Plasma-deposited Silicon Dioxide Research Articles

Subsurface imaging of metal lines embedded in a dielectric with a scanning microwave microscope**Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

We demonstrate the ability of the scanning microwave microscope (SMM) to detect subsurface metal lines embedded in a dielectric film with sub-micrometer resolution. The SMM was used to image 1.2 μm-wide Al–Si–Cu metal lines encapsulated with either 800 nm or 2300 nm of plasma deposited silicon dioxide. Both the reflected microwave (S11) amplitude and phase shifted near resonance frequency while the tip scanned across these buried lines. The shallower line edge could be resolved within 900 nm ± 70 nm, while the deeper line was resolved within 1200 nm ± 260 nm. The spatial resolution obtained in this work is substantially better that the 50 μm previously reported in the literature. Our observations agree very well with the calculated change in peak frequency and phase using a simple lumped element model for an SMM with a resonant transmission line. By conducting experiments at various eigenmodes, different contrast levels and signal-to-noise ratios have been compared. With detailed sensitivity studies, centered around 9.3 GHz, it has been revealed that the highest amplitude contrast is obtained when the probe microwave frequency matches the exact resonance frequency of the experimental setup. By RLC equivalent circuit modeling of the tip-sample system, two competing effects have been identified to account for the positive and negative S11 amplitude and phase contrasts, which can be leveraged to further improve the contrast and resolution.

Atomic layer deposition of tin dioxide sensing film in microhotplate gas sensors

We report the use of atomic layer deposition (ALD) to produce the gas-sensitive tin dioxide film in a microhotplate gas sensor. The performance of the device was demonstrated using ethanol, acetone and acrylonitrile as model analytes. Fast response times and low drift rates of the output signal were measured, indicating a structurally stable tin dioxide film and reflecting the capabilities of ALD in gas sensor applications. Fabrication of the microhotplate using tungsten metallization and plasma deposited silicon dioxide dielectrics is also detailed.

Electrical Properties of Plasma-Deposited Silicon Oxide Clarified by Chemical Modeling

Our study is focused on Plasma Enhanced Chemical Vapor Deposition (PECVD) of silicon dioxide films at low temperatures (< 150 oC) using Inductively Coupled (IC) High-Density (HD) plasma source. We recently fabricated Thin Film Transistors (TFTs) with high-quality ICPECVD gate oxides, which exhibited a competitive performance. For better understanding of the influence of deposition parameters on both the deposition kinetics and oxide quality, we have modeled the Ar-SiH4-N2O plasma system with 173 chemical reactions. We simulated concentrations of 43 reactive species (such as e.g. SiHx radicals and SiHx+ (x=0-3) ions, polysilanes, SiO, SiN, SiH3O, SiH2O, HSiO, etc., as well as atomic hydrogen, nitrogen and oxygen) in plasma. We further used our simulations to qualitatively explain (in terms of concentrations of the reactive species) the influence of SiH4/N2O gas-flow ratio and total gas pressure on film electrical properties and deposition rate.

Electrical Properties of Plasma-Deposited Silicon Oxide Clarified by Chemical Modeling

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Thermal and Kerr nonlinear properties of plasma-deposited silicon nitride/ silicon dioxide waveguides

We introduce and present experimental evaluations of loss and nonlinear optical response in a waveguide and an optical resonator, both implemented with a silicon nitride/ silicon dioxide material platform prepared by plasma-enhanced chemical vapor deposition with dual frequency reactors that significantly reduce the stress and the consequent loss of the devices. We measure a relatively small loss of approximately 4dB/cm in the waveguides. The fabricated ring resonators in add-drop and all-pass arrangements demonstrate quality factors of Q=12,900 and 35,600. The resonators are used to measure both the thermal and ultrafast Kerr nonlinearities. The measured thermal nonlinearity is larger than expected, which is attributed to slower heat dissipation in the plasma-deposited silicon dioxide film. The n2 for silicon nitride that is unknown in the literature is measured, for the first time, as 2.4 x 10(-15)cm(2)/W, which is 10 times larger than that for silicon dioxide.

