GaN etch rate and surface roughness evolution in Cl2/Ar based inductively coupled plasma etching

Cl₂/Ar based inductively coupled plasma (ICP) etching of GaN is investigated using photoresist mask in a consequential restricted domain of pressure < 1.2 Pa and radio frequency (RF) sample power < 100 W, for selective mesa etching. The etch characteristics and root-mean-square (rms) surface roughness are studied as a function of process parameters viz. process pressure, Cl₂ percentage in total flow rate ratio, and RF sample power at a constant ICP power, to achieve moderate GaN etch rate with anisotropic profiles and smooth surface morphology. The etch rate and resultant surface roughness of etched surface increased with pressure mainly due to dominant reactant limited etch regime. The etch rate and surface roughness show strong dependence on RF sample power with the former increasing and the later decreasing with the applied RF sample power up to 80 W. The process etch yield variation with applied RF sample power is also reported. The studied etch parameters result in highly anisotropic mesa structures with Ga rich etched surface.     

Comparison of dry etching of PMMA and polycarbonate in diffusion pump-based O2 capacitively coupled plasma and inductively coupled plasma

We report a comparison of dry etching of polymethyl methacrylate (PMMA) and polycarbonate (PC) in O₂ capacitively coupled plasma (CCP) and inductively coupled plasma (ICP). A diffusion pump was used as high vacuum pump in both cases. Experimental variables were process pressure (30-180 mTorr), CCP power (25-150 W) and ICP power (0-350 W). Gas flow rate was fixed at 5 sccm. An optimized process pressure range of 40-60 mTorr was found for the maximum etch rate of PMMA and PC in both CCP and ICP etch modes. ICP etching produced the highest etch rate of 0.9 μm/min for PMMA at 40 mTorr, 100 W CCP and 300 W ICP power, while 100 W CCP only plasma produced 0.46 μm/min for PMMA at the same condition. For polycarbonate, the highest etch rates were 0.45 and 0.27 μm/min, respectively. RMS surface roughnesses of PMMA and PC were about 2-3 nm after etching. Etch selectivity of PMMA over photoresist was 1-2 and that of PC was less than 1. When ICP power increased from 0 to 350 W, etch rates of PMMA and PC increased linearly from 0.47 to 1.18 μm/min and from 0.18 to 0.6 μm/min, while the negative self bias slightly reduced from 364 to 352 V. Increase of CCP power raised both self bias and PMMA etch rate. PMMA etch rates were about 3 times higher than those of PC at the same CCP conditions. SEM data showed that there was some undercutting of PMMA and PC after etching at 300 W ICP, 100 W CCP and 40 mTorr. The results also showed that the etched surface of PMMA was rough and that of PC was relatively smooth.     

Self-consistent global model for inductively coupled Cl2 plasma: Comparison with experimental data and application for the etch process analysis

The investigations of the influence of gas pressure and input power on the Cl₂ plasma parameters in the inductively coupled plasma system were carried out. The investigations combined plasma diagnostics by Langmuir probe and plasma modeling using the self-consistent global model with Maxwellian approximation for electron energy distribution function. From the experiments, it was found that an increase of gas pressure in the range of 0.27-3.33 Pa at 400-700 W input power results in decreasing both electron temperature (3.3-2.0 eV) and density (6.6 × 10¹⁰ − 3.0 × 10¹⁰ cm^− 3 for 400 W and 1.2 × 10¹¹ − 6.4 × 10¹⁰ cm^− 3 for 700 W). The model showed an outstanding agreement with the experiments and provided the data on densities and fluxes of active species. These data combined with the model of etch kinetics demonstrated the possibility of different etch rate behaviors depending on the input process parameters as well as on the properties of the etched surface.     

Etch characteristics of silver by inductively coupled fluorine-based plasmas

In this study, thin films of Ag deposited onto glass substrates were etched using inductively coupled fluorine-based plasmas. The effects of various process conditions on the Ag etch characteristics were evaluated to ascertain whether it would be possible to etch patterned Ag films with high etch rates and smooth sidewalls free of involatile etch products. It was found that involatile etch products remained on the substrate when films were etched in CF₄-based gas mixtures possessing either O₂ or N₂ as an additive. However, when Ar was added to either NF₃ or CF₄, a residue-free etch was obtained provided the partial pressure of Ar was no less than 50%. It is proposed that the residue-free Ag etch mechanism involves the formation of silver fluoride, which is physically sputtered by Ar⁺ ions. A Ag etch rate of 160 nm/min with a Ag to photoresist etch selectivity exceeding 1.1 was achieved with an inductive power of 1500 W, a d.c. bias voltage of −180 V and a chamber pressure of 0.8 Pa with 50-50 CF₄/Ar partial pressures obtained with 60 sccm CF₄/60 sccm Ar flows. In addition, these conditions produced smooth Ag sidewall etch profiles.     

