Radio Frequency Bias Research Articles

Diamond-like carbon film deposition by a 13.56 MHz hollow cathode RF–RF system using different precursor gases

Diamond-like carbon films (DLC) were deposited in a novel 13.56 MHz RF–RF system (Plasma Consult GmbH PlasCon HCD-System) at a substrate temperature of 60°C. Typically, a radiofrequency (RF) power of 400 W was used for plasma generation by a hollow cathode discharge plasma source (HCD). The substrate holder was also RF powered and had a negative DC self bias voltage in the range of 200–600 V. Both HCD and the RF-substrate holder are run with synchronized RF-power generators and automated impedance matching units. The carrier gas He was introduced into the primary hollow cathode discharge at a flow rate of typical 400 sccm. Methane and acetylene were used as a carbon source at flow rates between 15 and 100 sccm. An ion concentration of up to 2×10 11 cm −3 was measured in the plasma with a HCD and RF bias on. Without the HCD an ion concentration of approximately 5×10 10 cm −3 was achieved. Higher ion concentration had a positive influence on the deposition process and allowed to achieve a higher deposition rate. In the stationary mode deposition rates of 70–80 nm min −1 with methane as a precursor gas and 180–260 nm min −1 with acetylene as a precursor gas were measured. The films were investigated by micro-Raman spectroscopy, FTIR, ellipsometry and microhardness measurements. It was found that even in the stationary mode deposition the film thickness variations across a 5″ Si-wafer were lower than ±3.5%. The acetylene based DLC films have a refractive index of 2.1–2.15 at wavelength of 632 nm. The refractive index of DLC films, deposited with methane as a precursor, was between 2.2 and 2.3. Vickers microhardness of these films of up to 30 GPa were achieved.

Growth characterization and properties of diamond-like carbon films by electron cyclotron resonance chemical vapor deposition

Diamond-like carbon (DLC) films were deposited on radio-frequency (RF) biased substrates at low temperature by electron cyclotron resonance microwave plasma chemical vapor deposition using CH 4-Ar as reactant gas. The effects of gas composition ratio, microwave power and RF bias on growth rate, structure and hardness of the films were investigated. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and Vickers microhardness tests were used to determine the structural change and properties of the DLC films. By changing the microwave power, the growth rate shows a maximum value of 1.6 μm/h at 150 W. From Raman spectra of the films deposited at different microwave power and RF bias, the peaks centered at 1540±20 cm −1 for the deposited films, which was the characteristic peak of DLC coatings. By increasing the RF bias the CH n peaks in the FTIR spectra decreased because of hydrogen evolution. The film hardness increased with the increase in microwave power and RF bias. DLC films synthesized at a bias voltage of −(100–250) V and CH 4/Ar gas ratio of 2/7 exhibited extreme hardness of more than 3000 kg mm −2. The role of adding Ar in reactant gases on DLC deposition was also discussed.

Effect of radio-frequency bias voltage on the optical and structural properties of hydrogenated amorphous silicon carbide

Hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films have been deposited using the electron cyclotron resonance chemical vapor deposition process under varying negative rf-bias voltage at the substrate. The optical and structural properties of these films are characterized using Rutherford backscattering spectroscopy, transmittance/reflectance spectrophotometry, photothermal deflection spectroscopy, Fourier transform infrared absorption, Raman scattering, and room temperature photoluminescence (PL). These films deposited using a gas mixture of silane, methane, and hydrogen at a constant gas flow ratio showed a slight increase in the carbon fraction x, but very obvious structural transformation, at increasing rf induced bias voltage from −20 to −120 V. Near stoichiometric a-Si1−xCx:H films with a carbon fraction x of almost 0.5 are achieved at low bias voltage range from −20 to −60 V. Visible PL with relatively low efficiency can be observed from such films at room temperature. For larger bias voltages from −80 to −120 V, slightly C-rich a-Si1−xCx:H films (x>0.5) with larger optical gaps are obtained. These films have relatively higher PL efficiency, and the relative quantum efficiency was also found to depend strongly on the optical gap. Structurally, it was found that there is an increase in the hydrogen content and carbon sp2 bonding in the films at larger bias voltages. The latter leads to an increase in the disorder in the films. The linear relationship observed between the Urbach energy E0 and B factor in the Tauc equation suggests that the local defects related to microstructural disorder resulting from alloying with carbon dominate the overall defect structure of the films. Substrate biasing is noted to be crucial for the formation of Si–C bonds, as deduced from the Raman scattering results.

