Low bias dry etching of III-nitrides in Cl2-based inductively coupled plasmas

Cl₂-based inductively coupled plasmas (ICP) with low additional dc self-biases (?100V) produce convenient etch rates (500–1500Å·min^?1) for III-nitride electronic device structures. A systematic study of the effects of additive gas (Ar, N₂, H₂), discharge composition, process pressure, and ICP source power and chuck power on etch rate and surface morphology has been performed. The general trends are to go through a maximum in etch rate with percent Cl₂ in the discharge for all three mixtures, and to have an increase (decrease) in etch rate with source power (pressure). Since the etching is strongly ion-assisted, anisotropic pattern transfer is readily achieved. Maximum etch selectivities of approximately six for InN over the other nitrides were obtained.     

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.     

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.     

Plasma and wet chemical etching of In0.5Ga0.5P

Dry and wet chemical etching of epitaxial In_{0, 5}Ga_0.5P layers grown on GaAs substrates by gas-source molecular beam epitaxy have been investigated. For chlorine-based dry etch mixtures (PCl₃/Ar or CC1₂F₂/Ar) the etching rate of InGaP increases linearly with dc self-bias on the sample, whereas CH₄/H₂-based mixtures produce slower etch rates. Selectivities of ≥500 for etching GaAs over InGaP are obtained under low bias conditions with PCl₃/Ar, but the surface morphologies of InGaP are rough. Both CC1₂F₂/Ar and CH₄/H₂/Ar mixtures produce smooth surface morphologies and good (≥10) selectivities for etching GaAs over InGaP. The wet chemical etching rates of InGaP in H₃PO₄:HC1:H₂O mixtures has been systemically measured as a function of etch formulation and are most rapid (∼1 μn · min⁻¹) for high HCl compositions. The etch rate,R, in a 1:1:1 mixture is thermally activated of the formR ∝ , whereE _a = 11.25 kCal · mole⁻¹. This is consistent with the etching being reaction-limited at the surface. This etch mixture is selective for InGaP over GaAs.     

Wet Etching of (0001) GaN/Al2O3 grown by MOVPE

H₃PO₄, NaOH, and KOH solutions are found to be useful for removing nitrogen depleted layers or damage induced by high temperature annealing or dry etching of metalorganic chemical vapor deposition-grown (0001)GaN/Al₂O₃. Solutions are selective to the (0001)plane of GaN. However, certain flat planes etched without etch pits are exposed by wet etching.     

Plasma chemistries for high density plasma etching of SiC

A variety of different plasma chemistries, including SF₆, Cl₂, ICl, and IBr, have been examined for dry etching of 6H-SiC in high ion density plasma tools (inductively coupled plasma and electron cyclotron resonance). Rates up to 4500?-min⁻¹ were obtained for SF₆ plasmas, while much lower rates (≤800?·min⁻¹) were achieved with Cl₂, ICl, and IBr. The F₂-based chemistries have poor selectivity for SiC over photoresist masks (typically 0.4–0.5), but Ni masks are more robust, and allow etch depths ≥10 μm in the SiC. A micromachining process (sequential etch/deposition steps) designed for Si produces relatively low etch rates (<2,000?-min⁻¹) for SiC.     

N2 effect on GaAs etching at 150 mTorr capacitively-coupled Cl2/N2 plasma

The role of N₂ on GaAs etching at 150 mTorr capacitively-coupled Cl₂/N₂ plasma is reported. A catalytic effect of N₂ was found at 20-25% N₂ composition in the Cl₂/N₂ discharges. The peak intensities of the Cl₂/N₂ plasma were monitored with optical emission spectroscopy (OES). Both atomic Cl (725.66 nm) and atomic N (367.05 nm) were detected during the Cl₂/N₂ plasma etching. With the etch rate and OES results, we developed a simple model in order to explain the etch mechanism of GaAs in the high pressure capacitively-coupled Cl₂/N₂ plasma as a function of N₂ ratio. If the plasma chemistry condition became positive ion-deficient at low % N₂ or reactive chlorine-deficient at high % N₂ in the Cl₂/N₂ plasma, the GaAs etch rate is reduced. However, if the plasma had a more balanced ratio of Cl₂/N₂ (i.e. 20-25% N₂) in the plasma, much higher etch rates (up to 150 nm/min) than that in pure Cl₂ (50 nm/min) were produced due to synergetic effect of neutral chlorine adsorption and reaction, and positive ion bombardment. Pure Cl₂ etching produced 14 nm of RMS surface roughness of GaAs. Introduction of ?20% N₂ gas in Cl₂/N₂ discharges significantly reduced the surface roughness to 2-4 nm. SEM photos showed that the morphology of photoresist mask was strongly degraded. Etch rate of GaAs slightly increased from 10 to 40 nm/min when RIE chuck power changed from 10 to 150 W at 12 sccm Cl₂/8 sccm N₂ plasma condition. The surface roughness of GaAs etched at 12 sccm Cl₂/8 sccm N₂ plasma was 2-3 nm.     