Stress development in plasma-deposited silicon dioxide thin-films due to hydrogen evolution

The kinetics of stress change in plasma-enhanced chemical vapor deposited (PECVD) silicon dioxide dielectric films, during annealing, is studied. When silicon dioxide is deposited in excess nitrous oxide, a fraction of the oxide is in the form SiO 3/2OH, instead of SiO 2, which on annealing condenses to SiO 2 and H 2O. The condensed water subsequently diffuses out of the film inducing a stress change (increase in tension or decrease in compression) in the film. A theoretical model relating the hydrogen (or water) concentration to the stress change is developed. Secondary ion mass spectrometry analysis revealed a relatively flat hydrogen concentration profile along the thickness of the film, indicating that the stress change is not diffusion limited. The model thus accounts for first order kinetics for bond breaking accompanied by fast diffusion with a fraction of the atoms or molecules undergoing a trap and release process. From the theoretical fit for the experimental data, activation energy of 1.1 eV for bond breaking is obtained. The model allows predicting the time dependent stress evolution in the PECVD film subjected to a prescribed temperature history.

Interface creation and stress dynamics in plasma-deposited silicon dioxide films

The stress in amorphous silicon dioxide film grown by plasma-assisted deposition was investigated both during and after film growth for continuously and intermittently deposited films. It is shown that an intermittent deposition leads to the creation of interfacial regions during film growth, but also causes dynamical structural change in already-deposited film which results in a significantly different stress-thickness profile measured after deposition. Film growth in the continuously deposited film was also monitored using an in situ laser reflection technique, and a strong change in stress was detected at about 145nm which was attributed to the onset of island coalescence.

P Channel Mosfet Devices in Nanocrystalline Silicon

AbstractWe report on the growth and properties of p-channel nanocrystalline Si thin film transistor (TFT) devices. In contrast to previous work, the devices are fabricated in n-body nanocrystalline Si. The doping of the n-body is systematically changed by doping with ppm levels of phosphorous. The threshold voltage was found to change systematically as phosphorus content increased. The TFT devices are of the bottom-gate type, grown on oxidized Si wafers. Source and drain contacts were provided by using either plasma grown p type nanocrystalline layers, or by the simple process of Al diffusion. A top layer of plasma-deposited silicon dioxide was found to decrease the off current significantly. High on-off current ratios exceeding 106 were obtained. Hole mobilities in the devices were consistently good, with the best mobility being in the range of ~1.3 cm2/V-s.

Thickness-dependent stress in plasma-deposited silicon dioxide films

Thick silicon dioxide (SiO2) films up to 5 μm have been deposited by helicon activated reactive evaporation (plasma assisted deposition with electron beam evaporation source) as both bilayer and trilayer structures, and the film stress was investigated in the context of optical waveguide fabrication. A model for stress in the SiO2–Si bilayer as a function of film thickness is formulated and interpreted in terms of Volmer–Weber film growth mechanisms. We find that island coalescence begins at a film thickness of less than 165 nm and continues until about 700 nm. Above approximately 1 μm thickness, the film continues growth as a continuous film. The stress in a deposited SiO2 film in an SiO2–Si–SiO2 trilayer structure was investigated by adapting the established Stoney’s equation for a trilayer system, and comparing it with a thermally grown SiO2 trilayer. A constant value of stress is obtained for the deposited SiO2 film for film thickness &amp;gt;1μm which was consistently less than both measured and previously reported values of stress in thermally grown SiO2.

Plasma-deposited silicon oxide barrier films on polyethersulfone substrates: temperature and thickness effects

