Rf Bias Research Articles

Cone-Shaped Hard Carbon Films Grown by Inductively Coupled RF Plasma with RF or DC Bias Voltage

A simple method capable of producing uniform, large-area cone arrays of carbon films was found in a planar inductively coupled RF plasma source. The technique employs a DC or RF bias to substrate holder. Si substrates were mechanically pretreated using diamond paste. Cone-shaped carbon crystals preferentially nucleate and grow on the scratches using relatively low bias. Variation of the depositing conditions enables control of the cone density, geometry, and height. The cone arrays are believed to can significantly improve the field emission properties and have a tempting perspective in the microelectromechanical system (MEMS).

Influence of RF Bias on the Deposition of CrN Studied by OES

Chromium nitride coatings were synthesized by DC magnetron sputtering techniques. Optical emission spectroscopy (OES) was applied to investigate the role of the different level RF substrate biases. The results show that the main chemical reaction process occurred on substrate surface and OES signal ratio I(N 2 )/I(Cr) can be used to predict the composition evolution. Ion bombardment effects were identified, especially in the range from 60 V to 100 V, they provide benefits to make coating denser and improve mechanical properties.

Electrical Probe Diagnostics of the Hollow Cathode Plasma Jet System for Deposition of TiOx Thin Films

AbstractMeasurements of positive ion flux to a negatively biased substrate for deposition of TiOx thin films by the hollow cathode plasma jet system are presented. Different methods of obtaining the bias of substrate and measuring the resulting ion flux were used for different bias frequencies. Pulsed DC bias, middle frequency 500 kHz bias, and pulse‐modulated 13.56 MHz RF bias were compared. The measurements were performed for different bias and discharge conditions. Time‐resolved Langmuir probe technique was simultaneously used to observe the basic plasma properties during the ion flux measurements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Submicrometer- and nanometer-structure formation on the surface of epitaxial IV–VI semiconductor films by Ar-plasma treatment

A method is presented for fabricating submicrometer and nanometer structures on epitaxial films of a IV–VI compound semiconductor on a Si(111) substrate by sputtering with an RF-induction Ar plasma. The role is identified of threading dislocations and terraces on the film surface in the formation of submicrometer and nanometer hillocks. The relationship is determined of sputtering parameters to the RF bias and process time. The self-formation of submicrometer hillocks is traced to dislocation exit sites being masked by Al-containing components.

Directly modulated laser diode in the nonlinear regime for low‐cost digital frequency up‐conversion

AbstractThe purpose of this article is to demonstrate the extension of the modulation bandwidth of single mode laser sources to reach the RF frequency range of wireless access networks such as ultra‐wide band systems with low‐cost solutions. In this article, we have studied optical microwave mixing using nonlinear direct modulation of a distributed feedback (DFB) laser diode. First, the nonlinear behavior of the DFB laser diode near the threshold current has been analyzed and explained based on frequency mixing measurements made with CW signals. Then, we have experimentally demonstrated frequency up‐conversion of a QPSK signal. These results were compared to eye‐diagram simulations based on the laser static and dynamic characteristics. Subsequently, the frequency up‐conversion of a WLAN IEEE 802.11a signal was experimentally demonstrated. Best conditions in terms of DFB bias current and RF input power signals have been studied. Good conversion loss and EVM results have been obtained. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1214–1219, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23329

Tantalum Nitride Sputtering Deposition with Xe on Fluorocarbon for Cu Interconnects

TaN was sputter-deposited with radio frequency (rf) bias in Xe gas for a barrier film of Cu interconnects of advanced large-scale integration (LSI). Xe gas has been reported to reduce damage on substrates due to small ion bombardment. While crystalline TaN was formed by sputtering with rf bias, amorphous TaN was formed by sputtering without rf bias. TaN with rf bias shows superior barrier properties against Cu diffusion to that without rf bias. When Cu/TaN/fluorocarbon was annealed in temperatures higher than , delamination was found at the interface between the Cu and TaN. The SiCN cap layer over fluorocarbon improved adhesion, which increased the critical temperature to an acceptable range for LSI production. Assuming a simple center-of-mass elastic collision of the isolated particle, values of energy-transfer efficiency from an impinging ion to various kinds of substrate atoms were calculated. These result revealed that Xe ions efficiently transfer energy to TaN to build a crystalline structure without energy transferring to fluorocarbon.

