Etch characteristics of GaN and BN materials in chlorine-based plasmas

Reactive ion etching (RIE) was performed on GaN and BN thin films using chlorine-based plasmas. The optimum chemistry was found to be BCl₃/Cl₂/N₂/Ar and Cl₂/Ar at 30 and 40 mtorr for GaN and BN etching, respectively. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analysis of the GaN and BN etched surfaces show a decrease in the surface nitrogen atomic composition and an increase in chlorine impurity incorporation with increasing self-dc bias. A photo-assisted RIE (PA-PIE) process using an IR filtered Xe lamp beam was then used and resulted in improved etch rates and surface composition. Optical emission spectroscopy (OES) measurements have also shown photoenhancement of the etch process.     

Dry etching of GaN using chemically assisted Ion beam etching with HCI and H2/Cl2

The dry etching characteristics of GaN were investigated using chemically assisted ion beam etching (CAIBE) with HCI and H₂/Cl₂ gas. Etch rates using CAIBE/HC1 were investigated as a function of Ar ion beam energy and substrate temperature. These results were compared to CAIBE/C1₂. Etch rates were also investigated for CAIBE/H₂/Cl₂ for various ratios of H₂:C1₂. Highly anisotropic submicron lines are demonstrated using CAIBE/HC1. Auger electron spectroscopy was used to investigate surface stoichiometric changes of samples etched with CAIBE/HC1, CAIBE/H₂/Cl₂,, and CAIBE/C1₂. The diffusion of deuterium into GaN during etching was also investigated using secondary ion mass spectrometry.     

Reactive ion etching of gallium nitride films

Reactive ion etching (RIE) was performed on gallium nitride (GaN) films grown by electron cyclotron resonance (ECR) plasma assisted molecular beam epitaxy (MBE). Etching was carried out using trifluoromethane (CHF₃) and chloropentafluoroethane (C₂ClF₅) plasmas with Ar gas. A conventional rf plasma discharge RIE system without ECR or Ar ion gun was used. The effects of chamber pressure, plasma power, and gas flow rate on the etch rates were investigated. The etch rate increased linearly with the ratio of plasma power to chamber pressure. The etching rate varied between 60 and 500Å/min, with plasma power of 100 to 500W, chamber pressure of 60 to 300 mTorr, and gas flow rate of 20 to 50 seem. Single crystalline GaN films on sapphire showed a slightly lower etch rate than domain-structured GaN films on GaAs. The surface morphology quality after etching was examined by atomic force microscopy and scanning electron microscopy.     

Etch damage evaluation on (Bi4−xLax)Ti3O12 thin films during the etch process using inductively coupled plasma sources

The etching mechanism of (Bi_4−xLa_x)Ti₃O₁₂ (BLT) thin films in Ar/Cl₂ inductively coupled plasma (ICP) and plasma-induced damages at the etched surfaces were investigated as a function of gas-mixing ratios. The maximum etch rate of BLT thin films was 50.8 nm/min of 80% Ar/20% Cl₂. From various experimental data, amorphous phases on the etched surface existed on both chemically and physically etched films, but the amorphous phase was thicker after the 80% Ar/20% Cl₂ process. Moreover, crystalline “breaking” appeared during the etching in Cl₂-containing plasma. Also the remnant polarization and fatigue resistances decreased more for the 80% Ar/20% Cl₂ etch than for pure Ar plasma etch.     

Effect of gas mixing ratio on etch behaviors of Ba2Ti9O20 (BTO) and Pt thin films in Cl2/Ar inductively coupled plasma

An investigation of the Ba₂Ti₉O₂₀ (BTO) and Pt thin films etch mechanism in the Cl₂/Ar inductively coupled plasma was carried out. It was found that an increase in Ar mixing ratio at fixed gas pressure and input power causes a fast decrease in the BTO etch rate (26.9-1.2 nm/min for 0-100% Ar) while the Pt etch rate increases slightly from 17.4-23.0 nm/min. Langmuir probe diagnostics and zero-dimensional plasma modeling provided the data on plasma parameters, steady-state composition and fluxes of active species on the etched surface. From the model-based analysis of etch kinetics, it was shown that the behavior of the BTO etch rate corresponds to the reaction-rate-limited etch regime, where the etch rate is limited neither by physical sputtering of the main material nor by the ion-stimulated desorption of low-volatile reaction products. The etch process of Pt appears in the transitional regime and is controlled by the neutral and ion fluxes together.     

