Rf Bias Research Articles

Etching of sub-micron high aspect ratio holes in oxides and polymers

Sub-micron holes have been successfully etched into: PMMA/DR-1 on Teflon, BCB, Ta 2O 5, Nb 2O 5 and SiO 2, with the aim of forming photonic crystal structures. Pattern transfer from the NiCr masks into the material is achieved in an electron cyclotron resonance high-density plasma system, using a combination of the gases: CHF 3, CF 4, O 2, Ar and SF 6, and an RF bias on the sample. The etching of Ta 2O 5 and Nb 2O 5 is hindered by the high sublimation temperature, approximately 230 °C, of TaF 5 and NbF 5. This problem was resolved by heating the sample. For these materials aspect ratios of 5 were achieved, with holes of down to 200-nm wide being etched. Wall roughness was typically 30 nm, peak to trough. For SiO 2, aspect ratios of 8 were achieved and the wall roughness was about 10 nm, peak to trough. For the polymers: PMMA/DR-1 on Teflon and BCB aspect ratios of 16 are possible for 250-nm wide holes, along with high, 1 μm/min etching rates.

Investigation of the Alignment Phenomena on the a-C:H Thin Films by PECVD System using Ion-beam Alignment Method

We studied the nematic liquid crystal (NLC) aligning capabilities using the new alignment material of a-C:H thin film by plasma enhanced chemical vapor deposition (PECVD) system for 30 sec under 30W rf power at a gas pressure of 1.4＊10<TEX>$\^$</TEX>-1/ torr. A high pretilt angle of about 5 by ion beam exposure on the a-C:H thin film surface was measured. A good LC alignment by the ion beam alignment method on the a-C:H thin film surface was observed at annealing temperature of 250<TEX>$^{\circ}C$</TEX>, and the alignment defect of NLC was observed above annealing temperature of 300<TEX>$^{\circ}C$</TEX>. Consequently, the high LC pretilt angle and the good thermal stability of LC alignment by the ion beam alignment method on the a-C:H thin film by PECVD method as working gas at 30W rf bias condition can be achieved.

External rf substrate biasing during a-Si:H film growth using the expanding thermal plasma technique

ABSTRACTThe interchangeability of ion bombardment and deposition temperature during a-Si:H film growth at high deposition rates (10-42 Å/s) by means of the expanding thermal plasma has been studied. The ion bombardment is generated by applying an external rf bias voltage on the substrate. It is shown that the opto-electronic performance of the a-Si:H films improves considerably when a moderate rf substrate bias voltage (∼20-60 V) is applied, i.e. the photo response increases two orders of magnitude up to 106. Furthermore, it is also revealed that the additional energy supplied to the growth surface by the ion bombardment, makes a reduction of the deposition temperature by ∼100 °C possible, while preserving good material properties. On the basis of the results obtained, three effects caused by the rf substrate bias can be distinguished: creation of an additional growth flux, a reduction of the void incorporation, and an increase in the vacancy density.

Alignment Effects for Nematic Liquid Crystal Using a-C:H Thin Film as Working Gas at Bias Condition

We investigated the nematic liquid crystal (NLC) aligning capabilities using the new alignment material of a-C:H thin film as working gas at 30 W rf bias condition. A high pretilt angle by ion beam(IB) exposure on the a-C:H thin film surface was measured. A good LC alignment by the IB alignment method on the a-C:H thin film surface was observed at annealing temperature of 250°C, and the alignment defect was observed above annealing temperature of 300°C.

Liquid Crystal Alignment Effects Using a-C:H Thin Films Deposited at RF Bias Condition

The nematic liquid crystal (NLC) aligning capabilities using a-C:H thin film deposited at the three kinds of rf bias condition were investigated. A high pretilt angle of about 11° by the ion beam alignment method was observed on the a-C:H thin film (polymer-like carbon) deposited at 1 W rf bias condition, and the low pretilt angle the NLC than 1 W rf bias conditions was observed on the a-C:H thin film (diamond-like carbon) deposited at rf 30 W and 60 W bias condition. Consequently, the high LC pretilt angle and the good aligning capabilities of LC alignment by the ion beam alignment method on the a-C:H thin film deposited at 1 W rf bisa condition can be achieved. Also, the good thermal stability of LC alignment by the IB alignment method on the a-C:H thin film as working gas at rf bias condition can be achieved.