HfO2 etching mechanism in inductively-coupled Cl2/Ar plasma

Etching characteristics and the mechanism of HfO₂ thin films in Cl₂/Ar inductively-coupled plasma were investigated. The etch rate of HfO₂ was measured as a function of the Cl₂/Ar mixing ratio in the range of 0 to 100% Ar at a fixed gas pressure (6 mTorr), input power (700 W), and bias power (300 W). We found that an increase in the Ar mixing ratio resulted in a monotonic decrease in the HfO₂ etch rate in the range of 10.3 to 0.7 nm/min while the etch rate of the photoresist increased from 152.1 to 375.0 nm/min for 0 to 100% Ar. To examine the etching mechanism of HfO₂ films, we combined plasma diagnostics using Langmuir probes and quadrupole mass spectrometry with global (zero-dimensional) plasma modeling. We found that the HfO₂ etching process was not controlled by ion-surface interaction kinetics and formally corresponds to the reaction rate-limited etch regime.     

Dry etching process of GaAs in capacitively coupled BCl3-based plasmas

Dry etching of GaAs was investigated in BCl₃, BCl₃/N₂ and BCl₃/Ar discharges with a mechanical pump-based capacitively coupled plasma system. Etched GaAs samples were characterized using scanning electron microscopy and surface profilometry. Optical emission spectroscopy was used to monitor the BCl₃-based plasma during etching. Pure BCl₃ plasma was found to be suitable for GaAs etching at > 100 mTorr while producing a clean and smooth surface and vertical sidewall. Adding N₂ or Ar to the BCl₃ helped increase the etch rates of GaAs. For example, the GaAs etch rate was doubled with 20% N₂ composition in the BCl₃/N₂ plasma compared to the pure BCl₃ discharge at 150 W CCP power and 150 mTorr chamber pressure. The GaAs etch rate was ∼ 0.21 µm/min in the 20 sccm BCl₃ plasma. The BCl₃/Ar plasma also increased etch rates of GaAs with 20% of Ar in the discharge. However, the surface morphology of GaAs was strongly roughened with high percentage (> 30%) of N₂ and Ar in the BCl₃/N₂ and BCl₃/Ar plasma, respectively. Optical emission spectra showed that there was a broad BCl₃-related molecular peak at 450-700 nm wavelength in the pure BCl₃ plasma. When more than 50% N₂ was added to the BCl₃ plasma, an atomic N peak (367.05 nm) and molecular N₂ peaks (550-800 nm) were detected. Etch selectivity of GaAs to photoresist decreased with the increase of % N₂ and Ar in the BCl₃-based plasma.     

Effect of additive gases on the selective etching of ZrOx film using inductively coupled BCl3-based plasmas

In this study, the effect of BCl₃/C₄F₈ gas mixture on the ZrO_x etch rates and the etch selectivities of ZrO_x/Si were investigated and its etch mechanism was studied. The increase of C₄F₈ in BCl₃/C₄F₈ decreased the silicon etch rate significantly and finally deposition instead of etching occurred by mixing C₄F₈ more than 3%. In the case of ZrO_x, the etch rate remained similar until 4% of C₄F₈ was mixed, however, the further increase of C₄F₈ percentage finally decreased the ZrO_x etch rate and deposition instead of etching occurred by mixing more than 6%. Therefore, by mixing 3-4% of C₄F₈ to BCl₃, infinite etch selectivity of ZrO_x/Si could be obtained while maintaining the similar ZrO_x etch rate. The differences in the etch behaviors of ZrO_x and Si were related to the different thickness of C-F polymer formed on the surfaces. The thickness of the C-F polymer on the ZrO_x surface was smaller due to the removal of carbon incident on the surface by forming CO_x with oxygen in ZrO_x. Using 12 mTorr BCl₃/C₄F₈ (4%), 700 W of rf power, and − 80 V of dc bias voltage, the ZrO_x etch rate of about 535 Å/min could be obtained with infinite etch selectivity to Si.     