Low-Dimensional Waveguide Grating Fabrication in GaN with Use of SiCl<Subscript>4</Subscript>/Cl<Subscript>2</Subscript>/Ar-Based Inductively Coupled Plasma Dry Etching

Optimized fabrication of submicron-sized features in gallium nitride (GaN) with the use of inductively coupled plasma (ICP) dry etching, based on SiCl₄/Cl₂/Ar gas mixture, is presented. Dense periodic patterns, i.e., 400-nm-period gratings, were transferred into a gallium nitride waveguide under different etching conditions. ICP power, radiofrequency (RF) power, chamber pressure, and Ar/Cl₂ gas mixing ratio were altered during the experiment. Depths of fabricated grating couplers up to 670 nm were achieved. The most suitable etching conditions are discussed with the assessment based on etching selectivity, scanning electron microscopy (SEM) observation of grating tooth slope, hard-mask erosion process, and etched surface morphology.     

Selective dry etching of (Sc2O3)x(Ga2O3)1−x gate dielectrics and surface passivation films on GaN

(Sc₂O₃)_x(Ga₂O₃)_1?x films grown by molecular beam epitaxy show promise for use as surface passivation layers and gate dielectrics on GaN-based high electron mobility transistors. Completely selective, low-damage, dry etching of (Sc₂O₃)_x(Ga₂O₃)_1?x films with respect to GaN can be achieved with low-power inductively coupled plasmas of CH₄/H₂/Ar with etch rates in the range 200–300 Å/min. The incident ion energies are of order 100 eV, and no roughening of the underlying GaN was observed under these conditions. Similar etch rates were obtained with Cl₂/Ar discharges under the same conditions, but GaN showed rates almost an order of magnitude higher.     

Cl2基气体感应耦合等离子体刻蚀GaN的工艺

采用Cl2/He对GaN基片进行感应耦合等离子体刻蚀,并比较了相同条件下使用Cl2/He,Cl2/Ar对GaN基片进行刻蚀的优劣.实验中通过改变ICP功率、直流自偏压、气体总流量和气体组分等方式,讨论了这些因素对刻蚀速率和刻蚀后表面粗糙度的影响.实验结果表明,用Cl2/He气体刻蚀GaN材料可以获得较高的刻蚀速率,最高可达420nm/min.同时刻蚀后GaN材料的表面形貌也较为平整,均方根粗糙度(RMS)可达1nm以下.SEM图像显示刻蚀后表面光洁,刻蚀端面陡直.最后比较了相同条件下使用Cl2/He,Cl2/Ar刻蚀GaN基片的刻蚀速率、表面形貌,以及制作n型电极后的比接触电阻.     

Etching of silicon carbide for device fabrication and through via-hole formation

We have investigated the etching of SiC using inductively-coupled-plasma reactive ion etching with SF₆-based and Cl₂-based gas mixtures. Etch rates have been investigated as functions of bias voltage, ICP coil power, and chamber pressure. It will be shown, for the first time, that SiC surfaces etched in Cl₂-based plasmas yield better surface electrical characteristics than those etched in SF₆-based plasmas. We have also achieved SiC etch rates in excess of 1 μm/min which are suitable for micro-machining and via-hole applications. Through via-holes obtained in 140 μm thick SiC at an effective etch rate of 824 nm/min have been achieved. To the best of our knowledge, to date, this is the highest effective etch rate for a through via-hole etched with a masking process compatible with microelectronic fabrication.     

Patterning amorphous fluoropolymer films by reactive ion milling

Amorphous fluoropolymer films have low dielectric constants and high chemical resistance and, so, have potential to be used as the insulator for high speed interconnects and as protection layers. Many applications would require high resolution patterning of the fluoropolymer film. We have found that these films are easily etched by reactive ion beam etching using an O₂/Ar gas mixture. High etching rates of 600 nm/min with a 0.2 mA/cm^-2,500 eV ion beam were obtained. This technique allows good selectivity with typical underlayers such as Si, Au, and photoresist. We have also found that a short Arion milling of the fluoropolymer surface allows good wettability of the film by photoresist.     