Bias-graded deposition of diamond-like carbon for tribological applications

Hydrogen-free diamond-like carbon coatings of thickness 1.5 μm were deposited on 440C stainless steel substrates and silicon wafers via DC magnetron sputtering of graphite target at power density of 10.5 W/cm 2. RF bias voltage was applied in the range of −20 to −150 V in two configurations: constant bias and bias-graded. The structure, surface topography, hardness and tribological behavior of these coatings were investigated. The surface roughness ( R a) of the coating decreased from 5.8 to 3.4 nm as bias voltage increased. Results from nanoindentation showed that the hardness of the coatings deposited at high bias voltage reached at 30 GPa and toughness (plasticity during indentation deformation) from approximately 50 to 60%. The hardness of the coating deposited with bias-graded configuration was comparable to the coatings deposited at high bias voltage but the toughness and adhesion strength were significantly improved. Ball-on-disc tribotests were carried out under dry, water- and oil-lubrication conditions. The result showed that the bias-graded coating had a very low coefficient of friction (down to 0.07 in oil). Furthermore, after 2.5 km sliding against alumina and steel (6 mm in diameter) at a high load of 10 N with and without lubrication, no sign of damage was observed on the wear track. Bias-graded sputtering deposition is an appropriate technique to combine the hardness, toughness and adhesion strength into one coating for demanding tribological applications.

A Novel Plasma Etching Tool with RF-Biased Faraday-Shield Technology: Chamber Surface Reaction Control in the Etching of Nonvolatile Materials

A novel electro-magnetically coupled plasma (EMCP) etching tool for nonvolatile materials has been developed. The EMCP etcher is based on inductively coupled plasma generation and has a function for controlling surface reactions by supplying RF bias to a Faraday shield which covers a ceramic discharge dome. We investigated plasma controllability and chamber surface reaction controllability of the EMCP etcher and found that the RF-biased Faraday shield effectively keeps the internal surface of the dome clean in the etching processes of nonvolatile materials. Because of this feature, the EMCP etcher can be applied to the etching processes of various nonvolatile materials such as Pt, Ru, Ir, NiFe, Au, Mo, Ta, Al2O3, HfO2, ZrO2, and indium tin oxide (ITO).

바이어스 조건하에서 증착한 a-C:H 박막을 이용한 액정배향 효과

We studied the nematic liquid crystal (NLC) aligning capabilities using the new alignment material of a-C:H thin film as working gas at 30W rf bias condition. A high pretilt angle of about 5<TEX>$^{\circ}$</TEX> by ion beam(IB) exposure on the a-C:H thin film surface was measured. A good LC alignment by the IB alignment method on the a-C:H thin film surface was observed at annealing temperature of 250<TEX>$^{\circ}C$</TEX>, and the alignment defect of the NLC was observed above annealing temperature of 300<TEX>$^{\circ}C$</TEX>. Consequently, the high LC pretilt angle and the good thermal stability of LC alignment by the IB alignment method on the a-C:H thin film surface as working gas at 30W rf bias condition can be achieved.

Structural characteristics of carbon nanorods and nanotubes grown using electron cyclotron resonance chemical vapor deposition

Different types of carbonaceous materials were synthesized by electron cyclotron resonance chemical vapor deposition on Ni-coated glass substrates with radio-frequency (rf) self-biasing using a gas mixture of CH4 and Ar. Vertically aligned carbon nanorods and multiwalled carbon nanotubes (MWNTs) were obtained at the rf bias voltages of −100 and −200 V, respectively. High-resolution transmission electron microscopy indicated that the distance between two graphene layers of carbon nanorods is much larger than that of well-graphitized MWNTs having a typical value of graphite. Structural characteristics of carbon nanorods and well-graphitized MWNTs were investigated by electron-energy-loss spectra, in which the energy of π+σ plasmon peak obtained from carbon nanorods shifts to a lower value of 23.8 eV, compared to 25.5 eV from well-graphitized MWNT. Low-energy loss plasmon due to π electrons at 6 eV was not observed in carbon nanorods, but clearly defined for well-graphitized MWNTs. In addition, x-ray photoelectron spectroscopy showed that the delocalization of π electrons becomes more pronounced as the structure of carbonaceous films evolves into more crystalline phase from carbon nanowalls to well-graphitized MWNTs with increasing rf bias voltages. These results can suggest that ionic bombardment energy at high rf bias voltages provides sufficient internal energy for dehydrogenation from hydrocarbon molecules, and thus well-graphitized MWNTs can be synthesized even at low temperatures.

