Helical Resonator Research Articles

Low-frequency antiferromagnetic resonance of RbMnF 3 near the Néel temperature

Low-frequency, low-field magnetic resonance measurements made with a helical resonator based spectrometer for the temperature near the Néel temperature T N have revealed critical behavior of the sublattice magnetization of the cubic antiferromagnet RbMnF 3 in the spin-flop regime. The shift in the resonance position has been determined for 0.020 K < T N − T < 2.4 K for 503 < v < 2147 MHz. The results show δH r ∝ M( T) 2 ∝ ( T N − T) b with b = 0.680 ± 0.014, which yields the scaling exponent β = 0.340 ± 0.007.

Structural, Optical, and Field Emission Properties of Hydrogenated Amorphous Carbon Films Grown by Helical Resonator Plasma Enhanced Chemical Vapor Deposition

Hydrogenated amorphous carbon films have been prepared by helical resonator plasma enhanced chemical vapor deposition using CH4 and H2 mixtures. Films with various physical properties were obtained from different deposition conditions. The structural and optical properties of hydrogenated amorphous carbon (a-C:H) films were more sensitive to the substrate bias than the substrate temperature. This reflects that the energetic ion bombardment modified the films more effectively than the thermal energy. The a-C:H films deposited with no bias applied show characteristics of polymeric films with a large content of C–H bond while the a-C:H films deposited as a function of the substrate temperature at a bias of 40 W show characteristics ranging from diamond-like carbon (DLC) to graphitic nature with a significantly reduced C–H bond. From elastic recoil detection analysis, the hydrogen content in the films also significantly reduced with an increase of substrate temperature at a bias of 40 W. The field emission from bare Si emitters and a-C:H coated Si emitters has been examined in an ultrahigh vacuum chamber. The field emission characteristic of the a-C:H coated Si emitters is better than that of the bare Si emitters. For the a-C:H coated Si emitters, the emission current of the a-C:H coated (at 150°C/40 W) Si emitters is higher than the that of the a-C:H coated (at 260°C/40 W) Si emitters. This difference in field emission characteristic is attributed to the structural and optical properties as well as hydrogen content.

Structural and optical properties of amorphous hydrogenated carbon nitride films prepared by plasma-enhanced chemical vapor deposition

Thin films of amorphous hydrogenated carbon nitride ( a-CN x :H) were deposited from CH 4 and N 2 gases by plasma-enhanced chemical vapor deposition using a helical resonator discharge. The effects of substrate temperature on the structural and optical properties of the films have been systematically investigated by Raman spectroscopy and transmission ultraviolet-visible spectroscopy. The optical band gap of the a-CN x :H films was found to decrease from 1.72 to 0.95 eV as the substrate temperature is increased to 250°C. Through the relationship between optical band gap and Raman studies, it could be concluded that a progressive graphitization of the films occurs with increasing the substrate temperature.

Fabrication of amorphous-carbon-nitride field emitters

To improve silicon field emitters, an amorphous-carbon-nitride (a-CN) coating was applied by helical resonator plasma-enhanced chemical vapor deposition. By this process, a-CN was very uniformly coated on silicon tips without any damage. Microstructural and electrical investigation of the silicon and a-CN coated field emitters were performed. a-CN coating lowered turn-on voltage and increased emission current. Negative electron affinity of carbon nitride is suggested for enhancing emission current.