The etching characteristics of Al2O3 thin films in an inductively coupled plasma

In this study, the etching characteristics of ALD deposited Al₂O₃ thin film in a BCl₃/N₂ plasma were investigated. The experiments were performed by comparing the etch rates and the selectivity of Al₂O₃ over SiO₂ as functions of the input plasma parameters, such as the gas mixing ratio, the DC-bias voltage, the RF power, and the process pressure. The maximum etch rate was obtained at 155.8 nm/min under a 15 mTorr process pressure, 700 W of RF power, and a BCl₃ (6 sccm)/N₂ (14 sccm) plasma. The highest etch selectivity was 1.9. We used X-ray photoelectron spectroscopy (XPS) to investigate the chemical reactions on the etched surface. Auger electron spectroscopy (AES) was used for the elemental analysis of the etched surfaces.     

Thermal stability of amorphous molybdenum trioxide films prepared at different oxygen partial pressures by reactive DC magnetron sputtering

Thin films of MoO₃ were prepared on quartz and Si (1 0 0) substrates by reactive dc magnetron sputtering of a Mo target in an oxygen and argon atmosphere. The structural and optical changes induced in the films due to post-growth annealing have been systematically studied by Rutherford backscattering (RBS), X-ray diffraction (XRD), X-ray reflectivity (XRR) and by optical methods. RBS studies reveal no change in composition of the films upon annealing at high temperatures. Grazing angle XRD studies show that the as-deposited films are amorphous and crystallize to β-MoO₃ phase with small contribution of α-MoO₃ upon annealing at 300 °C. The film prepared at 0.40 Pa transforms to α-MoO₃ upon annealing at 650 °C, while the film deposited at 0.19 Pa still has some β-MoO₃ phase contribution. XRR measurements reveal that the film thickness decreases upon annealing with simultaneous increase of film density. The surface roughness of the films strongly increases after crystallization. The contraction of the film deposited at 0.40 Pa is much greater than the contraction of the film prepared at 0.19 Pa. The mass variation of the film deposited at 0.19 Pa and that deposited at 0.40 Pa are completely different. The optical properties of MoO₃ films deposited at 0.19 and 0.40 Pa are changed strongly by annealing.     

Crystallinity and photocatalytic activity of TiO2 films deposited by reactive sputtering with radio frequency substrate bias

TiO₂ films with thicknesses of 400-460 nm were deposited on the unheated non-alkali glass by radio frequency (rf) reactive magnetron sputtering using a Ti metal target. Depositions were carried out using a 3-in. 1000 G magnetron cathode with various rf substrate bias voltages (V_sb, dc component of self bias) of 10-80 V under total gas pressure of 1.0 or 3.0 Pa. The oxygen flow ratio [O₂/(O₂ + Ar)] and rf sputtering power were kept constant at 60% and 200 W, respectively. Photocatalytic activity on photoinduced oxidative decomposition of acetaldehyde (CH₃CHO) of the TiO₂ films showed a clear tendency to decrease with the increase in the V_sb during the deposition. Most of the films consisted of the mixture of anatase and rutile polycrystalline portions. It was confirmed that the rutile phase content increased and anatase phase content decreased markedly with increasing V_sb, where the crystallinity of anatase phase was much higher than that of rutile phase.     

The selective growth of rutile thin films using radio-frequency magnetron sputtering

Using unbalanced radio-frequency (RF) magnetron sputtering crystalline rutile films were synthesised on glass substrates at (combined Ar and O₂) pressures of 0.4 Pa or less, at RF powers of 500 and 600 W with substrate to magnetron distances of 40 mm or longer. Anatase films were deposited at the greater pressure of 1.2 Pa (substrate to magnetron distance of 40 mm) or shorter substrate to magnetron distance of 20 mm (at 0.4 Pa). A mixture of anatase and rutile was formed at 0.5 Pa with all other conditions being as for those required for rutile or the power was reduced along with the substrate to magnetron distance (500 W and 20 mm). The crystallite sizes of rutile obtained were 1 - 3 nm. It is proposed that the greater the energy imparted to the substrate surface by the impinging positive species the greater the activation energy to crystalline phase formation that can be overcome. Hence the formation of rutile over anatase is favoured at greater power, longer magnetron to substrate distances and decreased pressure. Moreover, not only is it possible to control the phase of TiO₂ formed it appears to be possible to control the degree of oxygen non-stoichiometry in the rutile films formed. Smaller O₂ partial pressures, shorter substrate to magnetron distances and greater RF power are believed to produce an environment of reduced reaction of sputtered Ti species with O₂ and to result in the formation of non-stoichiometric rutile structures resulting in increased band gap energies and decreased refractive indices.     

