Isotropic etch for SiO2 microcantilever release with ICP system

Inductively coupled plasma (ICP) system has been widely used for anisotropic silicon etching because it offers high aspect ratio with a vertical side wall. The isotropic etching capability of the ICP system, however, has not gained much attention, even though it possesses advantages in profile control and high etching rate over wet isotropic etching or conventional RIE (reactive ion etching). We report here an isotropic dry etching process to release microcantilever beams. Investigations have covered chamber pressure, plasma source power, substrate power, SF₆ (sulfur hexafluoride) flow rate relating to Si etching rate, undercutting rate, and isotropic ratio. The SiO₂ (silicon dioxide) cantilevers were successfully released from the Si substrate and the optimized silicon etching rate was 9.1 μm per minute. The etching profiles were analyzed by scanning electron micrographs (SEM).     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.     

Gallium arsenide surface chemistry and surface damage in a chlorine high density plasma etch process

In an effort to monitor ion-driven surface chemistry in the high density plasma etching of GaAs by Cl₂/Ar plasma chemistries, we have applied mass spectrometry and careful substrate temperature control. Etch product chlorides were mass analyzed while the substrate temperature was monitored by optical bandgap thermometry and as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. By ensuring that the substrate temperature does not deviate during process variations, the changes in product mass peak intensities are a direct measure of changes in the ionassisted surface chemistry which promotes anisotropic etching. Experimental results show that ion-assisted surface chemistry is optimum when sufficient Cl and Cl⁺ are present in the incident plasma flux. These conditions are met at low coupled microwave powers (<300 W) and low total process pressures (<1.0 mTorr) for input gas mixtures of 25% Cl₂ in Ar. Three mechanistic regions are identified for surface chemistry as a function of incident ion energy: 1) largely thermal chemistry for <50 eV; 2) ion-assisted chemistry for 50–200 eV; and 3) sputtering for >200 eV. Photoreflectance measurements of the surface Fermi level show significant damage for ion energies >75 eV. However, in situ and ex situ surface passivations can recover the surface Fermi level for up to 200 eV ion energies, in good correlation to the onset of sputtering and subsurface damage. Thus, anisotropic, low damage pattern transfer is possible for ion energies between 50 and 200 eV.     

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.     

The effect of ionization and displacement damage on minority carrier lifetime

Based on 1 MeV electrons and 40 MeV Si ion irradiations, the contribution of ionization and displacement damage to the decrease in the minority carrier lifetime of gate controlled lateral PNP (GLPNP) transistors is investigated by gate sweeping (GS) technique. Molecular hydrogen is employed to increase the ionization radiation sensitivity and help to understand the relationship between the minority carrier lifetime and ionization damage. Experimental results show that 1 MeV electrons mainly induce ionization damage to GLPNP transistors, 40 MeV Si ions primarily produce displacement defects in silicon bulk. For 40 MeV Si ions, with increasing the irradiation dose, the densities of interface trap and oxide charge are almost no change, the minority carrier lifetime obviously decreases. The decrease of the minority carrier lifetime is due to bulk traps induce by 40 MeV Si ions. For 1 MeV electrons, with increasing the irradiation dose, the densities of interface trap and oxide charge for the GLPNP with and without soaked in H₂ increase, and the minority carrier lifetime decreases. Compared with the GLPNP transistors without soaking in H₂, the density of the interface traps the irradiated GLPNP transistors by 1 MeV electrons and soaked in H₂ are larger and the minority carrier lifetime is lower. Therefore, both ionization and displacement damage can induce the decreases in the minority carrier lifetime including bulk minority carrier lifetime and surface minority carrier lifetime.     