Silicon oxide (SiO x ) films deposited on flexible polyethersulfone (PES) substrates by plasma-enhanced chemical vapor deposition (PEVCD) have been investigated for transparent barrier applications. Although the water vapor transmission rate (WVTR) of PES (∼28 g/m 2/day; thickness: 200 μm) is higher than that of the polyethylene terephthalate (PET; ∼16 g/m 2/day; thickness: 188 μm), the PES substrate can withstand process temperatures of up to 180 °C, providing more flexibility in the design of device processing. Details of the substrate-temperature and film-thickness effects on the SiO x /PES properties in terms of transmittance, refractive index, deposition rate, adhesion, roughness, and WVTR were described. When the substrate temperature increased from 80 to 170 °C, the deposition rate, adhesion, and roughness values were found to increase while the WVTR decreased to a value of near 0.3 g/m 2/day at 150 °C. With increasing the oxide thickness from 50 to 500 nm, the surface roughness increased from 2.71 to 5.84 nm. A lower WVTR value can be achieved under a barrier thickness of 200 nm. Further improvement was carried out by depositing a 100-nm-thick SiO x film on both sides of the PES substrate, which resulted in a minimum WVTR of 0.1 g/m 2/day. The double-sided coatings on PES could balance the stress and greatly improve the WVTR data.

Fabrication of micromirrors with self-aligned metallization using silicon back-end-of-the-line processes

Optical waveguides are being explored for on-chip purposes to overcome the speed limitations of electrical interconnects. Passive optical components like waveguides and vertical outcouplers are important components in such schemes. In this study, we fabricated planar waveguides with integrated vertical micromirrors using silicon back-end-of-the-line processes. Approximately 1.6 μm of a hybrid alkoxy-siloxane polymer with a refractive index of 1.50 at the intended wavelength of 830 nm was used as the core and plasma-deposited silicon oxide with a refractive index of 1.46 was used as the cladding. The angular face in the polymer waveguide that would function as the mirror surface was fabricated by a templated pattern transfer method, which involved transferring the angle in a template to the waveguide using anisotropic etching. The sidewall angle generated in Shipley® S1813, a positive photoresist, on patterning was used as the angle template. The photoresist sidewall angle was controlled using a two-step exposure method. A CF 4/O 2 plasma chemistry was used for the anisotropic reactive ion etch. A gas composition of 50/50 CF 4/O 2 was chosen to minimize the etch-related roughness of the alkoxy-siloxane polymer and the photoresist. The metallization of the mirror faces was done using two of the three proposed self-aligned techniques. A calibration-based technique was developed to measure the reflection efficiency of the micromirrors. In this technique, scattered light was used to estimate the incident power on the micromirror and a reflection efficiency of 83% was obtained at a wavelength of 650 nm.

Bond strain, chemical induction, and OH incorporation in low-temperature (350–100 °C) plasma deposited silicon dioxide films

New device concepts are being considered with very demanding requirements for low-temperature processing. In this article, infrared transmission and ellipsometry is used to compare silicon oxide films formed by plasma chemical vapor deposition using SiH4, N2O, and either He or H2 dilution gas between 350 and 100 °C. The Si–O asymmetric stretching mode is affected by bond strain and chemical induction, and monitoring the Si–O peak position gives insight into the effect of process conditions on local bond structure. Hydrogen is expected to affect surface processes during growth, for instance, to enable the removal of surface SiOH bonds through H-mediated abstraction, leading to improved bonding structure at low temperature. We find that exposing the surface to hydrogen atoms during growth helps eliminate isolated SiOH bonds, leading to Si–Si bond formation. However, an increase in associated SiOH bonding groups, stabilized by hydrogen bonding, is also observed. The density of associated SiOH groups is larger at low temperature where the rate of water desorption is reduced, suggesting that the associated OH is formed by physisorbed water produced during OH removal. Films deposited with hydrogen dilution show somewhat improved electrical performance at &amp;lt;200 °C, but further work is required to produce high quality films at very low temperatures.

Plasma-deposited silicon oxide and silicon nitride films on poly(ethylene terephthalate): A multitechnique study of the interphase regions

The “interphase” region between the deposited layer [e.g., plasma-enhanced chemically vapor deposited (PECVD) SiO2 or SiN] and the poly(ethylene terephthalate) (PET) substrate has been investigated and compared to physical vapor deposited (PVD) (electron beam evaporated) SiO2. Composition profiles determined by time-of-flight elastic recoil detection, electron microprobe analysis, and x-ray photoelectron spectroscopy all show an extended interphase region more than 50 nm in width, while the profile of the PVD SiO2 is narrower. However, since these analytical techniques are invasive and prone to artifacts, we have also examined ultrathin (about 10 and 20 nm) SiO2 and SiN PECVD layers on 50 nm spin-coated PET substrates by nondestructive infrared (IR) techniques. The IR spectra confirm that the thin PECVD deposits also comprise an organosilicon phase with Si–CHx bonds. We explain these observations in terms of a fragmentation/redeposition mechanism: During the earliest stage of PECVD, interaction between the plasma and the polymer surface produces volatile organic species, which intermix with the reagent gas feed, thus giving rise to the observed organosilicon-like deposit with gradually decreasing carbon content.