Characterization of amorphous hydrogenated carbon nitride films prepared by plasma-enhanced chemical vapor deposition using a helical resonator discharge

Amorphous hydrogenated carbon nitride thin films (a-CNx:H) have been prepared by plasma-enhanced chemical vapor deposition of N2 and CH4 gases using a helical resonator discharge. The structural and optical properties of the deposited a-CNx:H films have been systematically studied as a function of the substrate temperature and radio frequency (rf) substrate bias. The chemical structure and elemental composition of the a-CNx:H films were characterized by Fourier transform infrared spectroscopy (FT-IR), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The optical properties of the films were evaluated using transmission ultraviolet–visible spectroscopy. The morphology of the films was investigated by scanning electron microscopy and atomic force microscopy. The FT-IR and XPS studies demonstrate the presence of carbon–nitrogen bonds with hydrogenated components in the films. The film composition ratio N/C was found to vary from 0.127 to 0.213 depending on the deposition conditions. The Raman spectra, showing the G and D bands, indicate that the a-CNx:H films have a graphitic structure. It can be found that the optical band-gap Eg of a-CNx:H films is associated with graphitic clusters, while the decrease in Eg is correlated with an increase in the size and number of graphitic clusters. Combining the results of Raman and optical measurements, it can be concluded that a progressive graphitization of the films occurs with increasing the substrate temperature and rf substrate bias power, corresponding to bias voltage.

Constant Current N 2 O Plasma Anodization of Silicon

A helical resonator plasma source was used to perform constant‐current plasma anodization of silicon. Growth rates in plasmas were lower than those in plasmas. Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) was used to study nitrogen incorporation in the oxides as a function of oxidation time and annealing temperature. At an oxide thickness of 4.5 nm, a bonding structure was observed in the interfacial region. Growth of a 9.4 nm oxide revealed the same bonding structure at the interface, with low intensity and broad signals at higher binding energies in the bulk oxide. Distinct peaks at higher binding energies in the bulk oxide corresponding to oxynitride bonding structures emerged in 15 nm oxides. Both 9.4 and 15 nm oxides were etched to 4.5 nm in HF solution and analyzed using angle‐resolved x‐ray photoelectron spectroscopy for comparison of nitrogen incorporation within the same oxide/interfacial region. Within the 4.5 nm region, longer oxidation times yielded higher nitrogen concentrations and the formation of oxynitride bonding arrangements in addition to the bonding structure. Nitrogen annealing at temperatures above 500°C decreased the total nitrogen concentration, primarily due to a reduction of nitrogen in the oxynitride bonding arrangement.

Silicon and germanium nanoparticle formation in an inductively coupled plasma reactor

We have studied the formation of Si nanoparticles in a SiH4–Ar plasma discharge generated in a helical resonator type inductively coupled plasma reactor. It is observed that Si particles vary in sizes from 5 to 15 nm under different conditions. The particles were mostly spherical and made up of a crystalline core with a 1–2 nm thick amorphous shell. The size distribution was narrow for particles formed at a pressure of 200 mTorr, plasma power of 400 W and silane flow rate of 20 sccm (+980 sccm Ar). The effect of a dc bias applied to the particle collecting grids has also been studied. It is found that a negative bias (−25 to −100 V) applied to the grids used for particle collection results in a large increase in the number of Si nanoparticles collected, while a positive bias does not change the collection efficiency considerably, suggesting that the particles are positively charged. Under very low flow rates and under high plasma powers, the Si particle density decreases considerably and a film like deposition occurs on transmission electron microscopy grids placed in the reactor for collecting the particles. We have also studied the formation of Ge particles (50–200 nm) in a GeH4–Ar plasma generated in the same reactor. These particles are crystalline with a spherical morphology.

Determination of electron temperatures in plasmas by multiple rare gas optical emission, and implications for advanced actinometry

A method is described for determining the electron temperature of a low pressure plasma of the type used in microelectronics materials processing. A small amount of an equal mixture of He, Ne, Ar, Kr, and Xe is added to the process gas (in this example Cl2) and the intensities of optical emission lines from the Paschen 2p levels of the rare gases are recorded. The observed emission intensities are compared with those computed from a model that includes electron impact excitation from the ground state, as well as two-step electron impact excitation through intermediate metastable levels. This latter route is shown to be the dominant one for nearly half of the levels. Using adjusted, published electron impact excitation cross sections and assuming a Maxwellian electron energy distribution, the electron temperature (Te), the only adjustable parameter, was determined from the best match between the observed and computed intensities. For a high density, helical resonator Cl2 plasma at 10 mTorr, Te=2.2±0.5 eV was determined from this method. This value is about 1 eV lower than the typical values reported for high density inductively coupled Cl2 plasmas at similar pressures. While the precision of the method is expected to be high, the accuracy of Te determined from optical emission would likely be improved with more accurate cross section data. Implications for actinometric determination of species concentrations in plasmas are also discussed.