Iron nitride mask and reactive ion etching of GaN films

One of the major GaN processing challenges is useful pattern transfer. Serious photoresist mask erosion and hardening are often observed in reactive ion etching of GaN. Fine pattern transfer to GaN films using photoresist masks and complete removal of remaining photoresist after etching are very difficult. By replacing the etch mask from conventional photoresist to a sputtered iron nitride (Fe-8% N) film, which is easily patterned by wet chemical etching and is very resistive to Cl based plasmas, GaN films can be finely patterned with vertical etched sidewalls. Successful pattern transfer is realized by reactive ion etching using Cl (H) containing plasmas. CHF₃/Ar, C₂ClF₅/Ar, C₂ClF₅/Ar/O₂, SiCl₄, and CHCl₃ plasmas were used to etch GaN. The GaN etch rate is dependent on the crystalline quality of GaN. Higher crystalline quality GaN films exhibit slower etch rates than GaN films with higher dislocation and stacking fault density.     

Dry etch selectivity of Gd2O3 to GaN and AlN

Gd₂O₃ is a promising gate dielectric for GaN, but little is known of its dry etching characteristics. We achieved Gd₂O₃ etch rates up to ～600 Å · min^?1 in high density Cl2-based discharges, with maximum selectivities of ～15 over GaN and ～4 over AlN. Pure Cl₂ discharges produced reverse selectivities for both Gd₂O₃/GaN and Gd₂O₃/AlN, with typical values between 0.1–0.4. When a rare gas additive such as Ar or Xe was added to the plasma chemistry, the nitrides etched faster than the oxide. This indicates that volatile etch products (GaCl₃, AlCl₃, N₂) form in Cl₂-based plasmas once the GaN or AlN bonds are broken by ion bombardment, but that GdCl_x species are not volatile. In conjunction with the low efficiency for Gd₂O₃ bond-breaking at low ion energies, this leads to low selectivity.     

Effect of dry etching on surface properties of III-nitrides

Dry etched InAlN and GaN surfaces have been characterized by current-voltage measurement, Auger electron spectroscopy, and atomic force microscopy. Electron cyclotron resonance discharges of BCl₃. BCl₃/Ar, BCl₃/N₂, or BCl₃/N₂ plus wet chemical etch all produce nitrogen surfaces that promote leakage current in rectifying gate contacts, with the BCl₃/N₂ plus wet chemical etch producing the least disruption on the surface properties. The conductivity of the immediate InAlN or GaN surface can be increased by preferential loss of N during BCl₃ plasma etching, leading to poor rectifying contact characteristics when the gate metal is deposited on this etched surface. Careful control of plasma chemistry, ion energy, and stoichiometry of the etched surface are necessary for acceptable pinch-off characteristics. Hydrogen passivation during the etch was also studied.     

Surface etching of 6H-SiC (0001) and surface morphology of the subsequently grown GaN via MOCVD

The correlation between surface morphological properties of the GaN epilayers and the surface conditions of 6H-SiC (0001) substrates etched in H₂, C₂H₄/H₂, and HCl/H₂ was studied. Etching 6H-SiC in H₂ produced a high quality surface with steps and terraces, while etching in HCl/H₂ produced either a rough surface with many pits and hillocks or a smooth surface similar to that etched in H₂, depending on the HCl concentration and temperature. The GaN epilayers were subsequently deposited on these etched substrates using either a low temperature GaN or a high temperature AlN buffer layer via MOCVD. The substrate surface defects increased the density and size of the “giant” pinholes (2–4 μm) on GaN epilayers grown on a LT-GaN buffer layer. Small pinholes (<100 nm) were frequently observed on the samples grown on a HT-AlN buffer layer, and their density decreased with the improved surface quality. The non-uniform GaN nucleation caused by substrate surface defects and the slow growth rate of planes of the islands were responsible for the formation of “giant” pinholes, while the small pinholes were believed to be caused by misfit dislocations.     

Electron cyclotron resonance plasma etching of materials for magneto-resistive random access memory applications

Dry etching of multilayer magnetic thin film materials is necessary for the development of sensitive magnetic field sensors and memory devices. The use of high ion density electron cyclotron resonance (ECR) plasma etching for NiFe, NiFeCo, TaN, and CrSi in SF₆/Ar, CH₄/H₂/Ar, and Cl₂/Ar plasmas was investigated as a function of microwave source power, rf chuck power, and process pressure. All of the plasma chemistries are found to provide some enhancement in etch rates relative to pure Ar ion milling, while Cl₂/Ar provided the fastest etch rate for all four materials. Typical etch rates of 3000Å/min were found at high microwave source power. Etch rates of these metals were found to increase with rf chuck power and microwave source power, but to decrease with increasing pressure in SF₆/Ar, CH₄/H₂/Ar, and Cl₂/Ar. A significant issue with Cl₂/Ar is that it produces significant metal-chlorine surface residues that lead to post-etch corrosion problems in NiFe and NiFeCo. However, the concentration of these residues may be significantly reduced by in-situ H₂ or O₂ plasma cleaning prior to removal of the samples from the etch reactor.     

