DC Saddle Field Research Articles

Modified Saddle Field Ion Source Using A Ring Focusing Electrode

<b> </b>An experimental study of a dc saddle field ion source is reported with a view to maximizing the beam current and insuring extended operation of the source. The dependence of the beam current on geometrical and operational parameters is studied. The electrical characteristics of a dc saddle field discharge were studied using nitrogen gas. This study includes the experimental arrangement for ion beam production from nitrogen gas under the influence of discharge voltages applied to the focusing electrode. Also, different focusing hole diameters are examined and optimized to get a highest efficiency of the ion source. A focusing electrode in the form of a ring is placed at 4 mm from the ion source exit. Different ring electrodes with different exit hole diameters (5, 7, 9, and 11 mm) were used as focusing electrodes. The optimum hole diameter for the focusing electrode was found to be 9 mm where maximum ion beam current can be obtained. It was found that at 9 mm hole diameter, focusing voltage = - 3 kv, and pressure P<SUB>r</SUB> = 8x10<SUP>-4</SUP> mbar using nitrogen gas at distance between two anode rods= 6 mm, discharge region = 5 mm, an ion beam current of 243 µA could be obtained. These data are important for ion beam sputtering, ion beam thinning and in other several applications.

Interface properties of amorphous-crystalline silicon heterojunctions prepared using DC saddle-field PECVD

The planar conductance technique has been previously used in the study of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunctions, grown using conventional Radio Frequency Plasma Enhanced Chemical Vapor Deposition (RF-PECVD), to examine the existence of a strong inversion layer at the c-Si surface. In the present study such measurements were undertaken on a series of heterojunctions in order to provide insight into the nature of the electrical interface between the hydrogenated amorphous silicon, deposited using the DC Saddle Field PECVD system, and the underlying crystalline silicon wafer. The films showed good passivation and a strong inversion layer, indicating their amenability for device applications.

Influence of the Electrode Spacing on the Plasma Characteristics and Hydrogenated Amorphous Silicon Film Properties Grown in the DC Saddle Field PECVD System

ABSTRACTA new plasma deposition system was built with the capability of varying the electrode spacing in the DC Saddle Field plasma enhanced chemical vapor deposition system. An ion mass spectrometer was installed just below the substrate holder to sample the ion species travelling towards the substrate. Silane plasma and amorphous silicon film studies were conducted to shed light on the impinging ion species, ion energy distributions, and film properties with varying electrode spacing. The results indicate that decreasing the distance between the substrate and cathode leads to a reduction in the high energy ion bombardment.

Spectroscopic ellipsometry studies on hydrogenated amorphous silicon thin films deposited using DC saddle field plasma enhanced chemical vapor deposition system

Hydrogenated amorphous silicon (a-Si H) films deposited on crystalline silicon substrates using the DC saddle field (DCSF) plasma enhanced chemical vapor deposition (PECVD) system have been investigated. We have determined the complex dielectric function, ε(E)=ε1(E)+iε2(E) for hydrogenated amorphous silicon (a-Si:H) thin films by spectroscopic ellipsometry (SE) in the 1.5–4.5eV energy range at room temperature. The results indicate that there is a change in the structure of the a-Si:H films as the thickness is increased above 4nm. This is attributed to either an increase in the bonded hydrogen content and, or a decrease of voids during the growth of a-Si:H films. The film thickness and deposition temperature are two important parameters that lead to both hydrogen content variation and silicon bonding change as well as significant variations in the optical band gap. The influence of substrate temperature during deposition on film and interface properties is also included.

Passivation study of the amorphous–crystalline silicon interface formed using DC saddle‐field glow discharge

AbstractThe DC saddle‐field (DCSF) glow discharge method was used to deposit intrinsic a‐Si:H onto c‐Si to passivate the c‐Si surface. The effective minority carrier lifetime in the heterostructures as a function of the excess minority carrier density in the c‐Si wafers was measured. The results were then analyzed in the context of recombination associated with interface defect states using three known recombination models. The defect density and the charge density at the interface are inferred. In addition subsequent annealing of the samples was studied. It is shown that for our intrinsic a‐Si:H samples improvements in surface passivation are directly correlated with the reduction of interface defects and not the reduction of minority carrier concentration at the interface due to electric field. We have achieved excellent surface passivation with effective carrier lifetime &gt;4 ms for an intrinsic a‐Si:H sample deposited at a process temperature of 200 °C and thickness of about 30 nm. It is also demonstrated that subsequent annealing, at 240 °C, of the samples which were prepared at process temperatures &lt;240 °C greatly increases the effective lifetime.

Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a-Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritium outgassing with a time constant of 15h, seems to be related to surface desorption of tritiated water (HTO) with a free energy of desorption of 1.04eV. The second process, slow tritium outgassing with a time constant of 200–300h, appears to be limited by oxygen diffusivity in a growing oxide layer. This material of lowest H stability would lose half of the hydrogen after 60years.

Monte Carlo modeling of the dc saddle field plasma: Discharge characteristics of N2 and SiH4

Enhancements in rates of ionization, dissociation, and current in the dc saddle field (DCSF) glow discharge are studied using the direct Monte Carlo method (DMCM). The DCSF consists of a planar semitransparent anode positioned between two planar cathodes, producing a symmetric electric field that serves to lengthen the path of electrons. Simulations of current versus anode transparency of a N2 discharge agree very well with experimental results reported previously. Numerical results of the DCSF SiH4 discharge are also presented. At typical operating conditions (600V, 5cm cathode-anode spacing, anode transparency of 0.8), DMCM predicts a significant increase in the dissociation rate at pressures below 100mTorr in comparison to the dc diode. In consideration of its use for plasma enhanced chemical vapor deposition, the efficiency with which the DCSF operates at low pressures makes it attractive for the production of thin films whose qualities are sensitive to gas phase reactions.

