Low Drift Velocity Research Articles

A Comparison of Electromigration in Bulk and Thin - Film Aluminum

AbstractThe dominant mode of electromigration in polycrystalline Al thin - film conductors is along grain boundaries when the conductor width is significantly larger than the grain size. Integrated circuit feature sizes, however, have now decreased to the point where microstructures are no longer polycrystalline, but are near - bamboo. Electromigration must operate along pathways other than grain boundaries in the bamboo segments. Here drift velocity data is presented for bamboo microstructures with widths down to 0.6 μm and compared with drift data available in the literature for thin films with a variety of microstructures and bulk Al. Bamboo films show lower drift velocities and higher activation energies for drift than polycrystalline films. The data for bamboo microstructures is consistent with drift measurements performed on bulk Al indicating that the transport mechanism in bamboo films is identical to that in bulk Al.

Electron transport properties of Si/SiGe heterostructures: Measurements and device implications

We report electron transport properties of modulation-doped Si/SiGe at 300 and 77 K. Record mobilities of 2830 and 18 000 cm2/V s at 300 and 77 K, respectively, have been measured. Depending on the spacer layer thickness, the sheet resistance of the Si channel is in the range of 2000–10 000 Ω/⧠ at 300 K and 450–700 Ω/⧠ at 77 K. The low field electron drift velocity is 2–3 (5–10) times higher than the corresponding velocity measured in Si/SiO2 structures at 300 K (77 K). The saturation velocity is measured to be only 5% higher than in bulk Si, at both 300 and 77 K, but appears at a lower electric field. The effect of the enhanced transport properties in modulation-doped Si/SiGe on device design and performance is investigated.

The derivation of the parameters of coronal mass motions from noise storm observations

Based on a total of 152 events, we studied the spectral parameters of the continuum component of isolated noise storms (Lantos, 1982) subdividing the data set according to flare-related and non-flare-related events. There were typical differences between both classes with regard to starting frequency and frequency extent. The flare-related events can mostly be attributed to loops rising into the corona with velocities as being known for transients of moderate velocity and erupting prominences. The non-flare-related events must evidently be ascribed to a broad variety of mass motions but on the average these mass motions have a lower outward drift velocity than those associated with the flare-related events.

Studies of helium gas mixtures for low mass tracking detectors

Helium gas mixtures can improve traciing resolution for particles below 1 GeV/ c because of the reduction in multiple scattering. We have measured the drift velocity and spatial resolution of several interesting mixtures including helium:DME and helium:CO 2:isobutane. We discuss the advantages and disadvantages of using such a gas mixture for a detector at a B-meson factory, and conclude that helium:DME (80:20) and (70:30) mixtures will give significantly improved momentum resolution. The low drift velocity in such mixtures is a disadvantage in a high rate environment, but this is partly compensated by a low sensitivity to synchrotron radiation background.

D0 vertex drift chamber construction and test results

A jet-cell based vertex chamber has been built for the D0 experiment at Fermilab and operated in a test beam there. Low drift velocity and diffusion properties were achieved using CO 2(95%)-ethane(5%) at atmospheric pressure. The drift velocity is found to be consistent with [9.74+8.68(|; E|;−1.25)|; μm/ns where E is the electric field strength in kV/cm (1.0 kV/cm < |; E|; < 1.6 kV/cm. An intrinsic spatial resolution of 60 μm or better for drift distances greater than 2 mm is measured. The track pair efficiency is estimated to be better than 90% for separations greater than 630 μm.

Far-infrared studies of the plasmon resonance of a drifting 2DEG

We report far-infrared measurements of the shift of the plasmon resonance of a 2DEG drifting under a metal grating coupler of symmetric profile. Measurements were made as a function of lateral current through the GaAs AlGaAs heterostructure 2DEG in samples with different grating periods. Reference spectra were taken in a strong magnetic field which sweeps the (magneto-)plasmon resonance to higher energy. The plasmon energy is observed to be shifted downwards with increasing drift velocity, but this behaviour is non-linear. The results compare well with computations of the optical response of a drifting 2DEG near a metal grating performed using the Scattering Matrix method. The discrepancy can be accounted for by a slight decrease in the number density of carriers at high currents. The expected splitting of the plasmon caused by the presence of the grating is not observed at low drift velocity because the higher frequency mode is antisymmetric and is not coupled to radiation by a symmetric grating profile.

A shaping amplifier for a high-resolution drift chamber with flash-ADC readout

To fully exploit the spatial resolution capabilities of drift chambers operated at low drift velocities and readout by flash analog to digital converters, the use of shaping amplifiers becomes essential. The concept, the optimization study and results from tests of the shaping amplifier developed for the central tracking chamber of the L3 experiment at LEP are presented.