Impedance model of helical resonator discharge

Modeling and measurement results of the RF input impedance of a helical resonator discharge are given. A simple microstrip transmission line model is used to extract an equivalent circuit of a helical resonator discharge. The transmission line model consists of a grounded cylinder for RF shielding, a helical copper coil for RF heating, and a high density plasma. Transmission line parameters for an experimental helical resonator discharge chamber are extracted, and the results are compared with the measured results. The measured transmission line parameters for the experimental reactor are the characteristic impedance of 200 /spl Omega/, the attenuation constant of 6/spl times/10/sup -3/ Np/m, and the wave number of 0.36 rad/m, which have good agreement with the calculated values.

Ammonia Plasma Treatment of PTFE Under Known Plasma Conditions

Ammonia plasmas produced in a helical resonator have been investigated using an energy-selective mass spectrometer and an r.f.-compensated Langmuir probe. The effect of processing parameters on the plasma composition and ion energy distributions have been examined. The dominant ion present in the plasma was found to be NH 4 + , which varied in concentration between 95% and 65%, dependent upon operating conditions. Similarly, by relatively small variations in processing parameters it was possible to vary the NH 3 + ion concentration between 1.7 and 31%. In addition, no radicals (NH 2 . , NH . or N . ) were detected using threshold ionization mass spectroscopy. Polytetrafluoroethylene (PTFE) samples were treated under a variety of different plasma conditions, and the chemical changes induced were studied by in situ XPS. Defluorination was observed to be greater under conditions that yield low ion energies or high concentrations of NH 3 + ions. Conditions giving rise to significant concentrations of NH 3 + ions result in the production of NH 3 + F - groups. Evidence is provided suggesting that NH 3 + ions are much more reactive with PTFE surfaces than NH 4 + ions

The electromagnetic modes of a helical resonator

The electric fields of the helical resonator, both of the fundamental frequency and the odd harmonics up to the eleventh, have been probed using metallic copper and heavily doped Si:P disks by measuring the relative frequency shift Δf/f and the change in quality factor Δ(1/Q) with a network analyzer after insertion of the disk as a probe mounted on a dielectric support rod which could be translated along the z axis of the helix with a micrometer drive. The magnetic field component Hz was probed using the paramagnetic free radical DPPH and a simple transmission electron spin resonance (ESR) spectrometer. The Δf/f results were compared with calculated results using the Bethe–Schwinger expression and the plasma result for the change in dielectric response, while the Δ(1/Q) results were compared with a standard loss expression. The results, except for the role of the short but including the metallic disk thickness and diameter dependence, agree with the fields given by Pierce for an infinite helical current sheet, when adapted to the standing wave form. The ESR results demonstrate the optimum position for a paramagnetic sample. The ESR amplitude versus probe insertion depth is in qualitative agreement with an expression by Wilmshurst et al., but also exhibits an unexplained sign change.

Nitrogen incorporation in thin oxides by constant current N2O plasma anodization of silicon and N2 plasma nitridation of silicon oxides

A helical resonator plasma source was used to perform constant current N2O plasma anodization of silicon and N2 plasma nitridation of silicon oxides. The nitrogen bonding structure and distribution in the oxides were studied using angle resolved x-ray photoelectron spectroscopy. Nitrogen corresponding to a N–Si3 bonding structure was detected at the silicon side of the interface, the Si–SiO2 interfacial region, and the bulk oxide in a 4.5 nm N2O plasma grown oxide. The distribution profile of nitrogen in the oxide, determined from a normalized N 1s/Si 2p(ox) ratio, showed a continuous decrease from the silicon side of the interface towards the bulk oxide. Also, strong nitrogen peaks corresponding to either a N–Si3 or a N–Si2 bonding structure were detected throughout a 9.1 nm O2 plasma grown oxide after postanodization constant current N2 plasma nitridation.