Characteristics of C4F8 plasmas with Ar, Ne, and He additives for SiO2 etching in an inductively coupled plasma (ICP) reactor

We have characterized the effect of adding Ar, Ne, and He noble gases to C4F8 inductively coupled plasmas for SiO2 etching. The systematic variation of their ionization potentials, metastable energy levels, and mass of the dominant ion in C4F8/X% discharges (X=He, Ne, or Ar) containing a high percentage of the noble gas provides a means to evaluate the relative importance of gas phase and surface processes in the etching of SiO2. The total ion flux, ion composition, FC deposition and etching rates, and composition of the surface reaction layer formed on Si or SiO2 in these discharges show systematic differences as a function of process parameters because of the different electron impact ionization thresholds for Ar, Ne, and He gases, and differences in the mass of the dominant ion for discharges containing a large proportion of the noble gas. For given experimental conditions (600 W and 20 mTorr for most of this work), Ar addition gives rise to the largest ion current density, and He to the smallest ion current density. When the noble gases are added to C4F8, the Ar+ ion flux contributes the largest percentage and He+ the smallest percentage of the total ion flux for the same dilution of C4F8 with the noble gases. Ion compositional analysis shows that CF+, CF3+, and CF2+ are the dominant ionic fluorocarbon (FC) species (in order of importance), and that they show similar trends as a function of added Ar, Ne, and He percentage. The fluxes of the more highly dissociated C+, F+, and SiFx+/COFx+ ions are greater when Ne and He are added to C4F8 than for Ar addition. Ion energy distributions of all ions are displaced to lower energies for C4F8/Ar discharges as compared with C4F8/Ne or C4F8/He plasmas. Infrared laser absorption spectroscopy was used to determine absolute densities of neutral CF, CF2, and COF2 radical species as a function of gas composition. The densities of CF2 and COF2 radical species were enhanced when Ne and He were added to C4F8 in comparison to Ar addition. Fluorocarbon deposition rates on unbiased Si substrates were measured and greater for C4F8 discharges with Ne or He additives than when Ar was added. Upon rf biasing of the substrates, the ion energies required for etching to take place are lowest for C4F8/Ar discharges, followed by C4F8/He and C4F8/Ne discharges. A comparison of x-ray photoelectron spectra of SiO2 surfaces exposed to C4F8/X% discharges at −10 V self-bias voltage shows pronounced differences in the chemical bond distribution of fluorocarbon films which can be explained by differences in momentum transfer to the surface and the associated bond breaking of deposited fluorocarbon layers on the surface. A comparison of SiO2 and Si etching rates in C4F8/X% discharges shows that for C4F8/Ne discharges containing more than 70%Ne the highest SiO2/Si but lowest SiO2/resist etching rate ratio is obtained.

Formation of polycrystalline silicon germanium/HfO2 gate stack structure using inductively coupled plasma etching

A gate stack structure consisting of a polycrystalline silicon germanium (poly-SiGe) conductor and a HfO2 dielectric on a silicon substrate was formed by inductively coupled plasma etching using HBr/Cl2/O2. Etch rates of poly-SiGe with 46% Ge were 1.7–2.0 times higher than those of polycrystalline silicon, depending on processing conditions. In the small feature sized devices of 100 nm gates, the notching at the sidewall of poly-SiGe was pronounced up to the depth of 50 nm. The amount of notching increased with increasing inductive power and pressure, and decreasing rf bias power. A HfO2 etch rate of 950 Å/min was obtained at the condition of 550 W inductive power, 360 W rf bias power, and 10 mTorr pressure. Etch rates of HfO2 increased with increasing inductive power and rf bias power. Etching selectivity of poly-SiGe with respect to HfO2 increased significantly with the addition of 3.8% O2 to HBr, and it was possible to control the selectivity in the range of 15–70 by changing the rf bias power. The change in etching selectivity was considered mainly due to the change in HfO2 film property, originating from the incorporation of O into the remaining nonvolatile Hf and the reformation of HfOx during etching.

