Gas Utilization Efficiency Research Articles

Gas-efficient deposition of device-quality hydrogenated amorphous silicon using low gas flows and power modulated radio-frequency discharges

Hydrogenated amorphous silicon samples have been deposited by plasma-enhanced chemical-vapor deposition, using a square-wave amplitude-modulated radio-frequency excitation. In this article it will be shown that a combination of amplitude modulation and low gas flows improves the gas-utilization efficiency by a considerable amount. Using a conventional 50 MHz SiH4/H2 plasma with gas flows of 30 sccm, both for SiH4 and H2 at a pressure of 20 Pa, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s. The combination of amplitude modulation and gas flow reduction gives rise to sufficient ion bombardment and hydrogen dilution at low flows. Device-quality optoelectronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The electrical properties were also appropriate for solar cell application. The photo-to-dark-conductivity ratio varied between 105 and 107. The material exhibited a low defect density which is in the order of 1015–1016 cm−3. The Urbach energy amounted to 52 meV on the average.

Hyrogenated Amorphous Silicon with High Growth Rate, Gas Utilization and Homogeneous Deposition by Amplitude Modulated Vhf-Pecvd for Solar Cell Application

AbstractHydrogenated amorphous silicon samples have been deposited by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD), using a square-wave amplitudemodulated radio-frequency excitation. It is observed that the gas-utilization efficiency improves by a considerable amount when amplitude modulation is combined with a reduction in the gas flows. Using a conventional continuous wave (cw) 50 MHz plasma with SiH4 and H2 gas flows of 30 sccm each at a pressure of 0.2 mbar, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s; which is twice as large as compared to the deposition rate of a cw deposition. Device-quality opto-electronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The materials exhibited a low defect density (CPM) which is in the order of 3-8x1015 per cubic centimeter and photo-to-dark-conductivity ratio of 4-6x106. N-i-p solar cells of size 0.16 cm2 deposited on 10cmx10cm stainless steel (SS) substrate in the configuration SS/n-a-Si:H/i-a- Si:H/buffer/p-μc-Si/ITO/Ag grid (without back reflector) using amorphous silicon i-layer made by amplitude-modulated VHF plasma CVD showed an efficiency of 6.5%. This is a similar efficiency to the cell with standard device-quality cw a-Si:H in the same n-i-p structure, but at a high growth of 0.55 nm/s and gas utilization of ∼50%.

Development of Portable Pulsed Neutron Generators Utilizing a D-T or D-D Fusion Reaction

Prototypes of sealed neutron tubes in a D-T or D-D fusion reaction for logging while drilling (LWD) were developed; then operational tests were performed to check their functional properties. One of the prototypes passed most of the specified conditions for using LWD. Further studies were needed to put a sealed neutron tube into practical use. For applications to other fields, such as an in situ calibration source for neutron detector efficiencies and an in situ calibration source for fusion systems, a sealed neutron tube is needed to have higher-intensity neutron output and a long life. Thus, the performance of the ion source used in the neutron tube is improved to obtain high gas utilization efficiencies or low-pressure operation with high ionization efficiencies. The characteristics of the new ion sources used in the foregoing sealed neutron tube are discussed in terms of preliminary tests. The aforementioned performances are obtained.

高効率大直径ウェーハ作製のための大円盤形シートプラズマ

Ordinary plasmas for the silicon wafer production of large area and high deposition rate, are produced in large volume-shape under high gas pressures and large gas flows (above 1.33×10 Pa and a few 1000 sccM) where their ionization rates and gas utilization efficiencies are very low. Then, the gas load of vacuum pump and the gas disposition become very serious. To improve these problems, the process plasma must be produced in wide sheet-shape under lower pressures and smaller gas flows where a high ionization rate and a high gas utilization efficiency are possible. Thus, a large disklike (50 cmφ) sheet plasma in thickness of 4 mm is proposed in a plasma density of 1012/cc of H2 gas and an electron temperature of 30 eV where the H2 gas pressure is lower (2.66 × 10-1 Pa) and the gas flow is smaller (200 sccM). An outside plasma (1011/cc, and 4 eV in 50 cmφ) 3 cm apart from this plasma sheet will be very usefull as the much improved process plasma.