Beam-generated plasmas for processing applications

The use of moderate energy electron beams (e-beams) to generate plasma can provide greater control and larger area than existing techniques for processing applications. Kilovolt energy electrons have the ability to efficiently ionize low pressure neutral gas nearly independent of composition. This results in a low-temperature, high-density plasma of nearly controllable composition generated in the beam channel. By confining the electron beam magnetically the plasma generation region can be designated independent of surrounding structures. Particle fluxes to surfaces can then be controlled by the beam and gas parameters, system geometry, and the externally applied rf bias. The Large Area Plasma Processing System (LAPPS) utilizes a 1–5 kV, 2–10 mA/cm2 sheet beam of electrons to generate a 1011–1012 cm−3 density, 1 eV electron temperature plasma. Plasma sheets of up to 60×60 cm2 area have been generated in a variety of molecular and atomic gases using both pulsed and cw e-beam sources. The theoretical basis for the plasma production and decay is presented along with experiments measuring the plasma density, temperature, and potential. Particle fluxes to nearby surfaces are measured along with the effects of radio frequency biasing. The LAPPS source is found to generate large-area plasmas suitable for materials processing.

Use of radio frequency bias in the large area plasma processing system

In the large-area plasma processing system (LAPPS) scheme, rf bias can be used for either of two purposes: to provide energetic ion bombardment of a substrate, or to controllably raise the electron temperature (which is intrinsically very cool) to a desired value. The physics of rf bias in the LAPPS differs from the situation in conventional processing reactors for several reasons: (1) The plasma density adjacent to the substrate can be so high that the ion plasma frequency exceeds the microwave frequency. (2) Plasma transport to the substrate is across a magnetic field. (3) Ionization occurs only in a thin, well-defined planar sheet, and thus the volume occupied by plasma is very broad in two dimensions but thin in the third dimension. (4) The surface area of the substrate is comparable to that of the containment vessel. We discuss the modifications to the theory of rf bias that are needed to account for these factors in a LAPPS plasma.

Optical band gap and refractive index of c-BN thin films synthesized by radio frequency bias sputtering

Boron nitrogen (BN) films with the different cubic phase content were deposited on Si and fused silica substrates by radio frequency bias sputtering from a hexagonal BN target by using a two-stage deposition process. The BN films were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and UV-visible transmittance and reflection measurements. The optical absorption coefficient α and the refractive index n were calculated from the transmittance and reflection spectra. With increasing the c-BN content the absorption edge shifts to the higher energy, indicating that the optical band gap of the BN films increases with cubic BN content. The optical absorption behavior of BN films shows characteristics of amorphous materials. The dependence of α on the photon energy was fitted by the Urbach tail model and the band-to-band transition model at the two different energy regions, and the optical band gap of the BN films were obtained from the fits. In addition, the refractive index indicates obvious difference for the BN films with different cubic phase content.

Sheath model for radio-frequency-biased, high-density plasmas valid for all omega/omega(i)

A model is proposed for sheaths in high-density discharges, with radio-frequency (rf) bias applied at frequencies omega comparable to omega(i), the ion plasma frequency at the edge of the sheath. The model treats ion dynamics using fluid equations, including all time-dependent terms. Model predictions for current, impedance, and power were compared to measurements performed in high-density discharges in argon at 1.33 Pa (10 mTorr) at rf bias frequencies from 0.1 to 10 MHz (omega/omega(i) from 0.006 to 1.8) and rf bias voltages from 1 to 200 V. Model predictions were in good agreement with measurements, much better than that obtained by models that neglect time-dependent ion dynamics. In particular, differences of as much as 40-50 % between power measurements and the power predicted by previous models are now explained and eliminated. The model also explains why methods of extracting plasma parameters from electrical measurements using previous sheath models may fail, and it suggests more accurate methods of extracting these parameters.

Effects of fluorocarbon polymer deposition on the selective etching of SiO2/photoresist in high density plasma