Ions Bombardment in Thin Films and Surface Processing

Ions bombardment is very important in thin films and surface processing. The ion energy and ion flux are two important parameters in ion bombardment. The ion current density mainly dependent on the plasma density gives the number of energetic ions bombarding the substrate. The self-bias voltage in plasma sheath accelerates plasma ions towards the substrate. RF discharge can increase plasma density and RF bias can also provide the insulator substrate with a plasma sheath. In order to choose and control ion energy, ion density, the angle of incidence, and ion species, ion beam sources are used. New types of electrodeless ion sources (RF, MW, ECR-MW) have been introduced in detail. In the last, the effects of ion bombardment on thin films and surface processing are presented.

Etching silicon by SF6 in a continuous and pulsed power helicon reactor

The etch rate of silicon by SF6 in a helicon reactor has been measured along with simultaneous actinometric measurements of the concentration of atomic fluorine in the gas phase for a variety of gas flow rates resulting in pressures in the mTorr range. A bias rf power was applied to the substrate to investigate the effect of ion energy on the etch rate. The etch rate was found to be proportional to the fluorine concentration and independent of the bias for the higher gas flow rates. However, at lower flow rates, the situation was more complicated and no simple model can explain the measurements. Measurements of the etch rate were also made in the afterglow of a repetitively pulsed discharge so that the directed ion energy would be reduced to the thermal motion after the rapid collapse of the plasma potential. A simple model was developed to explain the temporal etching phenomena in terms of the lifetime of the atomic fluorine.

Study on self-bias voltage induced on the substrate by r.f. bias power in a high density plasma

Characteristics of the self-bias voltage induced on the substrate by a r.f. bias power in a high density helicon wave plasma were studied. The self-bias voltage induced on the wafer by r.f. bias of 300 kHz (less than the ion plasma frequency value) was directly measured with a Pt wafer using a high voltage probe. The self-bias voltage and the peak-to-peak voltage were influenced by both bias and source RF power in the high-density plasma. A new probe system was designed to trace these phenomena. It was found that these phenomena were related to the charging time of the blocking capacitor by the ion current. The role and effect of the negative ions on the etching process at low RF frequency are discussed. A method to apply the RF bias effectively without the influence of the source power was suggested for application in a real etching system.

Microwave power limits of AlGaN/GaN HEMTs under pulsed-bias conditions

Dynamic loadline analysis illustrating the microwave performance limits of state-of-the-art AlGaN/GaN high electron-mobility transistors (HEMTs) under pulsed RF biasing and drive conditions are presented. Calculation of dynamic loadlines concurrent with load-pull measurements show the increase of the device RF knee voltage with increasing drain voltage as the cause of reduced output power-added efficiency (PAE) at high drain biases. In this study, an 8-GHz saturated output power of 14.1 W (9.4 W/mm) is achieved on a 1500 /spl times/ 0.25 /spl times/ /spl mu/m/sup 2/ AlGaN/GaN HEMT at a pulsed drain bias of V/sub D/=40 V. The pulsed-bias conditions considered here preclude device self-heating as a mechanism responsible for the lower than expected output power and decrease in PAE with increasing drain bias. These data suggest electron trapping associated with the surface of the device between the gate and drain as the mechanism that limits the ultimate power, PAE, and linearity of AlGaN/GaN HEMTs.

