Etching of mesa structures in HgCdTe

Mesa structures were etched in HgCdTe using different Br₂/HBr/Ethylene glycol (EG) formulations. Etch rate and degree of anisotropy (A) were studied in detail for all of the combinations. Addition of EG to the conventional etchant gave A>0.5, with controllable etch rates. Optimum etchant composition was determined to be 2% Br₂ in a 3:1 mixture of EG:HBr. This composition resulted in a good anisotropy factor of ∼0.6 and a reasonably optimum etch rate of ∼2.5 μm/min, with rms surface roughness of ∼2 nm. Kinetics of the etching reaction have also been studied for the optimum etchant concentration and an etching mechanism has been proposed.     

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.     

A comparative study on a high aspect ratio contact hole etching in UFC- and PFC-containing plasmas

An etching of a SiO₂ contact hole with a diameter of 0.19 μm and an aspect ratio of 13, using C₄F₆/Ar/O₂/CH₂F₂ and c-C₄F₈/Ar/O₂/CH₂F₂ plasmas, was performed for a feasibility test of the use of unsaturated fluorocarbons (UFCs) as an alternative to perfluorocarbon (PFC) gases for a high aspect ratio contact hole etching. The etch profile of the contact hole obtained in the C₄F₆/Ar/O₂/CH₂F₂ plasma was shown to have 23% lower degree of bowing than that in the c-C₄F₈/Ar/O₂/CH₂F₂ plasma. The Kelvin and chain contact resistances of the contact holes etched in the C₄F₆/Ar/O₂/CH₂F₂ plasma were 10-12% higher than those in the c-C₄F₈/Ar/O₂/CH₂F₂ plasma, but they were within the device spec. The integration of device with 0.1 μm design rule using C₄F₆/Ar/O₂/CH₂F₂ and c-C₄F₈/Ar/O₂/CH₂F₂ plasmas during the contact hole etching was also conducted, and it was found that etch profiles, metal coverage, and bottom critical dimensions of the contact in the C₄F₆/Ar/O₂/CH₂F₂ plasma were nearly identical to those in the c-C₄F₈/Ar/O₂/CH₂F₂ plasma, suggesting that the use of C₄F₆ gas as an etchant gas for a high aspect ratio contact hole etching can be a good alternative to PFC gases.     

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.     

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.     

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.     

Thermally assisted ECR etching of CdTe in CCl2F2/Ar discharge under different gas flow ratio

II–VI compounds have attracted increasing attention, primarily because of the large range of energy band gaps available. ECR plasma etching of CdTe in a CCl₂F₂/Ar discharge with rf biasing were investigated at different temperature and different flow rate ratio. The etch rate increases with the increase in flow rate of reactive gas and temperature. The use of ECR conditions with additional rf biasing provides the good etching of the surface and fast etch rates. The etch depths were measured by Dektek profilometry and the surface morphology with scanning electron microscopy. This paper reports the thermal effect on the etch process of CdTe and the effect of various gas flow rates and ratio between CCl₂F₂ and Ar.     

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 UV light irradiation on SiC dry etch rates

Inductively Coupled Plasma etching of 4H-SiC under ultraviolet illumination was examined for SF₆/Ar and Cl₂/Ar chemistries. Etch rate enhancements up to a factor of 8 were observed with UV light irradiation during Cl₂/Ar etching. The enhancement mechanism is related to photodesorption of SiCl_x and CCl_x species. Surface morphologies were unchanged as a result of the UV enhancement with Cl₂/Ar discharges. By contrast, there was no effect of UV irradiation on the SiC etch rates in SF₆/Ar plasmas, but the surfaces were typically smoother than those obtained without the ultraviolet illumination. In the SF₆/Ar chemistry the rate-limiting steps are either Si-C bond-breaking or supply of fluorine radicals to the surface, and not desorption of the SiF_x and CF_x etch products.     

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.     

Dry etching of InGaAlP alloys in Cl2/Ar high ion density plasmas

Etch rates above 1 μm min⁻¹ are achieved for InGaP and AlInP under electron cyclotron resonance conditions in low pressure (1.5 mTorr) Cl₂/Ar discharges. Much lower rates were obtained for AlGaP due to the greater difficulty of the bond breaking that must precede formation and desorption of the etch products. The etched surface morphology and stoichiometry are strong functions of the plasma composition (i.e., the ion/neutral flux ratio). Under optimal conditions, there are no detectable chlorine related residues, in sharp contrast to reactive ion etching with this plasma chemistry.     

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.     

Plasma Passivation Etching for HgCdTe

Inductively coupled plasmas (ICP) are the high-density plasmas of choice for the processing of HgCdTe and related compounds. Most dry plasma process works have been performed on HgCdTe for pixel delineation and the p-to-n-type conversion of HgCdTe. We would like to use the advantages of “dry” plasma processing to perform passivation etching of HgCdTe. Plasma processing promises the ability to create small vias, 2 μm or less with excellent uniformity across a wafer, good run-to-run uniformity, and good etch rate control. In this study we developed processes to controllably etch CdTe, the most common passivation material used for photovoltaic-based HgCdTe devices. We created a process based on xenon gas that allows for the slow controllable CdTe etch at only 0.035 μm/min, with smooth morphology and rounded corners to promote further processing.     