Retarding field analyzer for ion energy distribution measurements at a radio-frequency biased electrode

A retarding field energy analyzer designed to measure ion energy distributions impacting a radio-frequency biased electrode in a plasma discharge is examined. The analyzer is compact so that the need for differential pumping is avoided. The analyzer is designed to sit on the electrode surface, in place of the substrate, and the signal cables are fed out through the reactor side port. This prevents the need for modifications to the rf electrode--as is normally the case for analyzers built into such electrodes. The capabilities of the analyzer are demonstrated through experiments with various electrode bias conditions in an inductively coupled plasma reactor. The electrode is initially grounded and the measured distributions are validated with the Langmuir probe measurements of the plasma potential. Ion energy distributions are then given for various rf bias voltage levels, discharge pressures, rf bias frequencies--500 kHz to 30 MHz, and rf bias waveforms--sinusoidal, square, and dual frequency.

High quality nanocrystalline silicon thin film fabricated by inductively coupled plasma chemical vapor deposition at 350 °C

High quality nanocrystalline silicon (nc-Si) film was deposited by inductively coupled plasma chemical vapor deposition (ICP-CVD) without substrate RF bias at 350°C. The nc-Si with a dense crystalline structure of the columnar type grew from the bottom to the top of the nc-Si film. A troublesome incubation layer did not exist at the bottom of the fabricated nc-Si film. A grain size of 40nm was measured by using a SEM image. When a RF bias of 100 and 200W was applied to the substrate to induce ion bombardment on the substrate, the crystalline structure and grains were not observed and a-Si deposition became dominant. The transition from nc-Si deposition into a-Si deposition can be attributed to ion bombardment which prevents nucleation and crystal growth at the surface of deposition. This shows that the reduction of ion bombardment can be a key factor to fabricate high quality nc-Si film. By using ICP-CVD with no substrate RF bias, ion bombardment can be reduced, while the density of plasma is kept high, so that high quality nc-Si can be fabricated due to the enhancement of crystalline growth on the surface.

Recent Results of a New Microwave SQUID Multiplexer

We present recent results of a prototype microwave SQUID multiplexer containing four SQUIDs coupled to GHz frequency resonant circuits and fed with a single microwave readout line. The system is operating at a readout frequency range of 8–10 GHz. All four SQUIDs share a common DC bias and modulation lines. A new modulation scheme is tested to eliminate the need for individual flux biasing of the SQUIDs, which extends the dynamic range of the readout. In this scheme a common modulation signal is imposed on each SQUID and the received signal is demodulated at one and two times the modulation frequency to maintain sensitivity at any flux state. We also demonstrated a microwave RF bias scheme eliminating the necessity of the DC current bias to the SQUID. Our preliminary performance tests at 4.2 K show that the input noise of the device is ∼5 pA/ \(\sqrt{\mathrm{Hz}}\) .

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400–800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.

Control of the Harmonics Generation in a Capacitively Coupled Plasma Reactor

The resonant growth of harmonics was observed in a dual-frequency capacitively coupled plasma etcher. The resonances of the second, third, and fourth harmonics of the bias RF were observed through current and voltage measurements. A fine tuning of the wafer-electrode reactance revealed that the reactance is the dominant factor controlling harmonic amplitude. The resonances take place when the sum of the wafer-electrode and plasma reactances reaches its minimum value at the harmonic frequency. A simple nonlinear equivalent circuit model well reproduces the experimental result. Significant changes in the radial distribution of electron density and the resultant etching rate were observed at resonance.

Proton exchange and diffusion in LiNbO3 using inductance coupled high density plasma

In this article, the underlying physical mechanisms of proton exchange (PE) in LiNbO3 with inductance coupled plasma (ICP) as proton source have been investigated according to quantitative calculations and simulations. It is shown that a high proton density layer near the sample surface acts as the source for proton diffusion. Experimentally, effects of parameters including working pressure, rf bias power, and ICP power are investigated by measurement of the PE depths with secondary ion mass spectrometry. And the variation of PE results has been linked to fundamental plasma characteristics such as ion density and ion flux through the variation in the measured dc self-bias values. A reproducible proton-exchange efficiency of 660nm∕h has been obtained at a relatively low sample temperature of 170°C and working pressure of 8mTorr, with high sample quality.

Influence of RF Bias on Hydrogenated Amorphous Silicon by High-Density Plasma Chemical Vapor Deposition

We investigate the microstructure and thermal stability of hydrogenated amorphous silicon thin films deposited by high-density plasma chemical vapor deposition (HDP-CVD) with various biases using radio frequency (rf) power. The films were prepared without external heating at a high rf power density of , with the rf power bias varying from 0 to . Because sputtering also occurs during deposition, hydrogen may be present in a variety of bonding configurations in the films with various biases. Fourier transform infrared spectra show that the HDP-CVD films possess more hydrogen content as well as stretching modes than the conventional plasma-enhanced CVD counterpart. The surface morphology of the films becomes smooth upon ion bombardment under rf bias. In addition, variations of the film stress against temperature correlate to the hydrogen desorption at different stages during thermal annealing. More importantly, the different bonding configurations affect the crystallization behavior upon thermal annealing.