Effect of reactive sputter etching of SiO2 on the properties of subsequently formed MOS systems

We investigated the effects of reactive sputter etching (RSE) of SiO₂ on the electrical properties of the SiSiO₂ system using CHF₃ in a commercial apparatus. SIMS and Auger spectrometry revealed contamination of RSE-exposed Si surfaces with carbon and heavy metals. The generation of crystal defects during thermal reoxidation was observed to be closely related to the level of metal contamination. C-V, C-t and I-V measurements on subsequently formed MOS structures showed, that oxide charge, interface state and mobile ion densities are nearly unaffected by the RSE process. However, minority carrier lifetime in the Si substrate and isolation behavior of the oxide layer are strongly degraded; our results suggest, that both effects are mainly due to metallic impurities. The use of inert cathode materials like quartz reduces metal contamination, but a non-negligible contribution from the grounded metal surfaces of the reactor remains. Carrier lifetime and insulating properties reach the values obtained on wet chemically etched samples only after extended times of plasma excitation in the apparatus. This is attributed to a passivation of the grounded surfaces by the formation of polymer films. Taking advantage of this effect MOSFETs were fabricated by the use of RSE without deterioration of their electrical performance.     

Plasma etch-induced conduction changes in gallium nitride

The effect of plasma-etching damage on carrier transport properties in GaN has been studied under various plasma conditions by monitoring the changes in sheet resistivity (ρ _s) and mobility (μ _s) or the resistivity (R). All the etching experiments were performed in an electron cyclotron resonance microwave plasma reactive ion etching (ECR-RIE) system. Consistent changes in the transport properties have been observed with increasing dc bias (ion energy) in all plasmas except in those containing chlorine. With noble gas plasmas, the largest change in conductance was created when Ar, the heaviest ion, was accelerated to its highest voltage. In these Ar sputtering cases, substantial surface micro-roughening has been observed. These surfaces also display considerable nitrogen deficiency as measured by Auger electron spectroscopy. These observations suggest that preferential sputtering of nitrogen from the surface of GaN is one form of ion damage. The other is displacement damage. Both of these forms of ion damage are considered to be the direct cause of the observed changes in the electrical properties.     

Carrier mobility in inversion layers and rf plasma induced radiation defects at the SiSiO2 interface

Radiation defects in the SiSiO₂ system caused by plasma etching or oxidation ($\text{d = 35}\text{A}\text{?}$) as well as rf O₂ plasma treatment of the thermal oxide ($\text{d = 50}\text{A}\text{?}$ and $\text{d = 1000}\text{A}\text{?}$) are investigated by making use of steady-state conductance measurements of inversion layers. It is found that the defects act as trapping centers leading to a rapid decrease of the channel conductivity at room temperature. The presence of positive charge in the ultrathin plasma oxide and an additional built-in negative charge in an oxygen plasma treated thermal oxide is observed. Conclusions on scattering processes are made from the temperature dependence of carrier mobility.     

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.     

Characterization of reactive ion etching of benzocyclobutente in SF6/O2 plasmas

This paper reports the reactive ion etching (RIE) characteristics of benzocyclobutene (BCB) in sulfur hexafluoride/oxygen (SF₆/O₂) plasmas. The dependence of etching rate and etch anisotropy on the processing parameters, including RF power, chamber pressure, and SF₆ concentration, are investigated comprehensively ranging from 50 to 200 W, 22.5 to 270 mTorr, and 0% to 80%, respectively. The BCB etching rate increases with chamber pressure and RF power in spite of nonlinearity, but decreases with the increase in SF₆ concentration. Anisotropic etching can be achieved using low chamber pressure, large RF power, and high SF₆ concentration. To avoid grass-like residue that happens at low pressure and large power fluorine-poor conditions, processing parameters with respect to residue-free etching are recommended. The etching mechanisms of the dependence of the etching characteristics on the processing parameters are discussed. Optimal processing parameters are presented as a guideline for isotropic etching of BCB as sacrificial layers to release structures and for anisotropic etching of BCB to precisely control etching dimensions and profiles.     

Inductively coupled plasma reactive ion etching of sapphire using C2F6- and NF3-based gas mixtures

Inductively coupled plasma reactive ion etching (ICP-RIE) of sapphire wafers using C₂F₆- and NF₃-based plasma was investigated as a function of ICP power, bias power, pressure, and plasma chemistry. Etch rate of about 150 nm/min in the case of C₂F₆ plasma and about 260 nm/min in the case of NF₃ plasma was obtained at the optimum condition, with anisotropic profiles and smooth surfaces. No chamber corrosion was observed after the etching, indicating that ICP-RIE using the fluorine-related gases is a promising technique for sapphire patterning.     