On the use of a neural network to characterize the plasma etching of SiON thin films

Using a generalized regression neural network (GRNN), plasma etching of oxynitride thin films was modeled. The etch process was characterized by means of a statistical experiment. A genetic algorithm was employed to improve prediction performance by optimizing multiparameterized training factors. Compared to a conventional GRNN model, the constructed etch rate model demonstrated an improvement of about 60% in the prediction performance. 3-D plots were generated to qualitatively interpret etch mechanisms while validating the predictions with experimental data. In separating physical and chemical effects, both dc bias and profile angle variations were effectively utilized. The source power affected significantly the etch rate irrespective of changes in the bias power or C₂F₆ flow rate. For pressure variations, the etch rate was estimated to be dominated by chemical etching. The complex effect of C₂F₆ flow rate could be explained by dominant chemical etching or polymer deposition.     

Optimization of sputtering parameters for deposition of Al-doped ZnO films by rf magnetron sputtering in Ar + H2 ambient at room temperature

The effects of sputtering pressure and power on structural and optical-electrical properties of Al-doped ZnO films were systemically investigated at substrate temperature of room temperature and H₂/(Ar + H₂) flow ratio of 5%. The results show that carrier concentration and mobility of the films show nonmonotone change due to the evolution of microstructure and lattice defect of the films caused by introduction of H₂ with increasing sputtering pressure and power. The transmittance of the films is also found to be related to the introduction of H₂ in addition to usually considered surface roughness and crystallinity. Finally, optimized sputtering pressure and power are 0.8 Pa and 100 W, respectively, and obtained minimum resistivity and highest transmittance are 1.43 × 10^− 3 Ω·cm and 90.5%, respectively. In addition, it is found that E_g of the films is mainly controlled by the carrier concentration, but crystallite size and stress should also be considered for the films deposited at different powers.     

Etching characteristics and mechanism of Ga-doped ZnO thin films in inductively-coupled HBr/X (X = Ar, He, N2, O2) plasmas

We investigated the etch characteristics and mechanisms of Ga-doped ZnO (Ga-ZnO) thin films in HBr/X (X = Ar, He, N₂, O₂) inductively-coupled plasmas. The etch rates of Ga-ZnO thin films were measured as a function of the additive gas fraction in the range of 0-100% for Ar, He, N₂, and O₂ at a fixed gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (40 sccm). The plasma chemistry was analyzed using a combination of the global (zero-dimensional) plasma model and Langmuir probe diagnostics. By comparing the behavior of the etch rate and fluxes of plasma active species, we found that the Ga-ZnO etch process was not limited by ion-surface interaction kinetics and appeared in the reaction rate-limited etch regime. In the HBr/O₂ plasma, the etch kinetics were probably influenced by oxidation of the etched surface.     

Surface morphology and growth mechanisms for sputtered amorphous silicon nitride thin films

Evolution of surface of sputter-deposited amorphous Si₃N₄ films growth on Si (100) substrates was investigated using atomic force microscopy (AFM). The scaling behaviors of the AFM topographical profiles were analyzed using the one-dimensional power spectral density. The results of root-mean-square surface height variation showed that there is a power law relationship between the surface roughness and deposition time. It is interesting to note that the growth exponent can be divided into one region and two regions, respectively, when Si₃N₄ films are deposited at different working pressures. A very low growth exponent of β = 0.07 ± 0.01 was found when Si₃N₄ films were deposited at a working pressure of 1.6 × 10^− 1 Pa. However, the growth exponent β can be divided into two regions, which is β₁ = 0.09 ± 0.01, β₂ = 0.24 ± 0.03 and β₁ = 0.09 ± 0.01, β₂ = 0.33 ± 0.04, when the films were deposited at a working pressure of 2.1 × 10^− 1 Pa and 2.7 × 10^− 1 Pa, respectively. The mechanisms of anomalous dynamic scaling exponents of Si₃N₄ films deposited at different working pressures were discussed.     