Drift chambers with controlled charge collection geometry for the NA34/Helios experiment

We report on the design and performance of a set of drift chambers for the NA34/HELIOS experiment at the CERN-SPS. The chambers are operated with CO 2 at low drift velocity and have a field geometry allowing a controllable track sample length and isochronous charge collection. In beam tests using a silicon microstrip hodoscope as external track reference, we measure a two-track separation of 600 μm and, at a drift velocity of 8.9 μm/ns, an absolute single track percision of 60 μ on average the total drift distance of 20 mm. The influence of the track sample length on the chamber performance is studied and a value of 2.5–3.0 mm is found as an optimum.

Large-signal time-of-flight experiments in polycrystalline tetrabenzofulvalene thin films

Time-of-flight experiments were carried out on the aromatic organic material tetrabenzofulvalene. It was shown that the response is a large signal. The transit times were resolved and the dependence of the drift velocity on the electric field was studied. The mechanism of low field drift velocity saturation in aromatic organic solids is reconsidered.

Lifetime and drift velocity analysis for electromigration in sputtered Al films, multilayers, and alloys

Besides high lifetimes and a low standard deviation of failure times, the electromigration drift velocity is an important parameter for metallization reliability. Here, lifetimes should be high and drift velocity low. For pure Al films, only properly chosen and optimized sputter conditions yield good film quality and electromigration data comparable to evaporated stripes. Annealing and oxide covering improve lifetime and reduce drift velocity of pure Al structures. For redundant AlTiAl multilayers, annealing yielded an enormous lifetime enhancement of 3 orders of magnitude. However, these stripes are not suited for practical applications because of whisker formation and of an increased drift velocity! These disadvantages do not appear for a lifetime-improving TiN intermediate layer instead of Ti. A further method of lifetime enhancement is Cu-addition which reduces the drift of stripes. Finally, for (Al + Cu)TiN(Al + Cu)-structures, lifetime is improved even further in combination with low drift velocity.

Special gas mixtures with very small drift angles at low drift velocities

Using the novel technique of UV laser-induced ionization, drift angles and drift velocities have been measured for various gas mixtures. In the region where the drift velocity is linearly dependent on the drift field and up to 10 kG, the measured values agree well with predictions by computations also described in this article. Two gas mixtures were found which give small drift angles at low drift velocities. This is important for the operation of a time expansion chamber (TEC).

Lower-hybrid drift instability saturation mechanisms in one-dimensional simulations

The linear properties and saturation mechanisms of the lower-hybrid drift instability have been investigated using a one-dimensional particle-hybrid simulation. For low drift velocities (vd≪vti), ion trapping and current relaxation (vd→0) are competing processes for stabilization. If the relative electron–ion drift velocity is kept constant in time, ion trapping causes the end of instability growth; if this drift is allowed to decrease consistent with momentum balance, then saturation is due to current relaxation.

Demonstration of an ultra-high mobility organic polymer

Abstract Experimental evidence is given for a high mobility (μ ∼ 20 m2 s−1 V−1) and a low field-saturated drift velocity (v d ∼ 2·2 × 103 m s−1) for carrier motion along a polymer chain. Photoconduction experiments in time domains from 10−9 to 102 seconds and fields from 102 V m−1 to 106 V m−1 are described. The polymer, the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol, is a single crystal in which the conjugated semiconducting carbon chains are all parallel to each other. Carriers travel a distance approaching 1 mm along a chain before trapping at a defect. It is possible to cut the chains and detect the resulting reduction of range of carrier motion.

Nonlinear frequency shift induced by the lower-hybrid drift instability

It is found that a finite perturbations of the ion orbits leads to a nonlinear frequency shift that reduces the mode frequency and has a weak stabilizing effect on the lower-hybrid drift instability. This result is obtained from a self-consistent solution of the Vlasov–Poisson equations using perturbation theory in which the nonlinear dielectric function and the nonlinear temporal evolution of a single unstable mode in the low drift velocity regime are calculated analytically. The nonlinear frequency shift does not seem to be a potent saturation mechanism in a collisionless plasma, but may be more relevant when there are ion-ion collisions.