Minimizing damage and contamination in RIE processes byextracted-plasma-parameter analysis

It is reported that important plasma parameters for reactive ion etching (RIE) processes, such as ion energy and ion flux density, can be extracted from a simple RF waveform analysis at the excitation electrode in a conventional cathode-coupled, parallel-plate plasma RIE system. This analysis does not introduce any contamination or disturbances to the process. By using the extracted plasma parameters, surface damage and contamination in Si substrates induced by reactive ion etching in a SiCl₄ plasma were investigated. Optimum RIE conditions were then confirmed by studying the relationship between these parameters and the etching performance. It is shown using the experimental data that low-energy high-flux etching is the direction for high performance RIE in future ULSI fabrication     

PECVD SiC材料刻蚀技术

通过对PECVD SiC进行不同条件下的反应离子刻蚀（RIE）和电感耦合反应离子刻蚀（ICP）实验研究，提出了使用SF6和He的混合气体进行RIE刻蚀，并讨论了功率和压强分别对刻蚀速率的影响. 进一步研究了SiC中H含量对于RIE刻蚀速率的影响，同时验证ICP刻蚀过程中负载效应的存在.     

Inductively coupled plasma etchingof in-based compound semiconductorsin CH4/H2/Ar

Inductively coupled plasma etching of InP, InSb, InGaAs, InGaP and InGaAsP was performed in CH₄/H₂/Ar plasmas as a function of CH₄-to-H₂ ratio ICP source power and rf chuck power. Etch rates as high as 6,000 Å×min⁻¹ were obtained for InP, but the surface is extremely rough (>70 nm root-mean-square roughness) under all conditions due to preferential loss of P. Optical emission spectroscopy shows efficient H₂ dissociation at even moderate ICP source powers, leading to the preferential group V loss. By contrast ternary and quaternary materials show excellent morphologies over a wide range of plasma conditions.     

Characterization of Via Etching in CHF3/CF4 Magnetically Enhanced Reactive Ion Etching Using Neural Networks

This study characterizes an oxide etching process in a magnetically enhanced reactive ion etching (MERIE) reactor with a CHF3/CF4 gas chemistry. We use a statistical 2^4‐1 experimental design plus one center point to characterize the relationships between the process factors and etch responses. The factors that we varied in the design include RF power, pressure, and gas composition, and the modeled etch responses were the etch rate, etch selectivity to TiN, and uniformity. The developed models produced 3D response plots. Etching of SiO₂ mainly depends on F density and ion bombardment. SiO₂ etch selectivity to TiN sensitively depends on the F density in the plasma and the effects of ion bombardment. The process conditions for a high etch selectivity are a 0.3 to 0.5 CF₄ flow ratio and a –600 V to –650 V DC bias voltage according to the process pressure in our experiment. Etching uniformity was improved with an increase in the CF4 flow ratio in the gas mixture, an increase in the source power, and a higher pressure. Our characterization of via etching in a CHF₃/CF₄ MERIE using neural networks was successful, economical, and effective. The results provide highly valuable information about etching mechanisms and optimum etching conditions.     

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.     

The effects of reactive ion etching-induced damage on the characteristics of ohmic contacts to n-Type GaN

The effects of reactive ion etching n-GaN surfaces with both SiCl₄ and Ar plasmas have been investigated using transmission line measurements. The measurements were made from ohmic contacts consisting of Al (as-deposited) and Ti/Al (as-deposited and rapid thermal annealed). The contact resistance, specific contact resistance, and sheet resistance were investigated as functions of the dc plasma self-bias voltage and etch time. The contact resistance extracted from contacts fabricated on surfaces etched with SiCl₄ was found to be improved over the unetched samples for all conditions investigated. Dry etching the surface with Ar severely degraded the contact resistance over the unetched sample except at the lower self-bias voltages. Rapid thermal annealing of etched samples prior to Al deposition was found to be effective in removing some of the reactive ion etching/SiCl₄-induced damage.     