A-Si:H and a-SiGe:H Alloys Fabricated Close to Powder Regime of RF PECVD

ABSTRACTWe have been observing that a-Si:H and a-SiGe:H films deposited under high chamber pressure or close to powder regime (500 to 2000 mT for a-Si:H and 200 to 1000 mT for a-SiGe:H) show many new features : the mobility lifetime product is 10 to 100 times higher and the density of states above Fermi level of a-Si:H and a-SiGe:H ([Ge] < 0.20) is lower than that of standard samples. This enhancement in photoconductivity can neither be attributed to autodoping nor to creation of sensitization centers in the samples. Hydrogen bonding of these films, is mostly monohydride and the density of microstructural (polyhydrides and microvoids) defects is low. Evidence for the presence of nanocrystals is noted. Though the films seems amorphous by all conventional techniques, they crystallize easily under low laser intensity. Kinetics of light induced degradation is very fast (less than 20 hrs), for a-Si:H and the value of photoconductivity in the light soaked state is comparable to that of the standard samples in the annealed state. The process gas utilization efficiency in this regime is also higher than in the low power and pressure regime.

Polysilicon RTCVD process optimization for environmentally-conscious manufacturing

In the semiconductor manufacturing industry, optimization of advanced equipment and process designs must include both manufacturing metrics (such as cycle time, consumables cost, and product quality) and environmental consequences (such as reactant utilization and by-product emission). We have investigated the optimization of rapid thermal chemical vapor deposition (RTCVD) of polysilicon from SiH/sub 4/ as a function of process parameters using a physically-based dynamic simulation approach. The simulator captures essential time-dependent behaviors of gas flow, heat transfer, reaction chemistry, and sensor and control systems, and is validated by our experimental data. Significant improvements in SiH/sub 4/ utilization (up to 7/spl times/) and process cycle time (up to 3/spl times/) can be achieved by changes in 1) timing for initiating wafer heating relative to starting process gas flow; 2) process temperature (650-750/spl deg/C); and 3) gas flow rate (100-1000 seem). Enhanced gas utilization efficiency and reduced process cycle time provide benefits for both environmental considerations and manufacturing productivity (throughput). Dynamic simulation proves to be a versatile and powerful technique for identifying optimal process parameters and for assessing tradeoffs between various manufacturing and environmental metrics.

Accumulation of polyphosphate and substrate gas utilization efficiency in PHB accumulation phase of autotrophic batch culture of Alcaligenes eutrophus ATCC17697T

Abstract The consumption of phosphate, formation of polyphosphate and the intracellular ATP level were compared in exponential phase and PHB-accumulating cells of Alcaligenes eutrophus . The phosphate consumption (mg per g dry cell weight) by PHB-accumulating cells was larger than that by exponentially growing cells. The phosphate consumed by the PHB accumulating cells was converted to polyphosphate, which was a storage substance for phosphorus. The intracellular ATP concentration in PHB-accumulating cells (expressed as μmol g −1 protein) was 1.5 times that in exponentially growing cells. The same proportional increase in H 2 oxidation was observed. From this result it was observed that the ATP pool size expressed in terms of ATP/g protein per mol H 2 product was the same in both culture phases.

Gas injection system in the Tara central cell

The gas injection system in the Tara central cell has been redesigned to overcome problems associated with fueling in the high field region at the ends of the central cell. Previous experimental studies indicated that plasma ions created in the region between the gas injection point and an ICRH antenna were lost radially before passing to the downstream side of the antenna. In the new system gas is injected at two diametrically opposite ports into a cylinder (‘‘gas box’’) located near the midplane, between the two double half turn antennas. A maximum in the magnetic field at the midplane divides the central cell into two R=2 mirrors. Ions trapped in one of these mirrors must scatter into the loss cone before passing into the fueling region, where the loss rate due to charge exchange is highest. This configuration produces separate regions for ionization (at the gas box) and for heating (near the magnetic field minimum). Pumping of neutral gas by the central cell plasma reduces the amount of neutral gas flux into the axicell to acceptable levels. Gas utilization efficiencies are roughly twice as high as in the original configuration.