Dry etching characteristics of In1-x-y Ga x Al y as alloys in CCl2F2:Ar and CH4:H2:Ar discharges

The etching characteristics of InGaAlAs alloys lattice-matched to InP were investigated using low pressure (1 mTorr) electron cyclotron resonance CH₄:H₂:Ar or CCl₂F₂:Ar discharges with additional radiofrequency biasing of the samples. Using CCl₂F₂:Ar discharges with ≥250V negative bias it is possible to obtain equi-rate etching of the material for all compositions between In_0.53Ga_0.47As and In_0.52Al_0.48As. At lower bias values, formation of A1F₃ on the surface leads to an inhibition of the etch rates. By making use of the differential etch rates of InGaAlAs layers of different compositions in CH₄:H₂:Ar mixtures, it is possible to choose dc bias values that allow one to stop the etching at a pre-selected depth in a multi-layer structure. For example, for -150 V bias, one can etch through In_0.53Ga_0.47As, In_0.53Ga_0.40Al_0.07As and Ino.53Ga_0.30Al_0.17As layers, and stop at an underlying layer with composition In_0.53Ga_0.20Al_0.27As.     

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.     

Deep dry etching of borosilicate glass using SF6 and SF6/Ar inductively coupled plasmas

Deep reactive ion etching (DRIE) of borosilicate glass was carried out using SF₆ and SF₆/Ar plasmas in an inductively coupled plasma (ICP) reactor. Electroplated Ni on Cu (≅50 nm)/Cr (≅100 nm)/glass structure using patterned SU-8 photoresist mask with a line spacing of 12-15 μm was used as a hard-mask for plasma etching. Plasma etching of borosilicate glass was performed by varying the various process parameters such as the gas chemistry, the gas flow ratio, the top electrode power, and the dc self-bias voltage (V_dc). In the case of using SF₆ gas only, the profiles of the etched channel showed the undercut below the Ni hard-mask due to a chemical etching and the microtrenching at the bottom of the etched channel. An optimized process using the SF₆ plasmas showed the glass etch rate of ≅750 nm/min. The addition of the Ar gas to the SF₆ gas removed the undercut and microtrenching but decreased the etch rate to ≅540 nm/min. The increasing and decreasing time-dependent etch rates with the etch depth in the SF₆ (200 sccm) and SF₆(60%)/Ar(40%) plasmas, respectively, were ascribed to the different ion-to-neutral flux ratios leading to the different etch process regime.     

Back channel etch chemistry of advanced a-Si:H TFTs

We report on the effects of back channel etch depth and etchant chemistry on the electrical characteristics of inverted staggered advanced amorphous silicon thin-film transistors. We found that the optimum amorphous silicon film thickness in the channel is about 800-1100 Å. Three dry etch, HBr + Cl₂, C₂F₆, and CCl₂F₂ + O₂, and one wet etch, KOH, chemistries are used for the back channel etch processing. We established that dry etch can be used for the back channel etch of amorphous silicon transistor without degrading its electrical characteristics.     

Use of electron cyclotron resonance plasmas to prepare CdZnTe (211)B substrates for HgCdTe molecular beam epitaxy

Without any additional preparation, Cd_1−yZn_yTe (211)B (y∼3.5%) wafers were cleaned by exposure to an electron cyclotron resonance (ECR) Ar/H₂ plasma and used as substrates for HgCdTe molecular beam epitaxy. Auger electron spectra were taken from as-received wafers, conventionally prepared wafers (bromine: methanol etching, followed by heating to 330–340°C), and wafers prepared under a variety of ECR process conditions. Surfaces of as-received wafers contained ∼1.5 monolayers of contaminants (oxygen, carbon, and chlorine). Conventionally prepared wafers had ∼1/4 monolayer of carbon contamination, as well as excess tellurium and/or excess zinc depending on the heating process used. Auger spectra from plasma-treated CdZnTe wafers showed surfaces free from contamination, with the expected stoichiometry. Stoichiometry and surface cleanliness were insensitive to the duration of plasma exposure (2–20 s) and to changes in radio frequency input power (20–100 W). Reflection high energy electron diffraction patterns were streaked indicating microscopically smooth and ordered surfaces. The smoothness of plasma-etched CdZnTe wafers was further confirmed ex situ using interferometric microscopy. Surface roughness values of ∼0.4 nm were measured. Characteristics of HgCdTe epilayers deposited on wafers prepared with plasma and conventional etching were found to be comparable. For these epilayers, etch pit densities on the order of 10⁵ cm⁻² have been achieved. ECR Ar/H₂ plasma cleaning is now utilized at Night Vision and Electronic Sensors Directorate as the baseline CdZnTe surface preparation technique.     

Comparison of etching characteristics of SiO2 with ArF photoresist in C4F6 and C4F8 based dual-frequency superimposed capacitively coupled plasmas

In this study, we compared the C₄F₆ and C₄F₈ based plasma etching characteristics of silicon dioxide and ArF photoresist (PR) in a dual-frequency superimposed capacitively coupled plasma (DFS-CCP) etcher under different high- and low-frequency combinations (f_HF/f_LF), while varying the process parameters such as the dc self-bias voltage (V_dc), O₂ flow, and CH₂F₂ flow rate in the C₄F₈/CH₂F₂/O₂/Ar and C₄F₆/CH₂F₂/O₂/Ar plasmas. The silicon oxide etch rates increased significantly in both chemistries with increasing f_HF and O₂ flow rate. The silicon oxide etch rates were higher in the C₄F₈/CH₂F₂/O₂/Ar than in the C₄F₆/CH₂F₂/O₂/Ar plasmas, but the PR etch rate was much higher in the C₄F₆/CH₂F₂/O₂/Ar than in the C₄F₈/CH₂F₂/O₂/Ar plasmas under the present experimental conditions. The slower oxide etch rate in the C₄F₆ based plasmas was attributed to the thicker steady-state fluorocarbon layer on the silicon oxide surface, while the faster PR etch rate in the C₄F₈ based plasmas was ascribed to the higher F radical density in the plasma.     

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.