Electron Cyclotron Resonance Conditions Research Articles

Comparison of dry etch chemistries for SiC

SiC has generally been plasma etched in polymer-forming chemistries such as CHF3/O2 or CF4/O2, often with addition of H2 to achieve acceptable surface morphologies. We find that under high ion density conditions gases such as SF6, NF3, IBr, and Cl2 produce smooth surfaces that are free of hydrogen passivation effects. Etch rates in excess of 1500 Å/min are achieved in electron cyclotron resonance (ECR) NF3 or Cl2/Ar discharges with low additional rf chuck powers (100–150 W); dc bias of −120 to −170 V. The rates are somewhat lower (factors of 2–4) with IBr and SF6 chemistries. Ion-induced damage is evident from Hall measurements for SiC exposed to rf powers >150 W (dc bias >−170 V) under ECR conditions and >250 W (dc bias >−275) under reactive ion etch conditions. Efforts to anneal damage at these higher powers reveals a major annealing stage is evident at ∼700 °C, with an activation energy of ∼3.4 eV, but there is significant damage remaining even after 1050 °C annealing. Hydrogen passivation is a problem only in p-type SiC, and is removed at similar temperatures to ion-induced damage by annealing at ∼700 °C under N2 ambients. This is strongly correlated with secondary ion mass spectrometry measurements on deuterated samples annealed at different temperatures.

Patterning of Cu, Co, Fe, and Ag for magnetic nanostructures

Wet and dry etching of thin metallic multilayer structures is necessary for the development of sensitive magnetic field sensors and memory devices based on spin–valve giant magnetoresistance elements. While it is well established that Cu, Co, and Fe are soluble in HNO3 and H3PO4 at room temperature, little effort has been made to investigate selective wet and dry etch chemistries. For example, we find Ag is not etched in H2SO4, HCl, or H3PO4 under conditions where etch rates for the other metals are in the range of 2000–60 000 Å/min. Electron cyclotron resonance (ECR) SF6/Ar plasmas provide etch selectivities of ⩾5:1 for Ag over Cu, Co, and Fe, while lower selectivities are obtained with CH4/H2/Ar. Cl2-based plasma chemistries leave significant metal–chlorine surface residues, which can be removed in situ by low ion energy H2 or Ar plasma treatments that eliminate corrosion problems. Cu etch rates in excess of 3000 Å/min at 25 °C can be obtained in ECR Cl2/Ar discharges because the high ion flux prevents formation of a CuClX-rich selvedge layer, which normally only is volatile for etch temperatures ⩾220 °C in conventional reactive ion etch systems. Photoresist masks suffer severe reticulation under ECR conditions, at least for microwave powers >400 W, and SiO2 or SiNX thin films offer much better etch resistance.

Damage investigation in AlGaAs and InGaP exposed to high ion density Ar and SF6 plasmas

Both electrical and optical properties of AlGaAs and InGaP were changed by exposure to high density Ar and SF6 plasmas. Capacitance–voltage measurements, photoluminescence, and sheet resistance measurements were used to characterize the degradation of the materials properties as a function of microwave power, rf power, exposure time, and chamber pressure. The results showed the n-type InGaP suffered more damage than the p-type InGaP in Ar plasmas, while the reverse was true for AlGaAs. Annealing of the damaged semiconductors needed high temperature (for example, ∼750 °C for InGaP), which is not suitable for typical III–V semiconductor processing because the high temperature may cause thermal degradation. Electron cyclotron resonance (ECR) SF6 plasmas produce more surface states than reactive ion etching (RIE) (i.e., rf power only). BothC–V measurements and photoluminescence data showed rf power (>50 W) and pressure (<2 mTorr) played important roles in introducing electrical and optical degradation of both AlGaAs and InGaP under ECR conditions, while there was little change as a function of rf power in the RIE case. The minimal overetching time of SF6 plasmas is necessary to avoid severe damage in the materials during opening the windows for the active semiconductors from photoresist, dielectric, or metal layers.