Selective etching of (Ba,Sr)TiO3 thin films over silicon in an inductively coupled plasma

In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductive coupled O₂/Cl₂/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O₂ addition of 1 sccm, 9.52 at a O₂ addition of 4 sccm into the Cl₂(30%)/Ar(70%) plasma, respectively. Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). These results confirm that the increased etch rates at O₂ addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect.     

Comparison of F2 plasma chemistries for deep etching of SiC

A number of F₂-based plasma chemistries (NF₃, SF₆, PF₅, and BF₃) were investigated for high rate etching of SiC. The most advantageous of these is SF₆, based on the high rate (0.6 μm·min⁻¹) it achieves and its relatively low cost compared to NF₃. The changes in electrical properties of the near-surface region are relatively minor when the incident ion energy is kept below approximately 75 eV. At a process pressure of 5 mtorr, the SiC etch rate falls-off by ∼15% in 30 μm diameter via holes compared to larger diameter holes (>60 μm diameter) or open areas on the mask. We also measured the effect of exposed SiC area on the etch rate of the material.     

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.     

Remote plasma etching of titanium nitride using NF3/argon and chlorine mixtures for chamber clean applications

To avoid plasma induced erosion of chamber hardware, the application of remote plasma sources to activate the etch gases was introduced. We present results on the etch behaviour of titanium nitride (TiN) using mixtures of NF₃, Cl₂ and argon. The gas mixture was excited in a remote plasma source and then routed through a reaction chamber to study the etch behaviour of TiN samples which simulate the situation at the chamber walls. The dependency of the TiN etch rate on temperature, gas flow, composition and pressure was examined. While the temperature (studied in the range 25-300 °C) turned out to be the most sensitive parameter, the general etch rate was mainly dependent on the availability of atomic fluorine. Etch products and NF₃/Cl₂ dissociation have been monitored by quadrupole mass spectrometry and infrared spectroscopy. While NF₃ showed a high decomposition up to 96%, chlorine decomposition was not observed. However the addition of chlorine increased the etch rates up to 260% in the low pressure/low temperature regime. Surface effects of chlorine addition are indicated by X-Ray Photoelectron Spectrometry and REM surface analysis.     

Very Low Pressure Magnetron Reactive Ion Etching of GaN and Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N Using Dichlorofluoromethane (Halocarbon 12)

In this work we report on the magnetron reactive ion etch (MRIE) technology for gallium nitride (GaN) and aluminum gallium nitride (Al _x Ga_1−x N) using dichlorodifluoromethane (CCl₂F₂), commonly known as halocarbon 12, with etch rates greater than 1,000 and 840 ?/min, respectively. Magnetic confinement of a very low pressure (10⁻⁴ Torr range) radio frequency (RF) discharge generates high-density plasmas, with low sheath voltages at the bounding surfaces, and very high dissociation of the source gas. Furthermore, the very low pressure of the etch process is characterized by long mean free paths so that sputtering contamination is reduced. MRIE chemistry has been monitored in situ by means of mass spectroscopy. Finally, we report on the successful fabrication of an indium gallium nitride (In _x Ga_1−x N) blue light emitting diode (LED), whose fabrication sequence included the MRIE etching of GaN in CCl₂F₂.     

Characterization of the CH4/H2/Ar high density plasma etching process for HgCdTe

High density plasma etching of mercury cadmium telluride using CH₄/H₂/Ar plasma chemistries is investigated. Mass spectrometry is used to identify and monitor etch products evolving from the surface during plasma etching. The identifiable primary etch products are elemental Hg, TeH₂, and Cd(CH₃)₂. Their relative concentrations are monitored as ion and neutral fluxes (both in intensity and composition), ion energy and substrate temperature are varied. General insights are made into surface chemistry mechanisms of the etch process. These insights are evaluated by examining etch anisotropy and damage to the remaining semiconductor material. Regions of process parameter space best suited to moderate rate, anisotropic, low damage etching of HgCdTe are identified.     