A new periodic two-step process composing SiO2 etching with high bias radio frequency (rf) power and fluorocarbon deposition with low bias rf power was studied for the highly selective etching of SiO2 to photoresist (PR). In this experiment, the time scale of each step is longer than the conventional time-modulation technique in order to maximize the protection layer on PR and prevent the etch stop. Many works have focused on the gaseous chemical species especially CF2 radicals for selective surface reaction. However, normally utilizing only the difference of stoichiometric surface reaction, they inherently limit the etching conditions such as dependence on the chemical composition of PR, densities, and impurities of SiO2 layers. And these conventional processes severely suffer reactive ion etching lag or etch stop in high selective etching. The new process utilizes fluorocarbon deposition with low bias rf power to increase the mask selectivity by enhancing the difference between the polymer thickness on the mask and that on the bottom surface of hole. After the etching step, the polymer film remains only on the mask, and then the higher selectivity of SiO2 to PR can be achieved. In this article it has been investigated whether the polymer deposition in the suggested process is governed by aspect ratio of holes, surface temperature, bias rf, and microwave powers. The ratio of the amount of etching to deposition is a very important factor in determining the selectivity. With the process a small and deep contact etching with thin PR is possible without shortage of mask thickness with the mask selectivity improved from 6 to 20. We can also find that the etch rate of this new process does not depend significantly on the aspect ratio.

Crystallography and structural evolution of cubic boron nitride films during bias sputter deposition

The mechanism and the crystallography of the growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrates by radio-frequency bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy. Particular attention has been paid to the atomic structure of graphitic (sp 2-bonded) BN and grain boundaries in the c-BN films. The c-BN films grow in the sequence of amorphous boron nitride (a-BN), turbostratic boron nitride (t-BN) and c-BN layers, similar to previous results. The sp 2-bonded BN material consists of small regions, 1–5 nm thick, forming in a layered manner parallel to the substrate surface. Each region consists of parallel lamellae in both the hexagonal and rhombohedral (h-BN and r-BN, respectively) configurations. Three orientation relationships between h-BN and r-BN phases were determined: OR-1: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (0001) h-BN‖(0001) r-BN; OR-2: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 1 ̄ ) r-BN ; OR-3: [2 1 ̄ 1 ̄ 0] h-BN ‖[2 1 ̄ 1 ̄ 0] r-BN , (01 1 ̄ 0) h-BN ‖(01 1 ̄ 2) r-BN . The r-BN crystallites within the sp 2-bonded BN grow preferentially in such a way that the (01 1 ̄ 2) r-BN ‖(01 1 ̄ 1 ̄ ) r−BN plane is almost parallel to the grain boundary. Twinning about both the basal planes and the {01 1 ̄ 1} r-BN planes is common within r-BN phase. The c-BN crystallites adjacent to the graphitic BN layer are highly twinned, the {111} c-BN twin planes being parallel to the basal planes of the sp 2-bonded BN. The c-BN phase nucleates on the oriented graphitic BN layer in a semicoherent manner and obeys specific orientation relationships with the hexagonal and rhombohedral phases, namely: OR-I: [ 2 ̄ 110] r-BN ‖[110] c-BN OR-II: [ 2 ̄ 110] r-BN ‖[ 1 ̄ 1 ̄ 0] c-BN (01 1 ̄ 1 ̄ ) r-BN ‖( 1 ̄ 11) c-BN ( interface plane) (01 1 ̄ 2) r-BN ‖(1 1 ̄ 1 ̄ ) c-BN ( interface plane) (0001) r-BN ≈‖(1 1 ̄ 1) c-BN (0001) r-BN ≈‖(1 1 ̄ 1) c-BN They are related by a rotation of 180° about their common axis, [0001] r-BN ‖[1 1 ̄ 1] c-BN , [2 1 ̄ 1 ̄ 0] h-BN ‖[110] c-BN (01 1 ̄ 2) h-BN ‖(001) c-BN (0001) h-BN ‖(1 1 ̄ 1) c-BN In the mainly c-BN region of the films, twins are present about more than one of the sets of {111} c-BN planes. The intrinsic microstructure of the c-BN films and the crystallography between the cubic and graphitic BN phases show that the structure of cubic boron nitride is directly related to the structure of the precursor phases. The atomic structure of an interface depends on the orientation relationship between adjacent c-BN grains and the boundary inclination. The grain boundaries consist of twin boundaries when two adjacent grains are oriented close to the [110] c-BN zone axis and the boundary plane is parallel to the {111} c-BN close-packed planes of both grains. However, a thin layer, 1–2 nm, of sp 2-bonded BN forms between the c-BN grains when the boundary plane inclines a few degrees from the {111} c-BN planes of the adjacent grains.