Oblique electron cyclotron emission for electron distribution studies (invited)

Electron cyclotron emission (ECE) at an oblique angle to the magnetic field provides a means of probing the electron distribution function both in energy and physical space through changes in and constraints on the relativistic electron cyclotron resonance condition. Diagnostics based on this Doppler shifted resonance are able to study a variety of electron distributions through changes in the location of the resonance in physical or energy space accomplished by changes in the viewing angle and frequency, and the magnetic field. For the case of observation across a changing magnetic field, such as across the tokamak midplane, the constraint on the resonance condition for real solutions to the dispersion relation can constrain the physical location of optically thin emission. A new Oblique ECE diagnostic was installed and operated on the PBX-M tokamak for the study of energetic electrons during lower hybrid current drive. It has a view 33° with respect to perpendicular in the tokamak midplane, receives second harmonic X-mode emission, and is constrained to receive single pass emission by SiC viewing dumps on the tokamak walls. Spatial localization of optically thin emission from superthermal electrons (50–100 keV) was obtained by observation of emission upshifted from a thermal cyclotron harmonic. The localized measurements of the electron energy distribution and the superthermal density profile made by this diagnostic demonstrate its potential to study the spatial transport of energetic electrons on fast magnetohydrodynamic time scales or anomalous diffusion time scales. Oblique ECE can also be used to study electron distributions that may have a slight deviation from a Maxwellian by localizing the emission in energy space. In “near-Maxwellian” plasmas, the Doppler broadening of the resonance means that observed oblique ECE from optically thick harmonics will originate from electrons with energies near or slightly above the thermal energy [(1–2)vth for example, depending on the observation angle and spatial gradients]. This is different from observed perpendicular ECE from optically thick harmonics that originate from electrons below the thermal energy. Experiments are presently being performed on the Tore Supra tokamak with two oblique views of ECE in equal and opposite toroidal directions (approximately 25° with respect to perpendicular) using two Michelson interferometers capable of observing second and third harmonic emission in X mode.

Comparison of ICl and IBr Plasma Chemistries for Etching of InGaAlP Alloys

ICl and IBr provide rapid etching of the ternary alloys InGaP, AlInP, and AlGaP under electron cyclotron resonance conditions. The rates are almost independent of microwave power in the range 400 to 1000 W, with typical values of ∼1.5 μm/min with ICl/Ar and ∼0.4 μm/min with IBr/Ar. At low microwave powers (⩽750 W), the etched surface morphologies are quite smooth and there is little degradation of photoresist masks. High Al content (x ⩾ 0.7) alloys appear to be good candidates as etchstop layers in InGaAlP device structures when using these plasma chemistries.

Selective Dry Etching of III‐V Nitrides in Cl2 / Ar , CH 4 / H 2 / Ar , ICl/Ar, and IBr/Ar

The selectivity for etching the binary (GaN, AlN, and InN) and ternary nitrides (InGaN and InAlN) relative to each other in , ICl/Ar, or IBr/Ar electron cyclotron resonance (ECR) plasmas, and or reactive ion (RIE) plasmas was investigated. Cl‐based etches appear to be the best choice for maximizing the selectivity of GaN over the other nitrides. GaN/AlN and GaN/InGaN etch rate ratios of ∼10 were achieved at low RF power in under ECR and RIE conditions, respectively. GaN/InN selectivity of 10 was found in ICl under ECR conditions. A relatively high selectivity (>6) of InN/GaN was achieved in under ECR conditions at low RF powers (50 W). Since the high bond strengths of the nitrides require either high ion energies or densities to achieve practical etch rates it is difficult to achieve high selectivities.

Cl2/Ar and CH4/H2/Ar dry etching of III–V nitrides

Electron-cyclotron-resonance (ECR) and reactive ion etching (RIE) rates for GaN, AlN, InN, and InGaN were measured using the same reactor and plasma parameters in Cl2/Ar or CH4/H2/Ar plasmas. The etch rates of all four materials were found to be significantly faster for ECR relative to RIE conditions in both chemistries, indicating that a high ion density is an important factor in the etch. The ion density under ECR conditions is ∼3×1011 cm−3 as measured by microwave interferometry, compared to ∼2×109 cm−3 for RIE conditions, and optical emission intensities are at least an order of magnitude higher in the ECR discharges. It appears that the nitride etch rates are largely determined by the initial bond breaking that must precede etch product formation, since the etch products are as volatile as those of conventional III–V materials such as GaAs, but the etch rates are typically a factor of about 5 lower for the nitrides. Cl2/Ar plasmas were found to etch GaN, InN, and InGaN faster than CH4/H2/Ar under ECR conditions, while AlN was etched slightly faster in CH4/H2/Ar plasmas. The surface morphology of InN was found to be the most sensitive to changes in plasma parameters and was a strong function of both rf power and etch chemistry for ECR etching.