Electrical properties of electron cyclotron resonance plasma-deposited silicon dioxide: effect of the oxygen to silane flow ratio

Silicon dioxide layers have been deposited at from electron cyclotron resonance plasma using pure oxygen and argon-diluted silane. The sign of the oxide charge depends on the flow ratio and also on the post-deposition processing. In the post-metallization annealed layers, net negative charge densities as low as have been obtained in the best conditions. Most of the charge seems to be trapped at oxide centres and the measured mobile charge is negligible. Post-oxidation treatments did not anneal out the oxide traps but released additional positive charge in the oxide.

Chromium and tantalum adhesion to plasma-deposited silicon dioxide and silicon nitride

Thin layers of Ta and Cr deposited onto plasma-deposited silicon dioxide (SiO2) and silicon nitride (SiN) surfaces promote adhesion of Cu to the underlying insulating surfaces. In this paper, the peel adhesion strength of the Ta and Cr layers to the reactive ion etched (RIE) and Ar-sputtered insulating surfaces is measured after Cu deposition. Peel tests, which provide a measure of the adhesion strength of the metallic (Cu + Ta or Cr) layer to the insulators, indicate that both Ta and Cr adhere better to SiN than to SiO2. Cohesive failure of the peel strip occurred for the Ta/SiN system rather than failure at the interface. The freshly exposed surfaces of the peeled metallic strip and the underlying insulator, which represent the locus of adhesion failure, were analyzed by X-ray photoelectron spectroscopy (XPS). Characterization of the chemistry at the surfaces and their relationship to adhesion is discussed.

InGaAs microwave switch transistors for phase shifter circuits

A new InGaAs insulated-gate FET (IGFET) with 1 /spl mu/m gate length and three different gate widths has been designed, fabricated and characterized as switch devices for microwave control applications in phase shifter circuits. The devices employed a plasma deposited silicon dioxide gate insulator and had multiple air bridged source regions. The details of the DC current-voltage (I-V) characteristics and small signal S-parameter measurements up to 20 GHz are presented. The switch IGFET's had a drain saturation current density of 300 mA/mm gate width with breakdown voltages of higher than 35 V. An insertion loss of 1.0, 0.6, and 0.4 dB at 10 GHz and 1.4, 0.8, and 0.4 dB at 20 GHz have been measured for the 300, 600, and 1200 /spl mu/m gate width IGFET's, respectively. Equivalent circuit models fitted to the measured S-parameters for IGFET's yielded on-state resistances from 10.7 to 3.3 /spl Omega/, off-state resistances from 734.4 to 186.8 R and off-state capacitances from 0.084 to 0.3 pF as the gate width is increased from 300 to 1200 /spl mu/m The simulation results using IGFET models for the phase shifter circuits indicated a maximum phase error of 0.11/spl deg/, 0.26/spl deg/, and 0.479 with 0.74, 0.96, and 1.49 dB maximum insertion loss and greater than 33, 26, and 19 dB return loss for the 11.25/spl deg/, 22.5/spl deg/, and 45/spl deg/ phase bits, respectively, over the 9.5-10.5 GHz frequency band.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The role of the gate insulator in the defect pool model for hydrogenated amorphous silicon thin film transistor characteristics