Forced oscillations generated by a point antenna in a drifting plasma

Drift effects on forced oscillations generated by an oscillating antenna are analysed in two cases. First, a Water Bag model with temperature T>or=0 is considered using a mixed analytical and numerical method. The numerical part is shown to be necessary to obtain results outside the perpendicular and parallel directions to the drift and radiation diagrams at different distances from the antenna give the behaviour of the potential. The existence of a radiation cone of the Cherenkov type is pointed out, and whilst outside of a cone centred on the drift direction with angle alpha , the potential is independent of the temperature, the bulk of the thermal effects are concentrated in an inner cone of smaller angle. Secondly a Maxwellian plasma represented by a Multi Water Bag (MWB) model with 60 bags is used and the normalisation of the distances is made classically in terms of the Debye length. The integration is performed by the same analytico-numerical method as used in the first case and the authors discuss the topology of the poles and the problems introduced by the choice of model. Radiation diagrams for low drift velocity (vD=0.35 vT) and w=1.2 wp are given at various distances from the source.

Radiation potential of a point antenna immersed in drifting cold or hot (hydrodynamical) plasma

Drift effects on forced oscillations generated by an oscillating antenna are analysed in two cases: 1.(i) For the first we consider a Water Bag model with temperature T ≥ 0 (with a special treatment for T=0). A non usual normalisation of the distances based on a parameter p equal to the thermal velocity divided by the drift one allows us to treat cold plasma in the W.B. formalism. A mixed analytical and numerical method is used. The numerical part is shown to be quite necessary to obtain results outside the perpendicular and parallel directions to the drift. Radiation diagrams at different distances from the antenna give the behaviour of the potential. The existence of a radiation cone of Cherenkov toe is pointed: outside a cone centered on the drift direction with angle α given by α = Arcsin √3ϱ, the potential is strictly independent of the temperature. On the other hand, the bulk of thermal effects are concentrated in an inner cone of smaller angle given by Bohm and Gross relation. Between these two cones, precursors effects appear.2.(ii) In the second case for which the normalisation of the distances is made classically in term of Debye length, we take a Maxwellian plasma represented by Multi Water Bag (M.W.B.) model with 60 bags. The integration is performed by the same analytico-numerical method. We discuss the topology of the poles, and the problems introduced by the choice of the model are pointed out. The difficulties lessen with increasing drift velocity. Radiation diagrams for low drift velocity (VD = 0.35 VT) and ω = 1.2 ωp are given at various distances from the source.

Oscillating echoes from the polar E layer

Radio waves with a frequency of 4 MHz transmitted vertically into the polar ionosphere and reflected in the E layer occasionally show very stable oscillations in intensity. This phenomenon, which occurs mainly in the evening sector under quiet conditions, has been studied at Ny‐Ălesund, Spitzbergen, by using a 4‐MHz transmitter and four spaced receiving antennas. The observed oscillations can be explained as interference of waves reflected from a pair of enhanced ionization regions some 30 km apart and moving with a horizontal velocity of the order of 100 m/s. The low drift velocity is consistent with low magnetic activity.

Lower hybrid drift instability at low drift velocities

The behavior of the lower hybrid drift instability is investigated in the low drift velocity regime by including the effects of the magnetic field on the ions. It is found that the mode is transformed from the nonresonant lower hybrid drift instability to the resonant ion cyclotron drift instability as the drift velocity Vd decreases and is finally stabilized when Vd/Vti∼ (me/mi)1/2.

Experimental study of the post-implosion phase of a theta pinch

Using 90° Thomson scattering and magnetic probes, the time and space resolved electron temperature and density and magnetic field profiles were obtained for a small, 15 kJ theta pinch. At times after the implosion phase, t≳0.45–0.5 μsec, the plasma is characterized by a high β, 0.5≲β≲1.0, Te/Ti&amp;lt;1, and vEϑ≲ vi (where vEϑ is the E×B drift speed and vi is the ion thermal speed). The current sheath width, ΔLs, and plasma resistivity in the sheath ηeff, were obtained from the measured temperature, density, and field profiles at these late times. The results show that ΔLs≲ 3c/ωpi≫c/ωpe, ηeff≈100 η⊥, and the effective collision frequency, ν′eff, ω1h≲ν′eff≪ωpi, where η⊥ is the classical, Spitzer–Härm resistivity, and ω1h is the lower hybrid frequency. These results indicate that plasma turbulence is an important factor when considering heating and transport, even in the low drift velocity regime. Theoretical estimates based on the lower-hybrid-drift instability are in reasonably good agreement with the measured resistivity and ion heating rate.

Effects of a magnetic field gradient on the lower hydrid drift instability

The effect of a magnetic field gradient on the lower hydrid drift instability is examined in the low drift velocity regime (i.e., of the order of the ion thermal velocity). The theoretical model includes density and temperature gradients, and considers only waves propagating perpendicular to the ambient magnetic field with wavelengths of the order of the electron Larmor radius. It is found, for the case of finite βe, that a magnetic gradient drift-wave resonance can be important and is stabilizing or destabilizing depending upon the physical conditions. Substantial plasma heating and a large anomalous resistivity can also occur.