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.

Dry etching of InGaP and AlInP in CH4/H2/Ar

Electron cyclotron resonance (ECR) plasma etching with additional rf-biasing produces etch rates ≥ 2,500 A/min for InGaP and AlInP in CH4/H2/Ar. These rates are an order of magnitude or much higher than for reactive ion etching conditions (RIE) carried out in the same reactor. N2 addition to CH4/H2/Ar can enhance the InGaP etch rates at low flow rates, while at higher concentrations it provides an etch-stop reaction. The InGaP and AlInP etched under ECR conditions have somewhat rougher morphologies and different stoichiometries up to ≈200 A from the sur face relative to the RIE samples.

Comparison of and plasma chemistries for dry etching of InGaAlP alloys

Etch rates above have been obtained for and plasma etching of , and under electron cyclotron resonance (ECR) conditions. The etched surface morphologies are a strong function of the ratio of chlorine radicals to total ion density in the discharges, and are smoother in . Non-optimized etching conditions produce rough surfaces characterized by the presence of chlorine and boron-containing residues and substantial post-oxidation. We believe that the high ion flux available under ECR conditions provides efficient sputtering of the normally involatile , and thus a selvedge layer of this material is prevented from forming. Therefore the etch rates are much higher and the surface morphologies smoother in ECR discharges. The etch rates of are much lower than the other two ternary compounds and are independent of composition over the range x = 0.05 - 0.70 for fixed plasma conditions.

Wet and Dry Etching of LiGaO2 and LiAlO2

and have similar lattice constants to GaN, and may prove useful as substrates for III‐nitride epitaxy. We have found that these materials may be wet chemically etched in some acid solutions, including HF, at rates between 150 and 40,000 Å/min. Dry etching with plasmas provides faster rates than or under electron cyclotron resonance conditions, indicating the fluoride etch products are more volatile that their chloride or metallorganic/hydride counterparts.

Cl2/Ar plasma etching of binary, ternary, and quaternary In-based compound semiconductors

A simple Cl2/Ar plasma chemistry can provide smooth, high-rate etching of InP, InAs, InGaAs, A1InAs, and InGaAsP at room temperature under conditions in which there is a balance between formation and sputter desorption of the normally involatile InCl3. When the neutral/ion ratio is either too high or too low, surface roughening is apparent due either to the presence of InCl3, or to preferential loss of the group V element. The etching has been investigated as a function of microwave power (600–1000 W), rf power (0–300 W), process pressure (1.5–10 mTorr), and Cl2:Ar ratio under electron cyclotron resonance conditions. Use of N2 or H2, rather than Ar, as gas additives to the chlorine, did not produce smooth, stoichiometric etched surfaces.

Cl2‐Based Dry Etching of GaAs, AlGaaAs, and GaP

‐based plasmas for etching GaAs, AlGaAs, and GaP have been examined as a function of gas additive (Ar, or ), radio frequency (RF) and microwave power, plasma composition, mask material, and process pressure. In a load‐locked reactor, smooth etched surface morphologies were obtained over basically all conditions investigated, with typical root‐mean‐square roughness of ⩽ 1.5 nm measured by atomic force microscopy. The etch rates for all three materials increase with RF power (ion energy), microwave power (ion current), percentage, and pressure, with controlled rates of ∼0.4 μm/min at a condition of , 850 W microwave power, 1.5 mTorr, and 100 to 150 W of RF power. Operating under electron cyclotron resonance conditions where the ion density is (measured by microwave reflection interferometry) produces rapid degradation of photoresist, and more robust mask materials such as , or W are necessary.

Thermal stability of dry etch damage in SiC

The introduction of dry etch damage into n-type SiC has been measured by monitoring the sheet resistance after exposure to Ar plasmas under both reactive ion etching and electron cyclotron resonance conditions. The threshold rf powers for measurable resistance changes in 1 μm thick films are ∼250 W for reactive ion etching (RIE) conditions, and ∼150 W for electron cyclotron resonance (ECR) conditions. A major annealing stage occurs with an activation energy of ∼3.4 eV, but some damage remains even after 1050 °C annealing.