Surface modification and contamination characterization of ion-enhanced, implanted photoresist removal

The authors study the dopant contamination removal effectiveness of blanket high-dose implanted photoresist wafers exposed to dual mode oxygen plasma, where a remote rf-plasma source is operated simultaneously with rf bias at low temperature. Based on total x-ray reflectance x-ray fluorescence spectrometry and secondary ion mass spectrometry analyses, sputtering effects are minimal during typical process times with most of the dopant remaining on the substrate after dual mode oxygen plasma processing. SC1-based (NH4OH:H2O2:H2O Standard Clean 1) wet clean treatments reduce dopant levels but 2×1013–4×1013at∕cm2 still remain unless the substrate is processed with hydrofluoric acid (HF) wet cleanings where the contamination level is less than 2×1011at∕cm2. Remote plasma low temperature fluorine post-treatments volatilize dopant residues but are less effective than HF wet cleanings. Dual mode forming gas (4%H2∕96%N2) plasma post-treatments are ineffective for dopant contamination removal. Low temperature dual mode oxygen plasma treatments followed by high temperature remote oxygen plasma treatments after wet cleaning have lower dopant levels than low temperature dual mode oxygen plasma processing alone. High temperature remote plasma forming gas post-treatments after dual temperature processing volatilize dopant residues. The increased dopant removal effectiveness with dry fluorine remote plasmas, HF wet cleanings, and dual temperature processing is consistent with the surface modification of the underlying silicon into an oxide from ion bombardment, which also drives dopant residues into the underlying oxide. The surface modification depth from exposure to dual mode oxygen plasma is about 49–53Å.

Arbitrary substrate voltage wave forms for manipulating energy distribution of bombarding ions during plasma processing

The energy distribution of ions (IED) bombarding a substrate during plasma etching has demonstrated effects on etch selectivity for integrated circuit fabrication. Accurate control of the IED is desired to better understand the nature of plasma–surface interaction and to control process outcomes. IED control can be achieved by tailoring the wave form shape of an rf bias applied to the substrate, using a programmable wave form generator in combination with a power amplifier. Due to the frequency dependence of the amplifier gain and the impedance of the plasma in contact with the substrate, it is not practical to predict the shape of the input wave form needed to produce a desired result at the substrate. Introduced here is an iterative approach using feedback control in the frequency domain to produce arbitrary wave form shapes at the substrate. A fast Fourier transform (FFT) of the substrate wave form is compared, one frequency at a time, with the FFT of a desired target wave form, to determine adjustments needed at the generator. This iterative procedure, which is fully automated and tested for several target wave form shapes, is repeated until the substrate wave form converges to the targeted shape.

Fabrication of Si mold with smooth side wall by new plasma etching process

A new silicon etching process for imprint mold fabrication is developed. A special sample bed, which is coated by teflon film, is placed on the RF bias electrode in the conventional ICP etcher. Silicon is etched by mixture gas plasmas of Ar, SF 6 and C 4F 8 (or CHF 3). The silicon side etching can be reduced by use of the teflon coated sample bed. When the switching process is used, fine and deep silicon patterns with smooth silicon surface can be obtained for the teflon coated sample bed. The silicon pattern of 0.1 μm wide with the aspect ratio of 30 can be fabricated. A silicon mold is fabricated by the new etching process. A fine pattern of 0.2 μm wide can be transferred to a polymethylmethacrylate (PMMA) film by the thermal imprinting.

Frequency Tuning and Spurious Signal Generation at Microwave Frequencies in Ferroelectric ${\hbox{SrTiO}}_{3}$ Thin-Film Transmission Lines

This paper evaluates the RF tuning and the microwave nonlinear response of SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin films by measuring the third harmonics and third-order intermodulation (IMD) products of several coplanar waveguide transmission lines fabricated directly on SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 </sub> thin-film samples. From these measurements, we obtained the distributed nonlinear capacitance per unit length as a function of RF bias voltage C(V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rf</sub> ) using an accurate equivalent circuit to model the spurious signal generation. A unique value of C(V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rf</sub> ) is used to describe the third-harmonic generation at frequencies 3f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and 3f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and the IMD products at frequencies 2f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> +f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and 2f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> +f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , where f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> represent the fundamental frequencies of the two-tone excitation signal. We compare C(Vrf) with the distributed nonlinear capacitance measured as a function of the dc-bias voltage C(V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dc</sub> ) and obtain excellent agreement at 50 and 76 K. From these results, we conclude that full tunability can be achieved on nanosecond time scales, and that spurious signals generated in SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ferroelectric transmission lines at microwave frequencies can be modeled based on dc-biased measurements