Effects of Ar and O2 additives on SiO2 etching in C4F8-based plasmas

Gas mixtures based on C4F8 are promising for the development of high-performance SiO2 plasma etching processes. Measurements of important gas phase species, thin film etching rates and surface chemistry changes were performed for inductively coupled plasmas fed with C4F8/Ar and C4F8/O2 gas mixtures. The addition of Ar to C4F8 causes a strong increase of the plasma density relative to that of pure C4F8 (by up to a factor of 4× at 90% Ar). For O2 addition the changes in plasma density are small up to 90% O2 relative to pure C4F8. Infrared laser absorption spectroscopy was used to determine the absolute densities of neutral CF, CF2 and COF2 radical species as a function of the gas composition. The densities of CF and CF2 were enhanced for certain operating conditions when Ar was added to C4F8 as long as the amount of Ar remained below 20%. For instance, the partial pressure of CF was 0.1 mTorr for a 20 mTorr 1400 W source power discharge for pure C4F8, and increased to 0.13 mTorr at 20% Ar. Above 20% Ar it decreased, roughly following the gas dilution. The CF2 partial pressure was about 5 mTorr for the same conditions, and increased by about 10% at 20% Ar. Above 20% Ar the CF2 partial pressure decreased roughly linearly with the amount of Ar added, to about 2 mTorr at 50% Ar. Of particular interest was the analysis of the difference in behavior of CF, CF2 and COF2 partial pressures over SiO2 and Si surfaces, with and without rf bias power (in the latter case a self-bias voltage of −100 V was used). For pure C4F8 discharges at 20 mTorr and 1400 W inductive power without rf bias the partial pressures of CF, CF2 and COF2 radicals are comparable over SiO2 and Si surfaces. Upon applying a rf bias, the CF2 partial pressure over a SiO2 surface is reduced much more strongly than for a Si surface. The overall reduction appears to be consistent with the relative SiO2/Si etch rate ratios observed for these conditions. These results indicate that CF2 is consumed during the etching of SiO2 and Si. We also measured fluorocarbon deposition rates without rf bias and etching rates of blanket SiO2, silicon, resist and deposited fluorocarbon films as a function of the rf bias and feed gas composition. Important differences in the response of the etching rates of those materials upon the addition of O2 and Ar to C4F8 were observed. In particular, we show that the SiO2/Si and SiO2/resist etching selectivities can be doubled by adding up to 90% Ar to C4F8, without inducing an unacceptably large reduction of the SiO2 etching rate. The change in etch rate ratios is at least in part due to strong surface chemical changes seen for Ar-rich fluorocarbon gas mixtures. The surface chemical changes of Si and SiO2 surfaces were investigated by real-time ellipsometry and x-ray photoelectron spectroscopy. A strong reduction of the fluorine content of the fluorocarbon steady-state layer and an increase in thickness is seen when up to 90% Ar was added to C4F8, and this coincides with an increase of the SiO2/Si etching selectivity. The change in fluorocarbon surface chemistry can be explained by the strongly increased ion/neutral flux ratio that is characteristic of Ar-rich C4F8/Ar gas mixtures.

An investigation of proton energy effects in SiGe HBT technology

We present the first investigation of low energy (1.75 MeV) proton irradiation in SiGe HBT's and discuss proton energy effects in SiGe HBT technology. The results show that after 1.75 MeV 1 /spl times/ 10/sup 14/ p/cm/sup 2/, a semi-insulating substrate is obtained and the peak quality factor of the monolithic inductors is improved by about 18% at 1.6 GHz. Although large current gain degradation for the SiGe HBT's was observed in the RF bias region after 1 /spl times/ 10/sup 14/ p/cm/sup 2/, the degradation in peak f/sub T/ is only about 11%. Proton energy studies from 1.75 MeV to 200 MeV in SiGe HBT's suggest that the conventional damage factor can be used to estimate energy-dependent proton-induced radiation damage in this technology.