Photocarrier Radiometric Lifetime Measurements of Intrinsic Amorphous-Crystalline Silicon Heterostructure

AbstractIntrinsic hydrogenated amorphous silicon films were deposited by the DC saddle field system on crystalline silicon wafers. The substrate temperature of the amorphous film, crystalline silicon surface cleaning schemes, and the native oxide etchant were varied. The transport parameters of the amorphous-crystalline silicon heterostructures were evaluated by Photocarrier Radiometric (PCR) lifetime measurements. PCR bulk lifetime estimates were obtained using the quinhydrone in methanol solution to passivate the crystalline silicon surface. We present the effectiveness of the PCR system in evaluating different surface passivation schemes.

The Reactive Neutral Beam Etching of SiC and its Application in p-n Junction Periphery Protection

This paper presents much more details on the process of etching n and p type SiC using a dc saddle field source. Here is described a method for stabilizing the dc discharge by adding controlled flow of O2 to SF6 in the source chamber. This kind of etching is used to fabricate 4H-SiC p-i-n diodes with a junction periphery protection. The effect of the junction periphery protection, the source power that terminates the etching process and testing environment on the breakdown voltage are investigated. The optimised p-i-n diodes exhibit a stable reverse bias operation with a breakdown voltage of 1700 V.

Optical properties of hydrogenated amorphous carbon thin films prepared by dc saddle field plasma-enhanced chemical vapor deposition

Hydrogenated amorphous carbon thin films were grown through dc saddle field plasma-enhanced chemical vapor deposition. All samples were grown in similar conditions, except that the chamber pressure and the substrate bias parameters were varied. The sample films were analyzed using Ultraviolet–Visible, Fourier Transform Infrared, and Raman spectroscopies. The Ultraviolet–Visible analysis was used in order to obtain the wavelength dependence of the index of refraction, the extinction coefficient, and the optical absorption coefficient. The index of refraction was found to increase as a function of the substrate bias voltage. Fourier Transform Infrared spectroscopic and Raman analyses were used in order to obtain the concentration of the sp 2 carbon bonds in the films. This measurement helped us interpret the relation between the sp 2 bond concentration and the optical absorption in the a-C:H thin films.

Electrical and Optical Properties of Amorphous and Microcrystalline Hydrogenated Silicon Films Deposited Using Saddle Field Glow Discharge

AbstractAmorphous (a-Si:H) and microcrystalline (μc-Si:H) hydrogenated silicon films were obtained using DC saddle field glow discharge. The structure of the films was determined by Raman spectroscopy, SEM and TEM. The optoelectronic characteristics of both a-Si:H and μcSi:H were investigated using FTIR, UV/VIS spectroscopy, dark electrical conductivity (σd) and photoconductivity (σph) measurements. Boron and phosphorous doping of a-Si:H and μc-Si:H films was also investigated. The results show that both a-Si:H and μc-Si:H undoped films are highly resistive (σd=10-8-10-10 Ω-1cm-1). The doping efficiency of μc-Si:H films is much higher than a-Si:H films. The Tauc gap for a-Si:H was in the range 1.8-1.9 eV and for μc-Si:H films it was in the range 1.9-2.5 eV. The photoconductivity measurements of undoped films indicate a higher photosensitivity of a-Si:H films (σph/σd=104)than that of μc-Si:H films (σph/σd=10-100).

Atmospheric aging and thermal annealing effects in a-C:H thin films

Atmospheric aging and thermal annealing effects have been studied in hydrogenated amorphous carbon (a-C:H) thin films deposited using a dc saddle field glow discharge technique with different ion energies (85–225 eV) during deposition. The a-C:H films grown with low ion energies showed aging effects when they were exposed to the ambient atmosphere. Infrared (IR) absorption due to O–H and CO vibration modes increased while the IR absorption due to C–H vibrations decreased with aging. The absence of absorption due to O–D vibrations in deuterated films indicates that the films reacted with water rather than oxygen when exposed to the atmosphere. The C–H and O–H bond concentrations decreased when the a-C:H films which had been exposed to the atmosphere for a one month period were thermally annealed. The a-C:H films grown with high ion energies exhibited neither atmospheric aging nor thermal annealing effects.

Diamond-like carbon films with extremely low stress

We in this paper report different ways to realise thick diamond-like carbon (DLC) films with stress values lower than 0.5 GPa, Thick DLC films grown by conventional r.f. self bias technique often delaminate from the substrates due to the presence of high compressive stresses of the order of 4–7 GPa. We have made an in-depth study of the delamination problem of DLC films at NPL and found that only for substrates kept away from the plasma (plume) it is possible to grow thick DLC films. This goes to show the heating of the substrates, when m contact with the plasma, appears to be one of the most important factors giving rise to the high stress values. Techniques that have produced consistently low stress values (0.2–0.5 GPa) in this laboratory are pulse plasma PECVD and the one using dc saddle field fast atom beam source. Electronic properties of the materials so produced have been estimated by evaluating Urbach energy using photothermal deflection spectroscopy (PDS) measurements. A correlation between the unbound hydrogen in these films, as measured by a nuclear technique (ERDA), and the stress levels has been found. Deposition rate, room temperature conductivity, optical bandgap and refractive index have also been measured for these films.