Wafer Redeposition Impact on Etch Rate Uniformity in IPVD System

The liner and seed deposition and feature coverage in an ionized physical vapor deposition (IPVD) technique depends on the background gas pressure and an RF bias power applied to the substrate electrode. Under such conditions, a redeposition (RD) occurs at the increased pressures, and it has a noticeable impact on the actual feature coverage and performance nonuniformity (NU) across the wafer. An analytical 2-D plasma fluid model, combined with analytical thermalization model, and experimental validation were used to investigate the impact of the wafer RD on the intrinsic and apparent etch rates (ERs) and their NU in the IPVD system. The model comprised the Cu+Ar collisional mechanism, including charge exchange and Penning ionization, considering gas rarefaction effect and variable ion mobility approach. The analytical approach was implemented to describe the mechanism of the thermalization of sputtered Cu. The experiment encompassed pressure range from 0.6 to 8.7 Pa and the table power from 100 to 800 W, showing the dramatic effect of the RD on the ER uniformity. Depending on the pressure, the apparent ER NU (one with contribution of the RD) can vary from ~6% (min-max) to over 20% (min-max). An intrinsic ER NU (contribution of the RD is extracted) is from 5% (min-max) to over 16% (min-max), shifted with respect to the pressure. The most significant impact of the RD effect has been observed at the edge of a 300-mm wafer. The evaluation on Ta (TaN) liners exhibits similar trends in RD, but a shift in the performance curves shows that the key factor in RD uniformity is the mean free path of the etched material

Deposition of dielectrics using a matrix distributed electron cyclotron resonance plasma enhanced chemical vapor deposition system

We have studied the deposition of silicon oxide, oxynitrides and silicon nitride in a novel, scalable, low-pressure high-density plasma system based on the ECR effect and the matrix arrangement of microwave antennas. Silane, nitrogen and oxygen gases were used as precursors. Films were deposited onto different heated and unheated substrates, including silicon, glass and polymer. The influence of the magnetic field configuration, gas flows, reactor pressure, microwave power and radio frequency bias on the uniformity and properties of the films was studied. Ex-situ and in-situ UV-visible spectroscopic phase-modulated ellipsometry and FTIR spectroscopy were used to study the properties of materials and the uniformity of the depositions, along with electrical measurements to evaluate the dielectric properties of the films. Optimizing the magnetic field configuration and applying an RF bias, we were able to achieve an uniformity better than 3.3% across the 200 mm wafers. Stoichiometric silicon oxide films with index values matching that of thermal silica were grown on unheated substrates with rates exceeding 2 nm/s.

Influence of the addition of silicon on the structure and properties of chromium nitride coatings deposited by reactive magnetron sputtering assisted by RF plasmas

CrN and Cr–Si–N films have been deposited by means of reactive magnetron sputtering in an Ar + N 2 mixture (20 and 10 sccm respectively) at a constant RF bias voltage of − 200 V. The silicon content of the films was controlled thanks to silicon coupons placed in the erosion track of the chromium target. The influence of the addition of silicon on the properties and on the structure of Cr–Si–N coatings is presented. It is shown that the high RF bias considered in this study favours the implantation of argon atoms into the coatings, associated to the re-sputtering of nitrogen atoms leading to the formation of very hard understoichiometric CrN and to internal compressive stress between 1 and 1.5 GPa. Then, the evolution of the composition and of the structure of the deposited coatings versus their silicon contents are presented thanks to GDOES and XRD analysis. The evolution of the hardness and of the Young's modulus versus the silicon content of the films exhibit a peak for a critical percentage of silicon (2.2 at.%), which is attributed to the formation of a nanocomposite structure. For this critical content of silicon, a basic model based on the Cr–Si–N ternary phase diagram and on an ideal 2D nanocomposite structure is proposed to calculate its fraction of amorphous phase. It is shown that the model calculations are in perfect agreement with the structure observed by means of high resolution transmission electron microscopy and that only a small amount of silicon (between 2 and 3 at.%) is needed for the formation of a nanocomposite structure in the Cr–Si–N system.