Comparison of advanced plasma sources for etching applications. V. Polysilicon etching rate, uniformity, profile control, and bulk plasma properties in a helical resonator plasma source

Etching of polysilicon features using a helical resonator plasma source is evaluated. Performance metrics consist of etching rate, etching rate uniformity, and profile control using HBr/O2–He gas-phase chemistry. The effect of source power, rf-bias power, and reactor pressure on etching rate and uniformity is examined using a response surface experiment. Feature profile control is determined by examining nested and isolated lines and trenches using oxide mask/polysilicon/oxide structures. Good uniformity and vertical profiles are obtained at low reactor pressures, high source power, and rf-bias between 50 and 60 W. The operating point for best uniformity is at 3.5 mTorr, 3000 W source power, and 53 W rf-bias power. At this point, the etching rate is 3700 Å/min and the nonuniformity is less than 1.0%, over 125-mm-diam wafers. Radial profiles of electron temperature and ion density near the wafer surface are presented as a function of source power, rf-bias power, and reactor pressure. The ion density was found to be in the mid-1011 cm−3 range and electron temperatures were 5–7 eV. An increase in source power and reactor pressure results in an increase in ion density; however, the electron temperature shows a weaker dependence. Finally, these results are compared to those using helicon and multipole electron cyclotron resonance plasma sources evaluated in previous studies. We found that all three plasma sources provide high ion density at low pressures to meet performance demands for polysilicon etching; however, the helical resonator source offers somewhat higher etching rate and better bulk plasma uniformity.

Metal stack etching using a helical resonator plasma

A low-pressure etching process for advanced aluminum metallization stacks was developed using a high-density helical resonator plasma source (Prototech model ESRF 600) mounted on a Lucas Labs cluster tool. The metallization stacks consisted of a 300 Å TiN antireflection layer on 6000 Å of Al (1% Cu) with a 1000 Å TiN diffusion barrier and a 100 Å Ti film to enhance adhesion to the underlying SiO2. The features widths were as small as 0.45 μm. The films were etched using gas mixtures of Cl2/BCl3. The BCl3 proved to be an important additive to reduce notching of the Al film at the interface between the Al and the top layer of TiN. Best feature profiles were obtained using 80–90 sccm Cl2 and 10–20 sccm BCl3 at the following reactor conditions: 2.0 mTorr, wafer platen temperature T=0 °C, 100 W rf bias power, and 1500 W source power. More anisotropic profiles are obtained by either decreasing the wafer platen temperature or increasing the rf-bias power. The photoresist is also stripped in the same process chamber using an oxygen plasma at 5 mTorr, 50 W rf-bias power, and 1500 W source power at a chuck temperature of 25 °C. Extensive application of real-time process diagnostics, including optical emission spectroscopy and full wafer interferometry, aided process development by identifying end points, etching rates, and etching rate uniformities.