Etching characteristics of ZnO thin films in chlorine-containing inductively coupled plasmas

This study examined the plasma etching characteristics of ZnO thin films etched in BCl₃/Ar, BCl₃/Cl₂/Ar and Cl₂/Ar plasmas with a positive photoresist mask. The ZnO etch rates were increased in a limited way by increasing the gas flow ratio of the main etch gases in the BCl₃/Ar, BCl₃/Cl₂/Ar and Cl₂/Ar plasmas at a fixed dc self-bias voltage (V_dc). However, the ZnO etch rate was increased more effectively by increasing the V_dc. Optical emission spectroscopy (OES) and X-ray photoelectron spectroscopy (XPS) analyses of the ZnO surfaces etched at various Cl₂/(Cl₂ + Ar) mixing ratios revealed the formation of the ZnO_xCl_y reaction by-products as a result of the increased etch rate with increasing Cl₂ addition, compared with 100% Ar⁺ sputter etching. This suggests that at Cl₂/Ar flow ratios ⩾20%, the ZnO etch process is controlled by an ion-assisted removal mechanism where the etch rate is governed by the ion-bombardment energy under the saturated chlorination conditions.     

Characterization of GaN films etched using reactive ion etching technique by secondary ion mass spectrometry

We investigated GaN films etched by using reactive ion etching (RIE) technique to fabricate the GaN-based devices. The samples were grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD), and Ti/Al contacts were formed on n-GaN surfaces after etching processes. The effects of the kinds of reactive gases were evaluated by secondary ion mass spectrometry (SIMS). The results showed that in the sample etched using BCl₃ gas, the signal from boron contaminations was strongly detected at the interface between the contact metal and n-GaN, and we found that additional etching in Cl₂ plasma after etching with BCl₃ gas was essential to make a good contact.     

ICP dry etching of ZnO and effects of hydrogen

Two different plasma chemistries for etching ZnO were examined. Both Cl₂/Ar and CH₄/H₂/Ar produced etch rates which increased linearly with rf power, reaching values of 1200 Å/min for Cl₂/Ar and 3000 Å/min for CH₄/H₂/Ar. The evolution of surface morphology, surface composition, and PL intensity as a function of energy during etching were monitored. The effect of H in ZnO was studied using direct implantation at doses of 10¹⁵–10¹⁶ cm⁻², followed by annealing at 500–700 °C. The hydrogen shows significant outdiffusion at 500 °C and is below the detection limits of SIMS after 700 °C anneals. SEM of the etched features showed anisotropic sidewalls, indicative of an ion-driven etch mechanism.     

Effect of O2/CHF3 plasma treatment on n-type GaN grown on sapphire by MOCVD

Plasma-induced damage of n-type GaN in Cl₂/CH₄/Ar reactants and its recovery by the O₂/CHF₃ plasma treatment in reactive ion etching (RIE) system were studied by etching rate, self-bias voltage and Hall measurement. RIE of n-type GaN was performed at a radio frequency power of 250 W in Cl₂/CH₄/Ar ambient prior to in the O₂/CHF₃ plasma treatment. The effect of O₂/CHF₃ plasma treatment on electrical characteristics of n-type GaN was investigated by changing the ratio of O₂/CHF₃ flow rate. It is found that the damage caused by conventional RIE processing could be partly recovered by CHF₃/O₂ plasma treatment.     

Final polishing of Ga-polar GaN substrates using reactive ion etching

Reactive ion etching of {0001} oriented plate-like GaN single crystals has been investigated using SiCl₄:Ar:SF₆ chemistry. The reactive ion etching process is highly chemical. Large anisotropy of the etching rate and of the morphology has been established on (000 ) N-polar and (0001) Ga-polar sides of the GaN crystals, with remarkably higher rate on the N-polar side. Atomic force microscopy measurements have shown smooth surface and good polishing effect obtained on Ga-polar side, while N-polar surface exhibits an increased roughness of a factor of 10 after RIE.