Epitaxial growth from organometallic sources in high vacuum

Herein we describe a process that combines some of the principal advantages of molecular beam epitaxy and metal-organic chemical vapor deposition (MOCVD) systems. We call this process vacuum chemical epitaxy (VCE). In this process, multiple group III-alkyl molecular beams are directed through a water-cooled gas distribution block onto wafers providing for the growth of uniform films over large areas with high group III-alkyl utilization efficiency. The group V source, on the other hand, is injected at a single point on one side of the deposition zone. The group V molecules are confined and undergo molecular flow across the deposition zone. The utilization efficiency of the group V source material can be enhanced by the use of a thermal cracker at the point of group V gas injection. This high group V gas utilization efficiency, combined with the fact that all of the reactants are contained within a stainless-steel vacuum chamber equipped with an air lock for wafer loading, leads to a safer MOCVD scaleup reactor. In this paper our VCE reactor is described in some detail along with the properties of III–V films grown with this equipment. The fabrication of a GaAsSb solar cell with an active area energy conversion efficiency of 26.7% demonstrates that VCE has the capability of producing high-performance devices.

High Deposition Rate Amorphous Silicon Alloy Xerographic Photoreceptor

AbstractA new multilayer amorphous silicon alloy photoreceptor has been deposited at rates exceeding 36 µm/hr. using 2.45 GHz microwave glow discharge. The device whose structure is Al/a-Si:H:F (B-300)/a-Si:H:F (B-10)/a-Si:H:F:C is deposited in a powderless plasma deposition process which exhibits gas utilization efficiency approaching 100%. The xerographic performance of a 28µm device is: Vsat∼1100 V for a +7 KV corona; dark half decay time ≃5 sec; and photosensitivity ∼0.3 µJ/cm2 at λ = 650 nm. Stable, high quality xerographic images are obtained with these photoreceptors.

Plasma studies on a hollow cathode, magnetic multipole ion source for neutral beam injection

A series of Langmuir probe studies has been undertaken on the hydrogen plasma of an 8×8-cm extraction area hollow cathode, magnetic multipole ion source intended for use in neutral beam injection systems. Five movable probes were utilized to measure plasma density, electron temperature, plasma potential, and primary electron density as a function of position in the plasma generator and performance of the ion source. The measurements indicate that at discharge current below 150 A, a fairly uniform plasma over the source volume, with a potential positive with respect to the anode, is generated. The performance of the plasma generator at low currents resembles the filament discharge, magnetic multipole confinement ion source. The ion source in this case has good efficiency and plasma uniformity, but atomic species fractions of less than 70%. As the discharge current is increased, the plasma separates into a high-density, high-ionization, positive potential region near the cathode, and a lower-density, negative potential plasma near the extraction grids. This plasma near the acceleration electrodes is characterized by a relatively low primary electron to plasma electron density ratio of about 2%. The performance of the source significantly increases over the uniform volume plasma case with atomic species output of typically 85% and gas utilization efficiencies of 40%–65%. The high-current, hollow cathode discharge produces a plasma configuration and output performance similar to that of the duoPIGatron ion source, but with lower noise levels.

Neutral Beam Line Improvements Resulting from a Reduction of Gas Flow

A D+ ion source with a gas utilization efficiency of 85% or greater is feasible and leads to dramatic performance improvements and simplifications of a high energy neutral beam line for fusion reactors. The source should be long compared to present sources and have magnetic multipole boundaries sufficiently intense to be highly reflecting for both ions and electrons. The benefits to be anticipated include the elimination of power dissipation at the acceleration electrodes, removal of the source from direct neutron bombardment, a great reduction in required pumping, and the elimination of reionization losses.

Mass transfer between a horizontal, submerged gas jet and a liquid

The rate of reaction between a horizontal, submerged gas jet and a liquid has been measured in a model system under conditions where mass transfer in the gas phase is rate limiting. The gas was 1 pct SO2 in air, and the liquid was a 0.3 pct solution of hydrogen peroxide in water. SO2 absorption rates were measured as a function of jet Reynolds number (10,000 < NRe < 40,000) and jet orifice diameter (0.238 < d0 < 0.476 cm). The product of the gas phase mass transfer coefficient and the interfacial area per unit length of jet trajectory, kSO2 α was found to increase linearly with increasing Reynolds number and to be a strong function of the orifice diameter. The ratio of kso2 α to volumetric gas flow rate was shown to be independent of Reynolds number for a given orifice diameter. Extrapolated values of kso2 α are lower than the coefficients measured for vertical CO jets blown upward through liquid copper. Extrapolation of the measured mass transfer data to the jet conditions in copper matte converting and in the gaseous deoxidation of copper has indicated that the gas utilization efficiencies in these processes should approach 100 pct if gas phase mass transport is rate controlling.