We demonstrate that differing transistor characteristics in the most important material systems can be explained by the defect pool model applied to the defects near the interface of hydrogenated amorphous silicon thin film transistors. Gate dielectrics used include plasma deposited silicon nitrides, plasma deposited silicon oxides, and thermally grown silicon oxides. The most important property of the gate dielectric is not the chemical composition but the fixed charge. In particular, as-deposited plasma deposited silicon oxide transistors can be made with similar properties to plasma deposited silicon nitride transistors or thermal silicon oxide transistors, by varying the fixed charge. After correcting the effects of the fixed charge variation, some differences still exist between the interface qualities. We introduce the parameter Ndb(min), i.e., the minimum density of dangling bonds (cm−2), which is a measure of interface quality independent of the fixed charge of the gate insulator. We propose that the variations in Ndb(min) are due to differences in disorder, perhaps caused by interface strain, leading to slight variations (∼5 meV) of the valence-band tail slope.

InGaAs field-effect transistors with submicron gates for K-band applications

Depletion mode InGaAs microwave power MISFETs with 0.7 mu m gate lengths and 0.2 mm gate widths have been fabricated using an epitaxial process. The devices employed a plasma deposited silicon dioxide gate insulator. The RF power performance at 18 GHz, 20 GHz, and 23 GHz is presented. An output power density of 1.04 W/mm with a corresponding power gain and power-added efficiency of 3.7 dB and 40%, respectively, was obtained at 18 GHz. This is the highest output power density obtained for an InGaAs based transistor on InP at K-band. Record output power densities for an InGaAs MISFET were also demonstrated to the stable within 3% over 17 hours of continuous operation at 18 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Real-time monitor for thin film etching in atomic oxygen environments

Various films have been deposited on gold-coated quartz crystals as part of a quartz crystal microbalance. In situ measurements of the effects of atomic oxygen on the net mass change in these materials have been determined by observing the frequency shift of the oscillator in conjunction with a mass-frequency calibration. Films were exposed to a neutral atomic oxygen beam produced by supersonic expansion of a lightly seeded helium plasma. Atomic oxygen fluxes as large as 10 17 cm -2 s -1 and incident kinetic energies up to 2.5 eV were typically used. Based on an assumed reaction path between amorphous carbon (a-C) and atomic oxygen, the reaction probability is 0.01. Hydrogenated amorphous carbon (a-C:H) films show a three-fold increase in etch rate. Layered coatings of amorphous carbon films can be identified by their differing etch rates. Amorphous carbon films were used to show how reaction rates vary with changes in reaction parameters. A 25% reduction in reactivity is found for a 25% reduction in incident kinetic energy. Synergistic effects were identified by comparing mass loss rates during exposure of amorphous and hydrogenated amorphous carbon films to a neutral atomic oxygen beam and to an oxygen plasma. Plasma-deposited SiO x , SiN x , a-Si:H and plasma polymer hexamethyldisiloxane are shown to be quite inert and are useful as protective coatings. The protective nature of a coating may be ascertained by overcoating a film of reactive material as shown for plasma-deposited silicon oxide and silicon nitride over amorphous carbon films. Films of aluminium have a uniform and adherent oxide layer demonstrated to be stable in atomic oxygen environments. This is in contrast to silver films, which indicate that a protective oxide layer is not formed.

Tapered etching of aluminum with CHF3/Cl2/BCl3 and its impact on step coverage of plasma-deposited silicon oxide from tetraethoxysilane

The tapered etching of aluminum CHF3/Cl2/BCl3 gas mixtures in a hexode reactive ion etching reactor is reported. The process involves sidewall polymer deposition during etching; Auger analysis of the deposit showed mainly carbon and chlorine. The metal sidewall angle (taper angle) was studied as a function of processing conditions, the spacing between Al lines, and photoresist thickness. The extent of taper increased with increasing CHF3 flow rates, saturating at high flow rates, and decreased with increasing Cl2 flow rates. The extent of taper increased with bias in the range −190 to −230 V, and decreased in the range −230 to −260 V. In addition, the extent of taper increased with increasing spacing and with decreasing photoresist thickness. The taper angle for a given spacing can be tailored in the range 60°–90° by adjusting processing conditions. As expected, the step coverage of plasma-enhanced TEOS oxide on taper-etched aluminum lines showed a marked improvement over that of vertical aluminum lines.