Study of surface reactions during plasma enhanced chemical vapor deposition of SiO2 from SiH4, O2, and Ar plasma

In situ attenuated total reflection Fourier transform infrared spectroscopy was employed in proposing possible surface reaction mechanisms during plasma enhanced chemical vapor deposition of SiO2 from a mixture of SiH4, O2, and Ar in a helical resonator plasma reactor. Infrared spectra taken during the oxide deposition revealed that the oxide surface is covered with OH when the deposition is conducted with a low SiH4 to O2 ratio (≪1). Both associated hydroxyls (SiOHassoc) and isolated hydroxyls (SiOHisol) were detected on the surface during growth. Two kinds of OH attached to Si exist on the surface: weakly bound OH that thermally desorb at 250 °C and strongly bound OH that desorb under ion bombardment. The thermal removal of weakly bound OH does not lead to SiO2 formation, but the ion-assisted removal of strongly bound OH by Ar+ ion bombardment results in oxide growth via the reaction 2SiOH(s)+Ar+→Si–O–Si(s)+H2O(g)+Ar+. Experiments undertaken to delineate the possible heterogeneous reaction paths between surface species and gas phase reactants showed that the molecular fragments of SiH4 (SiHx, x=1, 2, or 3) react with surface hydroxyl groups and produce surface hydrides such as HSiO3, H2SiO2, and H3SiO. The silane fragments also react with Si on the surface to give silicon hydrides whose Si constituent is backbonded to other Si atoms in addition to O [e.g., HSi(SiO2), H2Si(SiO), HSi(Si2O), H3SiSi, and H2SiSi2]. The latter reactions become more important as the OH on the surface is depleted, and the surface becomes silicon and silicon hydride rich. A subsequent exposure of the silicon- hydride-covered oxide surface to oxygen plasma creates SiOHassoc groups presumably by insertion of O into Si–H bonds of surface hydrides. The silicon hydrides are not observed during deposition using low SiH4 to O2 flow rate ratios (≪1): silane fragments, upon adsorbing onto the surface, are promptly oxidized by O atoms, leading to a SiOH-covered surface at steady state conditions. Bombardment of the surface by ions such as Ar+ and O+2 assists H2O desorption from SiOH forming Si–O–Si bonds.

Mass spectrometric measurements of neutral reactant and product densities during Si etching in a high-density helical resonator Cl2 plasma

Line-of-sight mass spectrometry was used to sample both stable and reactive neutral species near the walls in the downstream region of a high-density helical resonator Cl2 plasma during etching of Si. In this region, where the positive ion density is 1–2×1011 cm−3, the Cl2 number density at a pressure of 8 mTorr decreases by about 20% when the plasma is ignited. At constant pumping speed, this percentage increases with decreasing pressure, reaching 30% at 1 mTorr. A decrease of about 10% is due to expansion of the gas, heated by the plasma to a measured temperature of 400±50 K, integrated over a distance of one mean-free path from the wall. This, therefore, accounts for about one-half to one-third of the drop in Cl2 number density. The remaining half to two-thirds of the decrease in Cl2 number density upon ignition of the plasma can be ascribed to the formation of Cl atoms and SiClx etch products. Cl atoms are detected throughout this pressure range; their percentage increases at the higher pressures at constant pumping speed. SiCl4 is the main etching product in the chamber, though not necessarily a primary product. Smaller amounts of SiCl2 and possibly SiCl are also present in the plasma. Within experimental error, chlorine mass balance is found at all pressures, indicating an overall consistency in the mass spectrometric calibration methods. The percent dissociations measured in this study are much higher than earlier values derived from Cl-atom measurements, and are more in line with recent measurements and model predictions for high-density plasmas. Relationships between the mass spectrometer geometry and detection efficiency were also investigated. The detection configuration with the quadrupole axis perpendicular to the line of sight was found to be superior to one in which the quadrupole axis was parallel to the line of sight. In the latter configuration, signals from Cl and Cl2 are anomolously large due to charge exchange, producing a collimated beam of fast neutrals at the sampling orifice.