Charged species dynamics in an inductively coupled Ar/SF6 plasma discharge

The chemistry of high-density SF6 plasma discharges is not well characterized. In this article, a combination of computational modeling and experimental diagnostics has been utilized to understand charged species dynamics in an inductively coupled Ar/SF6 plasma discharge. The model is based on the two-dimensional Hybrid Plasma Equipment Model with a detailed plasma chemical mechanism for Ar/SF6. In the experiments, absolute electron density and total negative ion density have been measured using microwave interferometry and laser photodetachment, respectively. In addition, we have also utilized prior measurements of mass and energy resolved ion fluxes by Goyette et al. [J. Vac. Sci. Technol. A 19, 1294 (2001)]. Computational results show that all SFx+(x=0–5) ions are present in the plasma discharge. Important negative ions include SF6−, SF5−, and F−. Electron and positive ion densities increase with coil power due to enhanced ionization. However, negative ion densities decrease with coil power as the main negative ion precursor, SF6, is lost through neutral dissociation. An increase in SF6 concentration in the Ar/SF6 gas mixture decreases electron density due to enhanced electron loss through (dissociative) attachment, which enhances negative ion densities. RF bias power does not have an appreciable impact on the ion and electron densities for the parameter range of interest. Experiments show that electron density decreases with gas pressure while the total negative ion density increases up to 25 mTorr. This is due to a decrease in electron temperature, which enhances electron loss through (dissociative) attachment. Although the model is able to capture most of the experimentally observed trends, there are discrepancies regarding the impact of gas pressure on electron density and relative flux of large positive ions.

Semiempirical profile simulation of aluminum etching in a Cl2/BCl3 plasma

A semiempirical profile simulator to predict topographic evolution during Cl2/BCl3 plasma etching of photoresist patterned Al lines has been developed. Given incident flux distributions, the profile simulator uses a combination of a particle based Monte Carlo algorithm and analytic ray-tracing algorithm for solving feature-scale ion and neutral flux transport, respectively. We use angular and energy distributions for reflected ions that are consistent with experimental observation and molecular dynamic simulations. Etch yields with energy and angular dependence are experimentally determined for physical sputtering and ion-enhanced etching. The spontaneous etch rate of A1 by chlorine and the spontaneous desorption rate of Cl from photoresist are estimated from experimental results. Sticking coefficients for etchant, chlorine, and depositor, CClx, and depositing flux are determined by fitting simulated profiles to experimental data. A semiempirical site-balance model is developed to compute the surface coverage of etchant. The reaction probability of neutrals at surfaces is self-consistently determined from the surface coverage at incident location. Competition between etching and deposition on feature sidewalls is modeled. A shock-tracking method is used to advance the profile using computed etch/deposition rates. Simulation results are presented which demonstrate that facet formation, aspect ratio dependent etching, and critical dimension gain, are captured accurately by the calibrated profile simulator. In addition, the calibrated profile simulator along with results of a 23 design of experiments in which photoresist and Al etch rates were measured on open frame wafers have been used to create a plasma reactor model. The reactor model relates the operational parameters including inductively coupled power, rf bias and gas flow ratio to the flux variables, i.e., Cl flux, ion flux, ion energy, and depositor flux, that are used as inputs to the profile simulator. In this manner, calibration of the profile simulator requires a minimum of high resolution, expensive, patterned wafers. The reactor model so created is shown to be in quantitative agreement with results from the hybrid plasma equipment model (Ref. 1).

Resist trimming in high-density CF4/O2 plasmas for sub-0.1 μm device fabrication

Resist trimming using a CF4/O2 plasma generated by an inductively coupled high-density plasma source is investigated. Results show that the resist trimming process is applicable for the fabrication of a sub-0.1 μm polysilicon gate electrode with conventional 248 nm lithography. The trim rate depends strongly on gas composition, temperature, rf source power, bias voltage, reactor pressure and total gas flow. The trim rate increases with increasing temperature, rf source power and reactor pressure but decreases with higher bias voltage. For a typical trimming condition at gas composition of CF4 and O2 with a ratio of 3.3:1, the resist trim rates are 1.0 and 1.2 nm/s for dense and isolated lines, respectively. By examining the effect of various process parameters, it is found that the trimming process is dominated by the neutral reactant species. Angle-resolved x-ray photoelectron spectroscopy shows that a fluorinated carbon oxyfluoride polymer is deposited on the sidewall of photoresist. The degree of carbon fluorination of the film is influenced by the process parameters. A more fluorinated film, obtained at lower bias voltage and higher source power, gives more trimming.