Comparison of masking materials for high microwave power CH4/H2/Ar etching of III–V semiconductors

Patterned photoresist, SiO2, SiNX, and W were compared as masking materials for GaAs and other III–V semiconductors during CH4/H2/Ar plasma etching. The use of electron cyclotron resonance conditions is a much more severe test of the mask integrity in comparison to reactive ion etching. Ar flow rate, microwave power and rf power have significant effects on mask erosion (or polymer deposition on the mask), while process pressure is found to have little effect over the range 1.5–10 mTorr. SiNX retains high geometrical integrity and displays little erosion under electron cyclotron resonance conditions, while SiO2 is quickly coated with a polymer that can degrade critical dimensions. Morphological changes in both W and photoresist films during exposure to electron cyclotron resonance CH4/H2/Ar discharges indicate that these are less desirable choices than SiNX.

High density plasma etching of III–V nitrides

Two broad classes of plasma chemistry were examined for dry etching of GaN, AlN, and InN. The etch rates for CH4/H2-based plasmas are low (∼ 400 Å/min) even under high microwave power (1000 W) electron cyclotron resonance conditions. Halogen-based plasmas (Cl2, I2, Br2) produce rates up to ∼3000 Å/min with smooth stoichiometric surfaces. Preferential sputtering of N occurs for high ion energies, leading to rough GaN surfaces at rf power above ∼200 W. Etch rates for high ion density discharges are typically an order of magnitude faster than for conventional reactive ion etching.

BCl3/N2 dry etching of InP, InAlP, and InGaP

Etch rates for InP, InAlP, and InGaP can be controlled between ∼1500 Å/min and ≳1 μm/min by varying the microwave power, rf power, or N2 composition in BCl3N2 discharges under electron cyclotron resonance conditions. The surface morphology of InGaP is poor at low microwave power (250 W), but becomes very smooth at high powers (1000 W). InP has exactly the opposite dependence on microwave power, while InAlP shows little change in morphology over a wide range of powers. The high ion current under electron cyclotron resonance conditions enables etching of In-based semiconductors without the need for high sample temperatures to enhance desorption of InClX species.

Investigation of masking materials for high-ion-density plasma etching of GaAs

One of the most important steps in achieving good pattern transfer using dry etching of semiconductors is to find the best masking materials for the process. In particular, for high-ion-density plasma etching the high ion currents incident on the sample can provide a severe test of mask stability. For electron cyclotron resonance (ECR) etching of GaAs with plasmas, both microwave power and rf power are found to have significant effects on mask degradation while the Cl to Ar ratio and process pressure have a relatively constant effect over the given regions investigated (total flow rate for and Ar is 15 sccm, 1.5 - 10 mTorr). Of the potential masking materials of photoresist, , and W, is found to be the most stable under ECR conditions. The surface morphology of the photoresist and was significantly changed during ECR etching while sputtering of W creates micromasking on the GaAs.

First results of a high-current electron cyclotron resonance proton source

A 2.45 GHz electron cyclotron resonance (ECR) ion source for producing a 100 mA H+ beam is under study. The microwave power is coupled through a quartz window and a water-cooled 90° waveguide bend in order to avoid the heating of the electrons backstreaming from the source. The ECR condition is obtained with dc magnetic coils. Preliminary results reported here show that a beam current density of 75 mA/cm2 can be extracted at 10 kV from a 3-mm-diam aperture, with 1.2 kW of microwave power at 1×10−3 mbar of hydrogen pressure. The proton fraction is 75% with an alumina-coated plasma electrode and 85% with a nitride coating.

Etching of InP at ≳1 μm/min in Cl2/Ar plasma chemistries

A simple Cl2/Ar plasma chemistry without additional sample heating is found to produce etch rates above 1 μm/min for InP under high microwave power (1000 W) electron cyclotron resonance conditions. While the etch rate increases essentially linearly with Cl2 composition the root-mean-square (RMS) surface roughness measured by atomic force microscopy is strongly dependent on the Cl2-to-Ar ratio. Under optimized conditions (10Cl2/5Ar), RMS roughness of ∼2.6 nm is obtained on samples etched more than 1 μm, a typical unetched control sample displays a RMS value of 1.3–1.6 nm. The high ion current under ECR conditions appears to promote efficient sputter desorption of the InCl3 etch product and prevents buildup of the usual selvedge layer that requires elevated sample temperatures to desorb under more conventional reactive ion etching conditions. The result is a simplified plasma chemistry with avoidance of the polymer deposition and hydrogen passivation associated with CH4/H2 discharges.