A simple optical emission method for measuring percent dissociations of feed gases in plasmas: Application to Cl2 in a high-density helical resonator plasma

Optical emission was used to determine absolute Cl2 number densities in a Cl2 high-density, helical resonator plasma that was used to etch Si. Emission from Cl2 at 306 nm was monitored, and rare-gas actinometry was used to correct for changes in the electron energy distribution. This emission band of Cl2 is ascribed in this study to a state at 8.4 eV, or possibly to one at 9.2 eV. A state of Xe at 9.8 eV, emitting at 823.2 nm, was chosen as a suitable match to the energy of the emitting state of Cl2. Absolute number density measurements were obtained by extending the measurement to extremely low powers where little dissociation occurs. This simple self-calibration method has advantages over other more complicated techniques such as laser induced fluorescence that rely on external calibrations. It is also much easier to carry out than mass spectrometry and is more sensitive than optical absorption. It is applicable to feed gases with optical emission from states with energies above about 8 eV. In this study it was found that a Cl2 helical resonator discharge is nearly completely dissociated (95%) at a power density of 0.4 W/cm3 in the glass tube source region. The percent dissociation drops to 85% below the source and above the Si substrate. Near the stainless-steel walls, fast Cl-atom recombination reduces the dissociation to ∼10% (determined by line-of-sight mass spectrometry). The high degree of dissociation in the source and near-downstream region is in agreement with recent model predictions for high-density discharges.

Preparation and characterization of beryllium doped organic plasma polymer coatings

We report the formation of beryllium doped plasma polymerized coatings derived from a helical resonator deposition apparatus, using diethylberyllium as the organometallic source. These coatings had an appearance not unlike plain plasma polymer and were relatively stable to ambient exposure. The coatings were characterized by inductively coupled plasma mass spectrometry and x-ray photoelectron spectroscopy (XPS). Coating rates approaching 0.7 μm h−1 were obtained with a beryllium-to-carbon ratio of 1:1.3. There is also a significant oxygen presence in the coating as well which is attributed to oxidation upon exposure of the coating to air. The XPS data show only one peak for beryllium with the preponderance of the XPS data suggesting that the beryllium exists as BeO. Diethylberyllium was found to be inadequate as a source for beryllium doped plasma polymer, due to thermal decomposition and low vapor recovery rates.

Investigation of low temperature SiO2 plasma enhanced chemical vapor deposition

Low temperature (&amp;lt;270 °C) plasma enhanced chemical vapor deposition (PECVD) of SiO2 thin films using tetraethylorthosilicate (TEOS) and O2 plasma was investigated. Depositions were carried out in a PECVD reactor with a helical resonator discharge source. Transmission infrared spectroscopy, spectroscopic ellipsometry, and wet etch rate measurements were used to characterize the deposited films as a function of rf power, gas composition, and substrate temperature. Most pronounced effects were observed when the substrate temperature and TEOS:O2 flow ratio ℜR were varied. Good quality SiO2 films can be obtained at high temperature and/or low ℜR. For R≳0.1, while the deposition rate was weakly dependent on temperature between 260 and 100 °C, it increased almost by a factor of 2 between 100 and 45 °C. This is also accompanied by drastic changes in film properties such as refractive index, increase in OH and –OC2H5 content, and decrease in film density. Studies using in situ attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy indicated that stable good quality SiO2 films without any SiOH at higher temperature (250 °C) and with very little SiOH at room temperature could be deposited using very low R. Based on the understanding provided by ATR FTIR, films with properties approaching to those of thermal oxide have been deposited at room temperature.

High-Rate Rpecvd of a-Si:H Films by Means of a VHF Resonant Plasma Source

AbstractHigh deposition rate (up to 5 nm/s) a-Si:H films suitable for recrystallization were deposited using a λ/4 helical resonator source. Refractive index, Tauc-gap, photo- and dark conductivities were measured for film characterization. The metastable behaviour was characterized by the light- induced degradation of the photoconductivity. Hydrogen content and bonding configuration were analyzed by IR absorption and mass separated thermal effusion transients, film microstructure was studied by intentionally incorporating carbon and oxygen. Most of the hydrogen is located on internal surfaces in the otherwise dense material. Differences between the deposition from our highly excited plasma and the conventional remote PECVD process are discussed.