Dynamics of inductively-coupled pulsed chlorine plasmas in the presenceof continuous substrate bias

We report an analysis of the dynamics of a pulsed power,inductively-coupled chlorine plasma operated with continuous radio frequency(rf) bias applied to the substrate stage. A comparison of pulsed plasmasoperated with and without 12.5 MHz rf bias is performed through aninvestigation of the time dependences of electron (ne) and positive ion(ni+) densities and electron temperatures (Te), measured with aLangmuir probe in a 10 mTorr Cl2 plasma. There is no significant differencein the plasma characteristics with or without bias during the on portion ofthe power modulation of the source. Once the source power is turned off, Teinitially decreases rapidly, and ne and ni+ decay slowly, independentof the presence of the rf bias. About 25 µs into this decay, when Te isabout 0.5 eV (and would continue to decay in the absence of the rf bias), thepresence of the rf bias (70-100 W) causes Te to increase rapidly and reachvalues higher than those recorded at the end of the on portion. Meanwhile,ne and ni+ continue to decay, independent of substrate bias. Onlymuch later in the afterglow are charge densities affected by bias. About50 µs into the off period, ne with added substrate bias is lowerthan with no bias. If the off period is sufficiently long, the plasma in thepresence of the rf bias transforms into a so-called reactive ion etching (RIE)mode that is generated and sustained solely by the capacitively-coupled biaspower. This occurs rather abruptly when ni+ decays to the level ofpositive ion density for RIE operation (i.e. with the stage powered and thesource power off). Since ne at this instant has decayed well below theelectron density during RIE operation, it rapidly increases when thistransition occurs. The behaviour of Te in the off portion of the pulsedplasma indicates that the presence of rf bias prevents the sheath collapsethat occurs without bias. Since this will prevent negative ions (Cl-)from reaching the wafer, the reduced plasma-induced damage (with respect tocontinuous source operation) reported for pulsed plasma operation withhigh-frequency (e.g. ~13 MHz) bias cannot be caused by the dischargingof the wafer by negative ions.

Diamond-like carbon coatings applied to hard disks

Abstract Amorphous carbon (a-C:H) films were prepared on the surface of magnetic disks using an electron cyclotron resonance assisted microwave plasma chemical vapour deposition (ECR-MPCVD) system with variable radio-frequency substrate bias voltage. As indicated by the atomic force microscopy measurements, the surfaces of the hard disks coated with 10 nm thick films were smoother than those of the uncoated disks. The scratch resistance of coated disks increased at biases greater than −90 V. Corrosion resistance of the coatings was also studied by exposure to vapour of hydrochloric acid (HCl) for 24 h. The analysis showed that the radio-frequency bias voltage was a critical parameter affecting the chemical compositions, mechanical properties and corrosion resistance of the films.

Effects of radio frequency bias frequency and radio frequency bias pulsing on SiO2 feature etching in inductively coupled fluorocarbon plasmas

The effect of radio frequency (rf) bias frequency on SiO2 feature etching using inductively coupled fluorocarbon plasmas is investigated. It is found that the rf bias frequency can have an important effect on SiO2 feature etch rate, microtrenching phenomena, and SiO2-to-photoresist etch selectivity. In addition, the effect of rf bias pulsing on inductively coupled fluorocarbon plasma SiO2 etching has been studied and a model that describes the data well is presented. The model assumes that fluorocarbon deposition occurs while the rf bias is off, fluorocarbon etching occurs during the first part of time that the bias is on, and substrate etching occurs once the fluorocarbon material has been removed from the substrate.

Effect of radio frequency bias power on SiO2 feature etching in inductively coupled fluorocarbon plasmas

We present and discuss results obtained in studies of the mechanisms underlying various feature size dependencies of SiO2 etching in inductively coupled fluorocarbon plasmas. The variation of the fluorocarbon deposition rate and the SiO2 etch rate with both feature size and rf bias power has been measured in a variety of constant aspect ratio features for both an etch stop (C3F6/H2) and a nonetch stop (CHF3) feedgas chemistry.

The effects of proton irradiation on the RF performance of SiGe HBTs

The effects of proton irradiation on the RF performance of SiGe Heterojunction Bipolar Transistors (HBTs) are reported in this paper. Frequency response and broadband noise properties are investigated in SiGe HBTs for proton fluences up to 5/spl times/10/sup 13/ p/cm/sup 2/. The current gain in the radio frequency (RF) bias region and the cutoff frequency (f/sub T/) show little degradation at even extreme proton fluence (realistic space fluences are 1-5/spl times/10/sup 11/ p/cm/sup 2/). The slight degradation of NF/sub min/ is due to the irradiation-induced increase in the total emitter and base resistance. The associated available gain G/sub A, assoc/ at noise matching is 20.2 dB at f=2 GHz and I/sub C/=2.6 mA for a proton fluence of 5/spl times/10/sup 13/ p/cm/sup 2/. These results suggest that space-borne RF circuit applications of SiGe HBTs should be robust to proton irradiation.

