-valine

A nonlinear optical crystal of L-valine was grown from an aqueous solution containing a small amount of phosphoric acid by the slow evaporation method. The grown crystal was characterized by a single crystal X-ray diffraction to determine the unit cell parameters. The powder X-ray diffraction analysis also confirmed the lattice parameters to be a = 9.6687(7) A, b = 5.2709(4) A, c = 12.0371(10) A and β = 90.805(4)°. The results of the Inductively Coupled Plasma Optical Emission Spectrometry (ICPOES) indicate the presence of a small amount of phosphorus in the grown crystal. The Vickers micro hardness test was performed to study the mechanical strength of the crystals. Chemical etching studies were carried out to analyze the dislocation structure. The laser damaged threshold of the grown crystal was measured to be 11.11 GW/cm2 for 10 ns pulse at 1064 nm, which is higher than that of the standard nonlinear optical crystals like KDP. Second harmonic generation of the grown crystals was also 1.44 times that of KDP.

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons generated by Auger neutralization of low-energy ions, leaving photo-assisted etching as the only likely explanation. Experiments were carried out with light and ions from the plasma either reaching the surface or being blocked, showing conclusively that the “sub-threshold” etching was due to photons, predominately at wavelengths < 1700 Å. The photo-assisted etching rate was equal to the ion-assisted etching rate at 36 eV, causing substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. Under these conditions, photo-assisted etching likely plays an important role in profile evolution of features etched in Si with chlorine-containing plasmas, contributing to the commonly observed sloped sidewalls and microtrenches.

Blue light emission from a glass/liquid interface for real-time monitoring of a laser-induced etching process

An organic dye, Oil-Red-O, dissolved in p-xylene was used for laser-induced backside wet etching using a visible laser (visible-LIBWE) for the first time. Blue light (360–500 nm) emission from the glass/liquid interface was observed during the etching of borosilicate glass using a nanosecond Q-switched green laser. The emission was confirmed to accompany the etching process. The UV–visible spectrum consists of characteristic peaks of metals, which are the components of the glass. The maximal emission intensity occurs when the laser focusing is at the glass/liquid interface. The etching threshold measured by observing the blue light emission is comparable to that determined by the traditional method. We concluded that the emission is the plasma emission of the etched glass. By measuring the plasma emission, the occurrence of the etching and the crack formation in the glass can be monitored in real time.

Crack-free micromachining on glass substrates by visible LIBWE using liquid metallic absorbers

Laser induced backside wet etching (LIBWE) is a promising laser direct-write technique for etching transparent materials and for producing high precision and near-optical quality surfaces. In this study, visible LIBWE using gallium and eutectic indium/gallium as absorbers was used for crack-free microfabrication of sodalime and quartz. Eutectic indium/gallium (In/Ga) has a melting temperature lower than metallic gallium does and the etching rate by using In/Ga was found to be similar to that by gallium for visible LIBWE. When using the gallium absorber, the etching threshold of quartz by visible LIBWE was about one-third of that by UV LIBWE. The heat-affected zone of the quartz etching was negligible at the trench rim in the visible LIBWE process. The wettability of the metallic absorbers on the substrates affects aspect ratio and is a new important factor for LIBWE. In addition, etching rate decreased when repetition rate was increased.

Etching characteristics of Al 2O 3 thin films in inductively coupled BCl 3/Ar plasma

The investigation of Al 2O 3 etch characteristics in the BCl 3/Ar inductively coupled plasma was carried out in terms of effects of input process parameters (gas pressure, input power, bias power) on etch rate and etch selectivity over poly-Si and photoresist. It was found that, with the changes in gas pressure and input power, the Al 2O 3 etch rate follows the behavior of ion current density while the process rate is noticeably contributed by the chemical etch pathway. The influence of input power on the etch threshold may be connected with the concurrence of chemical and physical etch pathways in ion-assisted chemical reaction.