Realistic efficiency potential of next‐generation industrial Czochralski‐grown silicon solar cells after deactivation of the boron–oxygen‐related defect center

We measure carrier lifetimes of different Czochralski‐grown silicon (Cz‐Si) materials of various boron and oxygen concentrations and determine the maximum achievable lifetime after an optimized thermal treatment. We obtain very high and stable bulk lifetimes of several milliseconds, virtually eliminating the boron–oxygen (BO) defect complex, which previously limited the carrier lifetime in boron‐doped Cz‐Si materials after prolonged illumination. Based on these experimental results, we introduce a new parameterization of the bulk lifetime of B‐doped Cz‐Si after permanent deactivation of the BO center. Notably, we measure lifetimes up to 4 ms on 2‐Ωcm Cz‐Si wafers at an injection level of 1/10 of the doping concentration. Importantly, these high lifetime values can be reached within 10 and 20 s of BO deactivation treatment. A detailed analysis of the injection‐dependent lifetimes reveals that the lifetimes after permanent deactivation of the BO center can be well described by a single‐level recombination center characterized by an electron‐to‐hole capture cross‐section ratio of 12 and located in the middle of the silicon band gap. We implement the novel parameterization into a two‐dimensional device simulation of a passivated emitter and rear solar cell using technologically realistic cell parameters. The simulation reveals that based on current state‐of‐the‐art solar cell production technology, efficiencies reaching 22.1% are realistically achievable in the near future after complete deactivation of the BO center. Copyright © 2016 John Wiley & Sons, Ltd.     

Passivation of photonic nanostructures for crystalline silicon solar cells

We report on the optical and electrical performances of periodic photonic nanostructures, prepared by nanoimprint lithography (NIL) and two different etching routes, plasma, and wet chemical etching. Optically, these periodic nanostructures offer a lower integrated reflectance compared with the industrial state‐of‐the‐art random pyramid texturing. However, electrically, they are known to be more challenging for solar cell integration. We propose the use of wet chemical etching for fabricating inverted nanopyramids as a way to minimize the surface recombination velocities and maintain a conventional cell integration flow. In contrast to the broadly used plasma etching for nanopatterning, the wet chemically etched nanopatterning results in low surface recombination velocities, comparable with the state‐of‐the‐art random pyramid texturing. Applied to 40‐µm thick epitaxially grown crystalline silicon foils bonded to a glass carrier superstrate, the periodic‐inverted nanopyramids show carrier lifetimes comparable with the non‐textured reference foils (τ_eff = 250 µs). We estimate a maximum effective surface recombination velocity of ~8 cm/s at the patterned surface, which is comparable with the state‐of‐the‐art values for crystalline silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of differential etching and masking resist preparation on the quality of the reactive-ion-etched aluminum and aluminum alloys

The conventional method used for aluminum (Al) and aluminum alloy (Al + Si, Al + Si + Cu) delineation in integrated circuits is mainly by wet chemical etching. Because of its isotropic characteristic, wet chemical etching becomes inadequate for patterning Al metal lines with linewidths narrower than about 4 Μm. In this work, Al and Al alloys (Al + 2% Si; Al + 1% Si + 1% Cu) were reactively etched in SiCl₄ plasma using patterned photoresist as the etch-mask. Resist patterns were generated either by conventional processing methods or by tri-level resist techniques which included hard-baking (200°C, ≤ 30 min), and two consecutive reactive ion etchings in CF₄ and 0₂ plasmas. Masking resists prepared by the tri-level resist technique retained their integrity during exposure to a SiCl₄ plasma, and significantly improved resolution and fidelity of pattern transfer from resist to underlying Al or Al alloy film. The substrate surface of the reactively etched Al + Si + Cu sample was considerably rougher than that of the Al or Al + Si sample due to the high concentration of Cu accumulated at the metal/substrate region during RIE process.