Inductively coupled plasma reactive ion etching of SiC single crystals using NF<Subscript>3</Subscript>-based gas mixtures

Inductively coupled plasma reactive ion etching of SiC single crystals using NF₃-based gas mixtures was investigated. Mesas with smooth surfaces and vertical sidewalls were obtained, with a maximum etch rate of about 400 nm/min. Effects of CH₄ and O₂ addition to the NF₃ gas and the crystalline quality of substrates were studied during the SiC dry etching using various masks. Selectivity of the photoresist (PR) mask improved from about 0.2 to about 0.4 by the addition of 30% CH₄ during the RIE, although the etch rate decreased by 50–70%. Results also indicated that the substrate quality does not significantly affect the etch results.     

Atomic-layer-deposited Al2O3 and HfO2 on GaN: A comparative study on interfaces and electrical characteristics

Al₂O₃, HfO₂, and composite HfO₂/Al₂O₃ films were deposited on n-type GaN using atomic layer deposition (ALD). The interfacial layer of GaON and HfON was observed between HfO₂ and GaN, whereas the absence of an interfacial layer at Al₂O₃/GaN was confirmed using X-ray photoelectron spectroscopy and transmission electron microscopy. The dielectric constants of Al₂O₃, HfO₂, and composite HfO₂/Al₂O₃ calculated from the C-V measurement are 9, 16.5, and 13.8, respectively. The Al₂O₃ employed as a template in the composite structure has suppressed the interfacial layer formation during the subsequent ALD-HfO₂ and effectively reduced the gate leakage current. While the dielectric constant of the composite HfO₂/Al₂O₃ film is lower than that of HfO₂, the composite structure provides sharp oxide/GaN interface without interfacial layer, leading to better electrical properties.     

High-density plasma etch selectivity for the III–V nitrides

High-density plasma etching has been an effective patterning technique for the group-III nitrides due to ion fluxes which are 2–4 orders of magnitude higher than more conventional reactive ion etch (RIE) systems. GaN etch rates exceeding 0.68 μm/min have been reported in Cl₂/H₂/Ar inductively coupled plasmas (ICP) at −280 V dc-bias. Under these conditions, the etch mechanism is dominated by ion bombardment energies which can induce damage and minimize etch selectivity. High selectivity etch processes are often necessary for heterostructure devices which are becoming more prominent as growth techniques improve. In this study, we will report high-density ICP etch rates and selectivities for GaN, AlN, and InN as a function of plasma chemistry, cathode rf-power, ICP-source power, and chamber pressure. GaN:AlN selectivities >8:1 were observed in a Cl₂/Ar plasma at 10 mTorr pressure, 500 W ICP-source power, and 130 W cathode rf-power, while the GaN:InN selectivity was optimized at 6.5:1 at 5 mTorr, 500 W ICP-source power, and 130 W cathode rf-power.     

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.     

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.     

Bipolar resistive switching of chromium oxide for resistive random access memory

This study investigates the resistance switching characteristics of Cr₂O₃-based resistance random access memory (RRAM) with Pt/Cr₂O₃/TiN and Pt/Cr₂O₃/Pt structures. Only devices with Pt/Cr₂O₃/TiN structure exhibit bipolar switching behavior after the forming process because TiN was able to work as an effective oxygen reservoir but Pt was not. Oxygen migration between Cr₂O₃ and TiN was observed clearly before and after resistance switching from Auger electron spectroscopy (AES) analysis. Both low resistance state, ON state, and high resistance state, OFF state, of Pt/Cr₂O₃/TiN structures are stable and reproducible during a successive resistive switching. The resistance ratio of ON and OFF state is over 10², on top of that, the retention properties of both states are very stable after 10⁴ s with a voltage of −0.2 V.     

Comparison of ECR plasma chemistries for etching of InGaP and AlGaP

Etch rates for InGaP and AlGaP are examined under electron cyclotron resonance (ECR) conditions in Cl₂/Ar, BCl₃/Ar, BCl₃/N₂, ICl/Ar, and IBr/Ar discharges. All the plasmas except IBr/Ar provide rapid etching of InGaP at rates above 1 μm min⁻¹. ICl/Ar provides the highest etch rates. Unlike the Cl₂/Ar and BCl₃-based chemistries, the rates in ICl/Ar and IBr/Ar are almost independent of microwave power in the range 400–1000 W. Much lower rates were obtained for AlGaP in every discharge due to the greater difficulties in bond breaking that must precede formation and desorption of the etch products.