Dielectric properties of DC reactive magnetron sputtered Al2O3 thin films

Alumina (Al₂O₃) thin films were sputter deposited over well-cleaned glass and Si < 100 > substrates by DC reactive magnetron sputtering under various oxygen gas pressures and sputtering powers. The composition of the films was analyzed by X-ray photoelectron spectroscopy and an optimal O/Al atomic ratio of 1.59 was obtained at a reactive gas pressure of 0.03 Pa and sputtering power of 70 W. X-ray diffraction results revealed that the films were amorphous until 550 °C. The surface morphology of the films was studied using scanning electron microscopy and the as-deposited films were found to be smooth. The topography of the as-deposited and annealed films was analyzed by atomic force microscopy and a progressive increase in the rms roughness of the films from 3.2 nm to 4.53 nm was also observed with increase in the annealing temperature. Al-Al₂O₃-Al thin film capacitors were then fabricated on glass substrates to study the effect of temperature and frequency on the dielectric property of the films. Temperature coefficient of capacitance, AC conductivity and activation energy were determined and the results are discussed.     

Hydrogen atom density in a solar plasma reactor

A density of neutral hydrogen atoms was systematically measured in the MESOX solar plasma reactor at different MW powers and flow rates. The H-atom density was measured by a gold fibre optics catalytic probe. The H-atom density was in general increasing with increasing MW power. At a pressure of 40 Pa and a power of 500 W it was about 3.5 × 10²¹ m⁻³ and at a power of 1000 W it was about 4.1 × 10²¹ m⁻³. A degree of dissociation of hydrogen molecules was between 3% and 20% depending on pressure and power. A maximum degree of dissociation was obtained at a pressure of 40 Pa and 1000 W, while the lowest one at 130 Pa and 500 W.     

Nanoporous cluster-assembled WOx films prepared by radio-frequency assisted laser ablation

The influence of substrate temperature and ambient gas pressure-composition on the characteristics of WO_x films synthesized by radio-frequency assisted pulsed laser deposition (RF-PLD) are studied with the aim to obtain nanostructured films with large surface area that appear promising for gas sensing applications. A tungsten target was ablated both in chemically reactive molecular oxygen at 5 Pa and in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at 473, 673 and 873 K. Other deposition parameters such as laser fluence (4.5 J/cm²), laser wavelength (355 nm), radio-frequency power (150 W), and target to substrate distance (4 cm) were kept fixed. The sensitivity on the deposition parameters of roughness, morphology, nanostructure and bond coordination of the deposited films were analysed by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy. The role of the investigated process parameters to nanoparticle formation and to the development of an extended nanostructure is discussed.     

Optimized inductively coupled plasma etching for poly(methyl-methacrylate-glycidly-methacrylate) optical waveguide

Optical loss is a crucial quality for the application of polymer waveguide devices. The optimized oxygen inductively coupled plasma etching conditions, including antenna power, bias power, chamber pressure, O₂ flow rate and etching time for the fabrication of smooth vertical poly(methyl-methacrylate-glycidly-methacrylate) channel waveguide were systematically investigated. Atomic force microscopy and scanning electron microscopy were used to characterize the etch rate, surface roughness and vertical profiles. The increment of etch rate with the antenna power, bias power and O₂ flow rate was observed. Bias power and chamber pressure were found to be the main factor affecting the interface roughness. The vertical profiles were proved to be closely related to antenna power, bias power and O₂ flow rate. Surface roughness increment was observed when the etching time increased.     

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride coatings

Effects of substrate bias voltage and target sputtering power on the structural and tribological properties of carbon nitride (CN_x) coatings are investigated. CN_x coatings are fabricated by a hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron sputtering at various substrate bias voltage and target sputtering power in the order of −400 V 200 W, −400 V 100 W, −800 V 200 W, and −800 V 100 W. The deposition rate, N/C atomic ratio, and hardness of CN_x coatings as well as friction coefficient of CN_x coating sliding against AISI 52100 pin in N₂ gas stream decrease, while the residual stress of CN_x coatings increases with the increase of substrate bias voltage and the decrease of target sputtering power. The highest hardness measured under single stiffness mode of 15.0 GPa and lowest residual stress of 3.7 GPa of CN_x coatings are obtained at −400 V 200 W, whereas the lowest friction coefficient of 0.12 of CN_x coatings is achieved at −800 V 100 W. Raman and XPS analysis suggest that sp³ carbon bonding decreases and sp² carbon bonding increases with the variations in substrate bias voltage and target sputtering power. Optical images and Raman characterization of worn surfaces confirm that the friction behavior of CN_x coatings is controlled by the directly sliding between CN_x coating and steel pin. Therefore, the reduction of friction coefficient is attributed to the decrease of sp³ carbon bonding in the CN_x coating. It is concluded that substrate bias voltage and target sputtering power are effective parameters for tailoring the structural and tribological properties of CN_x coatings.