Modeling of fluorine-based high-density plasma etching of anisotropic silicon trenches with oxygen sidewall passivation J. Appl. Phys. 94, 6311 (2003); 10.1063/1.1621713Reduction of silicon recess caused by plasma oxidation during high-density plasma polysilicon gate etching Effects of gas distribution on polysilicon etch rate uniformity for a low pressure, high density plasma Evolution of etched profiles has been numerically studied during low-pressure, high-density ͑LPHD͒ plasma etching of Si in Cl 2 . The surface etch rates were calculated using a reaction model of synergism between incoming ions and neutral reactants, including the spread of ion angular distributions due to their thermal motions and the transport of neutrals arising from the reemission on surfaces in a microstructure. Etched profiles were then simulated using a so-called two-dimensional string algorithm to examine the effects of ion temperature kT i and energy ͑or sheath voltage͒ eV s on the etch anisotropy for different neutral-to-ion flux ratios ⌫ n /⌫ i toward the substrate. Numerical results indicated that in typical Cl 2 LPHD plasma etching environments, where the neutral-to-ion flux ratio is ⌫ n /⌫ i ϳ 1 and the ratio of sheath voltage to ion temperature is eV s /kT i ϳ 100, the chlorinated surface coverage is microscopically nonuniform in etched features: The coverage is very low at the bottom ͑␣ϳ0.1͒, whereas the sidewall surface ͑␣ϳ1͒ is almost saturated with neutrals. This microscopic nonuniformity of the coverage in etched features is the proposed mechanism responsible for the inversely tapered profiles that often occur in LPHD plasma etching. Additionally, the decrease in vertical etch rate in microstructures or the reactive-ion-etching lag due to neutral shadowing effects is also found to become significant in LPHD plasma etching. At such a low flux ratio of ⌫ n /⌫ i ϳ 1, more directional ions with a higher ratio of eV s /kT i տ 500 are required for the anisotropic etching; e.g., for an ion energy ͑or sheath voltage͒ of eV s ϭ 50 eV, the ion temperature in a plasma is required to be kT i Շ 0.1 eV.

The advantages and feasibility of neutral beams with Z > 3 formed from negative ions, accelerated to 0.5-1.0 MeV-amu" 1 , and neutralized, are investigated for use in tandem mirror reactor end plugs. A reactor plasma physics design incorporating these beams has been done with the result that such a reactor could produce Q's (ratio of fusion power to injected power) of 20-30. These beams thus might be a replacement for the currently proposed 200-500-keV neutral proton beams presently planned for tandem mirror reactors. Thus these Z > 3 neutral beams could increase the potential attractiveness of tandem mirror reactors by offering a possible substitute for difficult high-energy neutral-hydrogen end plug beams.

The RepE initiator protein (251 residues) is essential for mini-F replication in Escherichia coli and exhibits two major functions: initiation of DNA replication from ori2 and autogenous repression of repE transcription. Whereas the initiation is mediated by RepE monomers that bind to the ori2 iterons (direct repeats), the autogenous repression is mediated by dimers that bind to the repE operator, which contains an inverted repeat sequence related to the iterons. We now report that the binding of RepE to these DNA sites is primarily determined by the C-terminal region of this protein. The mutant RepE proteins lacking either the N-terminal 33 (or more) residues or the C-terminal 7 (or more) residues were first shown to be defective in binding to both the ori2 and the operator DNAs. However, direct screening and analysis of mutant RepEs which are specifically affected in binding to the ori2 iterons revealed that the mutations (mostly amino acid substitutions) occur exclusively in the C-terminal region (residues 168 to 242). These mutant proteins exhibited reduced binding to ori2 and no detectable binding to the operator. Thus, whereas truncation of either end of RepE can destroy the DNA-binding activities, the C-terminal region appears to represent a primary DNA-binding domain of RepE for both ori2 and the operator. Analogous DNA-binding domains seem to be conserved among the initiator proteins of certain related plasmids.The mini-F plasmid, like the parental F factor, is stably maintained in Escherichia coli with one or two copies per host chromosome. Replication of mini-F requires several host factors, including DnaA (14,22,31), HU (34,38), and a subset of heat shock proteins (DnaK, DnaJ, and GrpE) (6, 18), besides the plasmid-encoded replication initiator protein, RepE (251 residues, 29 kDa) (see reference 21). A minimal mini-F consists of an origin of replication (ori2), repE, and a drug resistance gene such as bla and exhibits high copy numbers (10 to 15 copies per chromosome) (19) due to the lack of incC, which negatively modulates the copy number. The ori2 region contains two DnaA boxes, an AT-rich region, and four direct repeats (iterons) of 19 bp, whereas the repE operator contains an inverted repeat whose half sequence (10 bp) resembles the ori2 iterons (8-bp matches) (Fig. 1). The RepE protein, a sequence-specific DNA-binding protein, binds to the ori2 iterons and to the operator (27,36). The binding to these separate but related DNA sequences plays a key role in the regulation of mini-F replication: RepE acts as an initiator of DNA replication through binding to ori2 and as an autogenous repressor of repE transcription through binding to the operator (see references 21, 30, and 37).We recently reported that the two functions of RepE are carried out by structurally distinct forms of the protein. One of the RepE mutants (RepE54), previously selected for their ability to replicate in the dnaJ mutant host (16), produced hyperactive RepE that cannot form dimers (17), unlike the wild-type protein that is fou...

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The effect of etch products on the Si etching characteristics in electron cyclotron resonance plasmas with Cl2 was investigated by using both plasma diagnostics and a numerical simulation. The etch products both in the gas phase and on the etched surface were observed by in situ Fourier transform infrared (FTIR) absorption spectroscopy. The concentration range of silicon tetrachloride SiCl4 detected as the gas-phase etch product during Si etching was about 1012 molecules/cm3 within the pressure range of 0.1–10 mTorr, while that of silicon oxide was 1010–1012 molecules/cm3. On the etched surface, unsaturated silicon chlorides SiClx (x=1–3) as well as SiCl4 were observed. Moreover, the effect of the etch products was numerically investigated by using a kinetic etching model. The calculated etch rate showed good agreement with the measured one within the pressure range of 0.1–10 mTorr even though the sticking coefficient of silicon oxide was varied from 0.001 to 0.5. The coverage of the SiClx layer saturated about one monolayer beyond pressure of 1 mTorr. The bare Si surface area and the actual coverage of the oxide layer decreased by increasing the gas pressure. However, the actual coverage of oxide layer at <1 mTorr varied according to the sticking coefficient of silicon oxide. The calculated concentration of the etch products was the same order of magnitude as the concentration obtained by in situ FTIR diagnostics. It is expected that the effect of etch products on the etching characteristic, especially etched profiles, becomes more and more important at pressure of less than 1 mTorr.