Highly Selective Etching of Poly-Si by Time Modulation Bias

Highly selective etching of poly-Si is possible by time modulation (TM) bias. Radio frequency (RF) bias applied to a substrate is pulse modulated. The energy of ions impinging on the wafer is varied by the peak-to-peak voltage of RF bias. The average flux of accelerated ions is controlled by the duty ratio of the pulse. The selectivity of poly-Si to SiO2 is about 1.5-fold that of continuous wave bias at a poly-Si etch rate of 100 nm/min. The SiO2 etch rate is suppressed and the selectivity increased, this may be caused by the selective deposition of reaction products including oxygen on the SiO2 surface during off periods of RF bias.

Thin Film Detection Employing Frequency Shift in Sheath Current Oscillation

For an end point detection of etching in ULSI fabrication processes, the behavior of the sheath oscillating current was theoretically and experimentally investigated under the radio frequency bias application. The oscillation of the displacement current in sheath was numerically calculated employing the fluid equations of plasma, and its frequency spectrum was analyzed in computational Fast Fourier Transformation. The results showed that the shift occurred mainly at lower regime than the center frequency under the condition of an relatively small bias voltage change. This behavior of the frequency shift was also confirmed by a simple experiment. A new method is proposed for detection technique of thin insulation film on substrate in process employing the frequency shift of this kind.

Optical absorption edge characteristics of cubic boron nitride thin films

Boron nitride films with different cubic phase contents were deposited in a radio frequency bias sputtering system by a two-stage deposition process. The Fourier transform infrared spectra and UV-visible transmittance and reflection spectra were measured. The optical absorption edge of BN films consists of a low energy region in which α increases exponentially as incident photon energy hν, and a high energy region, in which α varies as (hν)m, which is the characteristics of amorphous materials. These two regions are fitted by the Urbach tail model and the band-to-band transition model with an effective medium approach, and the Urbach energy and the optical band gap are determined from these fits. With an increase in the cubic boron nitride (c-BN) content, the absorption edge shifts to the higher energy and the optical band gap increases. For a BN film with 88% cubic phase, the optical band gap exceeds 6.0 eV, which is comparable to that of c-BN single crystal.

A Radio Frequency Driven DC Electron Beam Source

A 3 cm diameter disc shaped plasma generated electron beam source, employing ion-induced secondary electron emission from cold cathodes has been demonstrated. An inductively coupled plasma generated in 2–20 mTorr argon feedstock gas provides ions which impinge on the high secondary electron emission coefficient Al2O3 cathode. A separate 13.56 MHz radio frequency bias voltage is applied to the cathode and thereby provides the effective DC field for both ion impingement and electron beam acceleration as described herein. A differentially pumped retarding potential analyzer measures the electron beam energy spectra at a location 14 cm from the cold cathode. It is found that the maximum (negative) cathode voltage sets the maximum electron beam energy, and the electron beam current is roughly set by the inductively coupled plasma power, which corresponds to the ion flux to the cathode. A crude model relating the electron beam energy spectra to the time varying cathode sheath potential provided by the rf bias shows good agreement with measured results.

Radio frequency bias current scheme for dc superconducting quantum interference device

We have developed a radio frequency bias current scheme with reversal frequency up to 50 MHz for dc superconducting quantum interference devices (SQUIDs). The SQUID and the readout electronics, were connected via a quarter wavelength (/spl lambda//4) transmission line. This transmission line not only transported the SQUID signal, but also served as the resonant matching circuit. For a dc SQUID of inductance 115 pH (loop size 75/spl times/75 /spl mu/m/sup 2/), we measured a flux noise of 33 /spl mu//spl phi//sub 0///spl radic/Hz at 77 K in the flux locked loop configuration. The corner frequency of 1/f noise was less than 0.5 Hz.

Ion energy distributions and sheath voltages in a radio-frequency-biased, inductively coupled, high-density plasma reactor

Ion energy distributions were measured at a grounded surface in an inductively coupled, high-density plasma reactor for pure argon, argon–helium, and argon–xenon discharges at 1.33 Pa (10 mTorr), as a function of radio-frequency (rf) bias amplitude, rf bias frequency, radial position, inductive source power, and ion mass. The ground sheath voltage which accelerates the ions was also determined using capacitive probe measurements and Langmuir probe data. Together, the measurements provide a complete characterization of ion dynamics in the sheath, allowing ion transit time effects to be distinguished from sheath impedance effects. Models are presented which describe both effects and explain why they are observed in the same range of rf bias frequency.